1. Treatment of Nuclear Industry Waste,
using Magnox Dissolution and Liquid
Effluent Treatment Processes
Design Project
This Report is Submitted to
The University of Manchester
BUFORD, Lulu (7990845)
CHOONG, Heng (8187909)
MAHESHWARI, Sriyansh (7493671)
NANAYAKKARA, Jessica (7522073)
PATEL, Nikesh (7991224)
SILLERS, Adam (7876756)
TAN, Max (7500192)
WEBBER, Harvey (7635103)
April 2014

2. Part 1.......................................................................................................................................................1
Part 2......................................................................................................................................................67
Magnetic Separation Process...................................................................................68
Dissolution Process - Design 1..............................................................................102
Dissolution Process - Design 2...............................................................................134
Thermal Decomposition Process..........................................................................169
Kiln - Design 1.........................................................................................................202
Kiln - Design 2.........................................................................................................237
Carbonation Tower.................................................................................................272
Ion Exchange Process.............................................................................................305
Part 3................................................................................................................................................340

3. PART 1: Preliminary Process Design
1

4. Abstract
The design project considers a magnox dissolution process combined with a liquid effluent treatment
process (LEFT) for the treatment of approximately 800 m3 per year of magnox wastes and radioactive
liquid effluent waste respectively. In the magnox dissolution process, there are a number of steps
including filtration systems, dissolution, thermal decomposition and calcination which are used
to eliminate solid wastes and generate a useful product, magnesium oxide. In the liquid effluent
treatment process, carbonation and ion exchange are used to remove radioactive nuclides from the
liquid effluent wastes before being stored for further processing. The treatment plant is to be located
on the Sellafield site as it is in close proximity to both the magnox and liquid effluent wastes. The
utility requirement of the process was determined together with preliminary process integration
analysis. It was found that potentially, the total amount of hot utility required, 47.3 kW, could be
recovered from the kiln’s exhaust gases through calcination, whilst the heat recovery between other
processes reduces the cold utility requirement from 50.3 kW to 10.3 kW.
2

5. Table of Contents
1 Process Summary 9
1.1 Process Justification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2 Process Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2 Process Description 11
2.1 Filtration System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Magnox Dissolution Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.1 Dissolution Column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2.2 Thermal Decomposition Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2.3 Kiln . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3 Liquid Effluent Treatment (LEFT) Process . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3.1 Ion Exchange Column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3.2 Carbonation Tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.4 Ancillary Equipments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.4.1 Pumps and Compressors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.4.2 Heat Exchangers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3 Material, Energy and Radioactivity Balances 24
3.1 Material Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.1.1 Material Balance Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.1.2 Limitations of Simulation Packages . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.2 Process Flow Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2.1 Material Balance Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.2.2 Equipment List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.3 Energy Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.3.1 Dissolution Column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.3.2 Thermal Decomposition Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.3.3 Kiln . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.3.4 Ion Exchange Column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.3.5 Carbonation Tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.4 Radioactivity Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4 Health and Safety Assessment 42
4.1 Preliminary Health and Safety Assessment . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.2 Material Health and Safety Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.2.1 Carbon Dioxide Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.2.2 Magnesium Hydroxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.2.3 Magnesium Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.2.4 Magnesium Carbonate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.2.5 Clinoptilolite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.2.6 Caesium, Strontium, Plutonium, Americium and Cobalt . . . . . . . . . . . . . 46
4.2.7 Methane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.3 Personnel and Worker Safety Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.4 Equipment Health and Safety Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3

6. 4.4.1 Filtration System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.4.2 Dissolution Column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.4.3 Thermal Decomposition Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.4.4 Kiln . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.4.5 Ion Exchange Column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.4.6 Carbonation Tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.4.7 Pumping System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.4.8 Heat Exchange System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
5 Utilities and Process Integration 57
5.1 Utility Requirements without Heat Recovery . . . . . . . . . . . . . . . . . . . . . . . . 57
5.2 Process Integration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
5.2.1 Heat Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
6 Unit Allocation for Part Two 63
7 References 66
4

7. List of Tables
3.1 Mole and Mass Balance for Streams 1-10 . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.2 Mole and Mass Balance for Streams 11-20 . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.3 Mole and Mass Balance for Streams 21-30 . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.4 Mole and Mass Balance for Streams 31-35 . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.5 Equipment List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.6 Overall Energy Balance for the Dissolution Column . . . . . . . . . . . . . . . . . . . . 34
3.7 Heat of Reaction Calculation for the Dissolution Column . . . . . . . . . . . . . . . . . 34
3.8 Overall Energy Balance for the Thermal Decomposition Reactor . . . . . . . . . . . . . 35
3.9 Heat of Reaction Calculation for the Thermal Decomposition Reactor . . . . . . . . . . 36
3.10 Overall Energy Balance for the Kiln . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.11 Overall Energy Balance for the Ion Exchange Column . . . . . . . . . . . . . . . . . . . 38
3.12 Overall Energy Balance for the Carbonation Tower . . . . . . . . . . . . . . . . . . . . . 39
3.13 Heat of Neutralisation Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.14 Comparison between Feed Radioactivity and Permissible Limits in Tera Becquerels
per annum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.15 Radioactivity Balance Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.1 Filtration System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.2 Dissolution Column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.3 Thermal Decomposition Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.4 Kiln . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.5 Ion Exchange Column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.6 Carbonation Tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.7 Pumping System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.8 Heat Exchange System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
5.1 Heating Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5.2 Cooling Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5.3 Process Integration Stream Information . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
5

8. List of Figures
2.1 Block Flow Diagram for the Magnox Waste Treatment Process . . . . . . . . . . . . . . 11
2.2 Mass Transport through a Membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3 Typical Flow Diagram for a Membrane Filter . . . . . . . . . . . . . . . . . . . . . . . . 13
2.4 The amount of metal adsorbed with respect to initial pH (initial metal concentration
300 mg dm-3, equilibration time 24 h, temperature 20 ◦C, solid to solution ratio 1:200) 19
2.5 Carbonation Process via Bubbling Carbon Dioxide in a Tank (Linde Group
SOLVOCARB R -B Process) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.6 Rotary Drum Dryer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.1 Overall Process Flow Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
5.1 Composite Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.2 Cascade Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.3 Grand Composite Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
6

9. List of Reactions
Reaction 2.1 Carbonic Acid Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Reaction 2.2 Magnesium Bicarbonate Formation . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Reaction 2.3 Magnesium Bicarbonate Dissociation . . . . . . . . . . . . . . . . . . . . . . . . . 16
Reaction 2.4 Bicarbonate Ion Dissociation Reaction 1 . . . . . . . . . . . . . . . . . . . . . . . . 16
Reaction 2.5 Bicarbonate Ion Dissociation Reaction 2 . . . . . . . . . . . . . . . . . . . . . . . . 16
Reaction 2.6 Water Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Reaction 2.7 Magnesium Carbonate Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Reaction 2.8 Magnesium Hydroxide Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Reaction 2.9 Magnesium Hydroxide Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Reaction 2.10 Magnesium Carbonate Decomposition . . . . . . . . . . . . . . . . . . . . . . . . 17
Reaction 2.11 Magnesium Hydroxide Decomposition . . . . . . . . . . . . . . . . . . . . . . . . 17
Reaction 2.12 Caesium-134 Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Reaction 2.13 Caesium-137 Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Reaction 2.14 Strontium-90 Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Reaction 2.15 Cobalt-60 Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Reaction 2.16 Carbonic Acid Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Reaction 2.17 Carbonic Acid Disassociation to Bicarbonate . . . . . . . . . . . . . . . . . . . . . 20
Reaction 2.18 Carbonic Acid Disassociation to Carbonate . . . . . . . . . . . . . . . . . . . . . . 21
Reaction 3.1 Dissolution Column Overall Reaction . . . . . . . . . . . . . . . . . . . . . . . . . 34
Reaction 3.2 PFR Reaction 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Reaction 3.3 PFR Reaction 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Reaction 3.4 Sodium Carbonate Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
List of Equations
Equation 2.4.1 Power Calculation for Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Equation 3.3.1 Energy Balance Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Equation 3.3.2 Energy In . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Equation 3.3.3 Energy Out . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Equation 3.3.4 Energy Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Equation 3.3.5 Energy Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Equation 3.3.6 Energy Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Equation 3.3.6 Enthalpy of Reaction 1 in Dissolution Column . . . . . . . . . . . . . . . . . . 36
Equation 3.3.6 Enthalpy of Reaction 2 in Dissolution Column . . . . . . . . . . . . . . . . . . 36
Equation 3.3.7 Energy Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Equation 3.3.8 Temperature Elevation in Kiln . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
7

10. Nomenclature
Qh,min Minimum hot utility requirement kW
Qc,min Minimum cold utility requirement kW
CP Specific heat capacity kJ kg-1 K-1
CP Flowing heat capacity kW ◦C-1
Qi Heat duty (stream i) kW
∆f H0
298K Standard enthalpy of formation kJ kmol-1
∆Tmin Minimum approach temperature ◦C
n Mass/mole fraction -
Ein Energy in kW
Eout Energy out kW
Ereaction Energy of reaction kW
∆Tb Deviation in boiling point ◦C
Tb Boiling point temperature before solids addition ◦C
x2 Mass fraction of solids -
R Ideal has constant J mol-1 K-1
∆hvap Enthalpy of vaporisation J mol-1
P Power required (at 100% efficiency) kW
γ Ratio of specific heats -
Q1 Volumetric flowrate at inlet m3 s-1
P1 Pressure at inlet Pa
P2 Pressure at outlet Pa
Abbreviations and Acronyms
CSTR Continuously Stirred Tank Reactor
HSE Health and Safety Executive
LEFT Liquid Effluent Treatment Process
NRTL Non-Random Two Liquid
NTU Nephelometric Turbidity Units
ONR Office for Nuclear Regulation
PFR Plug Flow Reactor
PRSV Peng-Robinson Stryjek-Vera
PVDF Polyvinylidene Di-fluoride
RK-S Redlich-Kwong-Soave
SOLVOCARB Solutions for Dissolving Carbon Dioxide
TLV Threshold Limit Value
TR Traces
UNIFAC UNIversal Functional-group Activity Coefficients
UNIQUAC UNIversal QUAsiChemical
8

11. 1. Process Summary
The purpose of this project is to create an economical, safe and environmentally sustainable process
for the treatment of both; legacy waste produced by the UK’s first generation of magnox type nuclear
reactors as well as liquid effluent produced by normal operational nuclear reactors. The plant is
to be located on the Sellafield site in Cumbria. Magnox waste can be separated into solid and liquid
streams; the solid material will be encapsulated in concrete whilst the liquid stream will be treated for
certain radionuclides before being stored for further treatment. Magnesium oxide will be extracted
from the liquid stream and used as a packing material for transuranic waste. Existing magnox waste
stockpiles will enable approximately 800 m3 of material to be treated each year for minimum of five
years. The liquid effluent produced by standard nuclear reactors will be treated for the presence of
radionuclides before being disposed of at sea.
1.1 Process Justification
The magnox breed of nuclear reactor is one that is unique to the UK, the first magnox reactor began
operating in 1956 and the final reactor is due to be decommissioned in 2014. The fuel rods used in
magnox reactors are cladded in an alloy consisting primarily of magnesium and aluminium; these
materials have the benefit that they have low neutron capture cross sections, however they can
create problems for long term storage and disposal. When the uranium fuel rods are spent, they
are removed from the reactor core and stored underwater until their heat has dissipated sufficiently.
When magnox fuel rods were stored underwater, the magnesium cladding reacted with the water
and over time this resulted in the formation of a thick sludge known as magnox sludge which
consisted predominantly of magnesium hydroxide. This legacy waste which the process will be
treating is currently being stored on the Sellafield site; locating our facility here will mean that the
potentially hazardous and costly operation of transporting the waste can be avoided.
Standard liquid waste produced by nuclear reactors is being produced in large quantities each day;
this waste is often highly contaminated with radionuclides and must be treated for safe disposal.
Nuclear waste is a difficult and dangerous material to store and contain, it is perhaps the largest
inconvenience to the industry and whilst its eventual treatment may be delayed it must eventually
be confronted. The treatment of nuclear waste is a service, the raw materials of which come at
no cost; unfortunately the same can be said for the products which are effectively impossible to
sell in conventional markets due to their association with the nuclear industry. Finding innovative
ways to incorporate these products with other nuclear processes can save costs and improve process
sustainability. Thanks to the value of the service provided by this facility it may be possible to apply
for public funding to pay for the initial construction and day-to-day operation of the plant.
1.2 Process Overview
The treatment process begins by combining the two waste forms each of which will have been stored
in their own separate storage tanks. A series of increasingly fine filters will remove effectively all
the solid material from the feed. Of the solid removed by the filters only magnesium hydroxide is
retained for further treatment, all other solids are sent for encapsulation in concrete. The concentrated
suspension of magnesium hydroxide in water is treated by the magnox dissolution process whilst the
water permeate which is highly contaminated by radionuclides is sent to the liquid effluent treatment
process (LEFT).
9

12. The magnox process begins by using a dissolution column to convert the magnesium hydroxide into
magnesium bicarbonate in addition to removing hazardous radionuclides. This is followed by a
plug flow reactor (PFR) which further converts the bicarbonate to carbonate. A final membrane filter
concentrates the magnesium hydroxide and magnesium carbonate suspension by removing water
and magnesium bicarbonate. An evaporator is used to dry the concentrated slurry of magnesium
hydroxide and magnesium carbonate whilst thermally decomposing any remaining magnesium
bicarbonate. The dry magnesium hydroxide and magnesium carbonate then undergo thermal
decomposition in a kiln to produce magnesium oxide. This process reduces the volume of waste
to be disposed of by burial by a factor of 50 and creates a solid magnesium oxide product.1 All the
liquid removed by the process will be sent through the LEFT process.
The liquid permeate that is separated by the filter systems at the beginning of the process will be
treated using the LEFT process. This process makes use of an ion exchange column to remove
the radioactive isotopes of strontium, caesium and cobalt. A carbonation tower is then used to
reduce the pH of the effluent. The ion exchange process allows for the removal of up to 99% of the
strontium, caesium and cobalt but as other radioactive isotopes will be present such as plutonium
and americium; the stream will need to be sent for further processing before it can be discharged into
the sea.
10

13. 2. Process Description
This section describes the function, design and operating conditions of all units and ancillary
equipment which form part of the treatment process, shown in Figure 2.1
Figure 2.1: Block Flow Diagram for the Magnox Waste Treatment Process
2.1 Filtration System
This process involves the use of three sets of filters; two sets process the feed stream (which is a
combination of the magnox sludge and standard liquid effluent) and a third separates the desirable
products from the suspension which exits the plug flow reactor (PFR). This section will provide a
detailed explanation of the function of each.
F-101
This represents a combination of four filters of varying mesh size, it includes; a mechanical bar
screen, a vibrating screen, a coarse screen and a micro screen.
The bar screen removes large items of solid material which are present in the open air storage ponds
of the standard liquid effluent as well as shards of spent fuel rods and other miscellaneous pieces of
metal debris which are present in the magnox storage ponds. Bar spacing is limited to between 15 to
40 mm and a maximum flow velocity of 0.9 m s-1 is imposed to ensure material is not forced through
the bars. This screen will be inclined at an angle of 70◦ to the horizontal and will be mechanically
cleaned to prevent the build-up of material which could potentially block the inflow.
Smaller items of debris will be removed by vibrating screens.2 These screens will be designed for
solid particles of a size greater than 10 mm and work by vibrating at frequencies in the range of 1000
to 7000 Hz. These screens can achieve high separating efficiencies at high throughput rates which is
important because if sharp, pointed items of debris were to pass through they could puncture the
membrane filters which follow or damage the intake pump.
The coarse and micro screens which follow act as the final pre-treatment for the membrane filters;
they will have mesh sizes of 2000 and 120 µm respectively.3
The solid waste removed by each of these filters will be sent for encapsulation in concrete.
F-102
This is a membrane designed for ultrafiltration.3 Ultrafiltration can very successfully remove
suspended particles of a size greater than 5 nm almost completely independent of the quality of the
feed, as shown in Figure 2.2. For mass balance calculations it has been assumed that the smallest
11

14. molecule to be separated from the slurry, magnesium hydroxide has a fixed size of 10 nm and
can therefore be completely removed from the feed stream by the membrane; this is a suitable
assumption to make given that ultrafiltration works by using size exclusion to physically reject
material.4 The high turbidity of the feed stream prevents the use of finer more complex and
expensive filtration methods such as nanofiltration or reverse osmosis,5 each of which would
become clogged almost immediately due to the high solids content of the slurry (acceptable turbidity
levels for nanofiltration systems are typically accepted to be less than 0.3 NTU; NTU=Nephelometric
Turbidity Units.6 The material of choice for the construction of this membrane will be polyvinylidene
di-fluoride (PVDF)).5 This material experiences minimal degradation within a pH range of 2-11, it
has a high tolerance to a majority of oxidants and is easy to clean thanks to its high resistance to
fouling. Ultrafiltration has the benefits of operating at the low transmembrane pressures of 0.15-1 bar
and low applied pressures of 0.7-2 bar, which reduce pump operating costs.
The membrane F-102 separates the feed stream into water (which is highly radioactive due to the
presence of soluble radionuclides) and a solid cake of magnesium hydroxide. The magnesium
hydroxide cake which accumulates on the membrane is periodically removed by backwashing the
membrane with a stream of pure (uncontaminated) water, creating a highly concentrated slurry of
magnesium hydroxide which is passed to the dissolution column R-201. The backwash is performed
for 2 minutes at the beginning of each 30 minute period. To allow for backwashing the two streams
entering and the two streams leaving the membrane filter must have isolation valves such that
during normal operation the backwash line is disabled and during a wash cycle the normal flow
line is disabled. Backwashing disrupts the flow of material to the ion exchange (part of the LEFT
process) and dissolution columns, so to ensure a steady continuous flow to these units buffer tanks
are installed to collect the permeate and backwash material after the filter.
The water permeate, which is highly contaminated with radionuclide particles due to its long
exposure to contaminated magnox waste or its use in the nuclear reactor, is passed from its buffer
tank to the ion exchange column R-301 in the LEFT process.
Figure 2.2: Mass Transport through a Membrane7
12

15. F-201
This is also a membrane filter similar to that of F-102; it employs ultrafiltration to separate
insoluble magnesium hydroxide and magnesium carbonate from the stream of water and magnesium
bicarbonate ions which leaves the PFR.
The solid cake of magnesium hydroxide and magnesium carbonate is removed from the membrane
by backwashing 50% of the permeate (not pure water as with F-102) at the same time interval and
duration as filter F-102. This creates a highly concentrated slurry of magnesium hydroxide and
magnesium carbonate, which will also contain small amounts of magnesium bicarbonate. This high
percentage of backwash liquid is required to ensure that all the solid cake is removed from filter and
whilst it may seem an excessively large proportion of the overall permeate flow one must consider the
large mass of solid material which must be removed from the membrane. This concentrated slurry is
passed to an evaporator which removes the water and decomposes the magnesium bicarbonate ions.
The permeate consisting of water and magnesium bicarbonate ions is passed to the ion exchange
column R-301.
A typical ultrafiltration membrane system can be seen in Figure 2.3. It shows how a pre-screen (or
in our case a serious of pre-screens) is used as an initial treatment for the feed, then three membrane
units operated in parallel are used for further treatment. This figure also shows how the permeate,
backwash and waste streams connect with the membranes.
Figure 2.3: Typical Flow Diagram for a Membrane Filter5
2.2 Magnox Dissolution Process
The magnox dissolution process begins after the filtration system, taking in the solid filtrates
from F-102. The feed is passed through the dissolution column, R-201, followed by the thermal
decomposition reactor, R-202. The products of R-202 are passed through the filter F-201, described in
Section 2.1. The filtrate from F-201, is passed through the kiln, R-203, which is the final unit operation
in the magnox dissolution process.
13

16. 2.2.1 Dissolution Column
Process Description
The dissolution column follows the filters F-101 and F-102 and is fed with the suspension of
magnesium hydroxide in water created during the backwash cycle of the membrane filter. The
magnesium hydroxide needs to be treated to remove radionuclides and converted into magnesium
bicarbonate which will in later processes be used to create magnesium oxide. Magnesium hydroxide
is insoluble in water of pH≈7 so it is reacted with a dilute solution of carbonic acid, producing
magnesium bicarbonate. The dissolution column is a bubble reactor, in which there are two reactions
taking place. The first is the reaction between carbon dioxide and water to form carbonic acid.
The second is the reaction between magnesium hydroxide and carbonic acid to form magnesium
bicarbonate. The dissolution column is also responsible for the removal of insoluble radionuclides,
mainly strontium, plutonium and americium from the magnesium hydroxide particles. Soluble
radionuclides such as caesium that are absorbed by the magnox sludge are also removed from
magnesium hydroxide during the dissolution process.
The removal of radionuclides is made possible because the insoluble radionuclides form weak
ionic-like bonds with the polar solvent, water, due to the higher electronegativity of the oxygen ion.
This formation of weak ionic bonds between the water and the radionuclides, results in complete
separation of the radioactive nuclides that were present on the surface of the magnesium hydroxide.
The soluble radionuclides, such as caesium, are trapped inside the magnox sludge particles, along
with some of the insoluble radionuclides. The radionuclides that are trapped inside the magnox
sludge are called non-exchangeable radionuclides. Some of the non-exchangeable radioactive
nuclides present in the feed are brought to the surface of the solids due to the dissolution of the
previous layer and transferred to the water, which is treated by the LEFT process. Still trapped
non-exchangeable radionuclides will be treated later in the process.
In addition to the separation of the radioactive nuclides, water acts as a solvent for dissolving the
carbon dioxide gas that is bubbled through the column. The equilibrium reaction between water
and carbon dioxide forms carbonic acid, as shown in Reaction 2.1. Carbonic acid is also referred
to as carbon dioxide solution, as carbonic acid is the equilibrium between the H+ and HCO3
- ions.
This dissociation reaction can extend to the dissociation of carbonic acid into 2 H+ and CO3
- ions.8
Dissolving a gas in a liquid is typically an exothermic process, this reaction follows this trend and is
slightly exothermic.9
H2O(l)+CO2(g) −−−− 2H+
(aq)+CO−
3 (aq) (Reaction 2.1)
Magnesium hydroxide Mg(OH)2, while insoluble in water and difficult to react with carbon dioxide,
readily reacts with carbonic acid. The reaction between the two compounds is highly endothermic
as calculated by Hess’ law. This reaction leads to the formation of magnesium hydrogen carbonate,
more commonly known as magnesium bicarbonate. Like the carbonic acid, magnesium bicarbonate
exists in equilibrium between the MgHCO3
+ and HCO3
- ions in the aqueous phase. Further
dissociation results in equilibrium between a single Mg2+ and two HCO3
- ions in solution, as shown
in Reaction 2.2. As magnesium bicarbonate is thermally unstable, it cannot be isolated as a solid
under normal conditions,9 it is only found in solution.
2H2CO3(aq)+Mg(OH)2(s) −−−− Mg(HCO3)2(aq)+2H2O(l) (Reaction 2.2)
As the two are reactions in series, it is essential to have excess carbonic acid to propagate the forward
reaction, responsible for the dissolution of magnesium hydroxide.
14

17. Design Considerations10
The bubble reactor sees the interaction of all three phases in one reactor; magnesium hydroxide
enters as a solid, water enters as a liquid and carbon dioxide enters as a gas. Under these conditions
it becomes very difficult to achieve high conversion; hence, a conservative target conversion of 75%
was selected for the reactor. The choice and design of this reactor requires a detailed analysis of the
reaction kinetics of each reaction in order to maximise conversion.
If for example the rate constant for the second reaction is higher than the first, then the carbonic acid
is being consumed faster than it is being produced; in such a scenario it is advisable to use a CSTR,
which would help reduce the concentration of magnesium bicarbonate inside the reactor and prevent
the backward reaction from becoming dominant.
If for example the rate constant for the first reaction is higher than that of the second, a batch or plug
flow reactor would be preferred; this would ensure that a higher concentration of carbonic acid was
available for the favoured second reaction. The reverse of the first reaction would not be harmful as
it is an intermediate reaction.
As for the reaction conditions, they can be determined from the reaction equations and would require
optimisation based upon the conversion set. However, based on the reactions described above, it can
be inferred that both operate better at low temperatures. Magnesium hydroxide requires carbonic
acid in which to dissolve, the highest possible concentrations of carbonic acid are therefore required.
High carbonic acid concentrations are achieved under conditions which favour carbon dioxide
forming a solution with water, which as is the case with most gases occuring at low temperatures
because the reaction is exothermic. Ideal operating conditions could therefore be a pressure of 1 atm
and a temperature of 0◦C. However, to obtain such a low temperature, refrigeration would be
required, resulting in increased energy consumption and operating costs. To avoid this situation it
is preferable to use feed which is at ambient temperature (15◦C). To ensure the forward propagation
of the second reaction it advisable to use a heating jacket around the column to provide heat to
the reaction; this will help with overcoming the activation energy barrier and avoid the situation
where the reaction stops completely because it has reached a temperature which is too low. The first
reaction which produces carbonic acid can be propagated forward by ensuring its reactants are in
excess and as carbonic acid consumed by the second reaction this will be maintained. To improve the
production of carbonic acid in the first reaction, stirring or mixing can be used in combination with a
low temperature and moderate gas pressure. An overall reactor temperature of 10◦C and the carbon
dioxide gas pressure in the range of 3.5-5 bar is therefore preferable.11
The temperature in the reactor was allowed to change from 15◦C to 10◦C, as the heat of reaction
is endothermic and if the reactor is kept under adiabatic conditions, the predicted temperature
change was found to be around −7◦C (using heat of reaction and average specific heat capacity
values). In order to maintain the reactor temperature at a constant value of 10◦C, a heating jacket
is recommended to avoid killing the reaction. The two reactions occurring inside the reactor are
opposite to each other in the sense that the carbonic acid formation reaction is exothermic in nature
and the magnesium bicarbonate formation reaction is endothermic. The first reaction would prefer
the use of low temperatures, whereas the second reaction works better at higher temperatures. As the
two reactions are in series it is essential to favour the first reaction over the second reaction and hence
a lower temperature of 10◦C is suggested. As suggested by theory, the conversion of pressurised
carbon dioxide to carbonic acid can be reasonably high, and this should provide sufficient heat and
feed for the second reaction.
In case the feed into the reactor is lower than the recommended 15◦C, the heating jacket around the
reactor should still be able to maintain the temperature of the reactor at 10◦C, in the extreme case
where the feed is at a very low temperature that leads to no reaction, heat can be added by using
either a feed pre heater, increased jacket duty or hot shots.
The use of a catalyst in the reactor was not considered due to the presence of organics and radioactive
15

18. nuclides in the reactor. The effects of these components on the physical and chemical integrity of the
catalyst were not known, hence their use is not recommended. In addition, the use of a catalyst
would introduce a fourth phase (vapour-liquid-solid1-solid2) to an already complex reactor.
Aside from the magnesium hydroxide which has been assumed to be the main component in the
feed, there are traces of other organic components. These organics can range from soluble salts and
gases to insoluble solids; the effect of the organics can be substantial and will affect the design of
the dissolution column. However, the effects cannot be predicted based on the data available and
experiments need to be conducted to predict their effects with reasonable certainty. For the design of
the dissolution column it has been assumed that the organics dissolved in the feed will have no effect
on the two main reactions taking place. It was assumed that the potential side reactions between the
organics and water or carbon dioxide will lead to inert by-products that will not affect the overall
integrity of the process. Due to the lack of data, the mass balances performed assumed no presence
of organics. However, their presence in the feed was considered and their treatment discussed. The
organics and the by-products can be passed through the whole process as inert components, before
being incinerated in the kiln resulting in the treatment of the radioactive organics present in the
magnox feed.
Alternative designs, using two separate reactors, with one reactor concentrating on the formation of
carbonic acid and the other on the dissolution of magnesium hydroxide could be considered in the
case of overly complex design. Ideally however it would be preferable to conduct the dissolution
process in a single reactor as this reduces capital, maintenance and operating costs, whilst containing
the radioactive material within fewer units.
In conclusion, rate constant data is required in order to select the type of reactor. However, as the
overall treatment process is continuous, a continuously stirred reactor or a plug flow reactor would
be preferred over a batch reactor. The operating conditions of the reactor were estimated using the
data available; the reactor temperature was taken to be 10◦C, with the feed entering at 15◦C and
the operating pressure was estimated to be 1 atmosphere. The carbon dioxide gas would enter the
reactor at a higher pressure to ensure better mixing and a higher yield of carbonic acid.
2.2.2 Thermal Decomposition Reactor
Process Description
The stream of water containing a suspension of magnesium hydroxide and dissolved magnesium
bicarbonate enters the reactor at 10 ◦C; when it enters the PFR it is allowed to heat up to 50 ◦C
a temperature which is maintained using cooling water in order to prevent a runaway reaction.
This increased temperature allows for the thermal decomposition of magnesium bicarbonate into
magnesium carbonate, magnesium hydroxide, water and carbon dioxide. The reactor can achieve a
magnesium bicarbonate conversion of 88% with a selectivity of 90%.12
The reaction can be split into two stages, the first being the ionisation of the magnesium bicarbonate
which leads to the formation of water, carbon dioxide, and ions of magnesium, hydroxide and
carbonate. This is shown in Reaction 2.3 to Reaction 2.6.12
Mg(HCO3)2(aq) −−−− Mg2+
(aq)+2HCO−
3 (aq) (Reaction 2.3)
HCO−
3 (aq) −−−− H+
(aq)+CO2−
3 (aq) (Reaction 2.4)
HCO−
3 (aq) −−−− OH−
(aq)+CO2(g)↑ (Reaction 2.5)
16

19. OH−
(aq)+H+
(aq) −−−− H2O(l) (Reaction 2.6)
In the second stage magnesium ions react with the carbonate and hydroxide ions to form magnesium
hydroxide and magnesium carbonate as shown in the Reaction 2.7 and C2.8.12
Mg2+
(aq)+CO2−
3 (aq) −−→ MgCO3(s)↓ (Reaction 2.7)
Mg2+
(aq)+2OH−
(aq) −−→ Mg(OH)2(s)↓ (Reaction 2.8)
The overall reaction can be expressed in reaction Reaction 2.9.
(x+y)Mg(HCO3)2(aq) −−→ xMgCO3.yMg(OH)2.zH2O(s)+(x+2y)CO2(g)↑ +(x−z)H2O(l)
(Reaction 2.9)
Design Considerations
As the ionisation of magnesium bicarbonate is a reversible reaction, Le Chatelier’s principle should
be considered. In order to improve the conversion of the magnesium bicarbonate it is appropriate to
keep its concentration as high as possible; this will favour the forward reaction of reaction one.
The process is to be run continuously, so based on Le Chatelier’s principle a PFR would be preferred
over a continuously stirred tank reactor (CSTR). This is because the system needs to maintain a
high concentration of magnesium bicarbonate to push the equilibrium to the right and increase the
conversion of magnesium bicarbonate. If the reaction was carried out in a CSTR, the conversion
would be much lower because the concentration of the bicarbonate would much lower.
The level of agitation in the reactor has to be considered, it has been shown in experiments carried
out in a stirred tank reactor that an impeller speed of over 600 rpm has little effect on the conversion.
With the reaction being carried out in a PFR, as long as the flow is kept in the turbulent region then
the flow can be considered well mixed in the radial direction.
As the reaction is exothermic the reactor will require a cooling jacket in order to stop a runaway
reaction. The feed to the reactor will be at 10 ◦C which will be allowed to heat up to 50 ◦C as this is
the temperature required to achieve the desired conversion of magnesium carbonate.
2.2.3 Kiln
Process Description
To achieve a product of magnesium oxide, the dried magnesium carbonate and magnesium
hydroxide must undergo thermal decomposition. The equipment used to provide this heating is
a rotary kiln which is essentially a heterogeneous non-catalytic gas-solid reactor. The stoichiometric
equations for the decomposition of magnesium carbonate and magnesium hydroxide to produce
magnesium oxide are shown in Reaction 2.10 and Reaction 2.11.
MgCO3(s)
heat
−−→ MgO(s)+CO2(g)↑ (Reaction 2.10)
Mg(OH)2(s)
heat
−−→ MgO(s)+H2O(l) (Reaction 2.11)
The thermal decomposition of magnesium carbonate takes place at a temperature of 660 ◦C
whilst magnesium hydroxide will decompose at the lower temperature of 330 ◦C.13 The operating
temperature of the kiln should therefore be maintained in excess of 660 ◦C.
17

20. The pressure of the kiln will vary depending on the length of the column which will be decided later
in the design process.
Design Considerations
Since the feed entering the kiln will be a granular solid, this will be conveyed using an air operated
pneumatic conveyor or a belt conveyer, and will be determined based on the size of the particles.
Another consideration is whether the heating will be direct or indirect. Since the material needs to
be heated to a specific temperature for a given amount of time, it may be more appropriate to use
indirect heating because the control would be easier (outside of the shell).14 It is likely that methane
will be used as the fuel to produce a flue gas which will be contacted with the solid and provide
sufficient heat.
The kiln could be inclined at a small angle and gravity used as the main method of transporting the
solids, which allows the assumption to be made that the solids are sufficiently “concentrated”.
The hydrocarbon that can be combusted to create the fire used in the kiln will be methane but may be
replaced when the design is considered further in terms of efficiency. The pressure of the kiln is likely
to be negative (relative to atmospheric pressure), so that the gas used in the kiln does not leak out.
However if there is a leakage in the kiln, this could mean air enters the kiln and potentially lowers
the temperature which will lead to inefficient production of the desired product magnesium oxide.
Furthermore, since the flue gas is now at a high temperature, this can be re-used in such a way as
to heat water to produce steam to be used in maintaining the isothermal reactors at their required
temperature. The flue gas will be used for co-generation, where the heat recovery steam generator
will be used to generate steam and a steam or gas turbine will generate power.
2.3 Liquid Effluent Treatment (LEFT) Process
The LEFT process comprises of the ion exchange column, R-301 and the carbonation tower, R-302.
2.3.1 Ion Exchange Column
Process Description
The primary aim of the ion exchange column is to remove strontium-90, caesium-134, caesium-137
and cobalt-60 from the feed stream through ion exchange to below the acceptable limit for discharge
to the environment. There will be two streams entering the batch ion exchange column, with one
stream at conditions of 25 ◦C and 101 kPa (stream 26),15 and the other at conditions of 15 ◦C and
101 kPa (stream 27). The ion exchange resin to be used is a zeolite named clinoptilolite, which has
been chosen for its selectivity to remove the required radioactive isotopes from the liquid effluent
feed. The unit cell formula of a natural zeolite can be given as:
(Li, Na, K)a (Mg, Ca, Sr, Ba)d [Al (a+2d) Sin-(a+2d) O2n].mH2O
Where the unit cell of clinoptilolite is usually characterized on the basis of 72 oxygen atoms (n=36)
and 24 water molecules (m=24).16 The required amount of clinoptilolite will be determined upon
detailed design of the column from adsorption isotherms.
The ion exchange reactions which take place are detailed by reactions, shown in Reaction 2.12 to
Reaction 2.15.
134
Cs+
(aq)+RX(s) −−−− RCs(s)+X+
(aq) (Reaction 2.12)
137
Cs+
(aq)+RX(s) −−−− RCs(s)+X+
(aq) (Reaction 2.13)
18

21. 2 90
Sr2+
(aq)+2RX(s) −−−− 2RSr(s)+2X+
(aq) (Reaction 2.14)
2 60
Co2+
(aq)+2RX(s) −−−− 2RCo(s)+2X+
(aq) (Reaction 2.15)
Where R represents the ion exchange resin, and X represents the resin cation for exchange, dependent
on the selected clinoptilolite used.
The optimum pH for adsorption of strontium-90, caesium-134, caesium-137 and colbalt-60 using
clinoptilolite under the operating conditions selected, is greater than pH 10, as shown in Figure 2.4.
Figure 2.4: The amount of metal adsorbed with respect to initial pH (initial metal concentration 300 mg dm-3,
equilibration time 24 h, temperature 20 ◦C, solid to solution ratio 1:200)15
As the pH of the feed stream is weakly alkaline,17 the optimum pH should be taken into
consideration, and potentially measured and altered prior to entering the ion exchange column.
The loading and unloading of clinoptilolite has not been shown in the PFD, however after the
ion exchange capacity has been reached, the column will be taken offline, the spent clinoptilolite
unloaded and directed into the cementation process, and fresh clinoptilolite loaded into the column.
The clinoptilolite will become radioactive during operation; an automated procedure will therefore
be required to replace it.
19

22. Design Considerations
As clinoptilolite is a very inexpensive ion exchange resin costing around £5/tonne, contrary to
most ion exchange processes, it is more economical to replace the resin upon saturation instead
of regenerating the resin. However, due to the nature of the process, a method for loading and
unloading the resin with no manual operator interaction will need to be devised.
There are numerous options for the adsorption column configuration; firstly whether to use batch or
continuous processing. The residence time for batch adsorption is significantly shorter to achieve the
same conversion, which will directly impact the required labour hours and rate at which the stored
waste can be processed. In contrast a plug flow reactor would adsorb all the caesium and strontium
ions until it reaches breakthrough, this is not the case in a batch reactor, extending the volume of
liquid effluent which can be processed per gram of zeolite.
Design of the ion exchange columns will be determined by calculation of the resin volume required,
ion exchange capacity required, number of columns, column height and pressure drop through the
column.18 Additionally, prior to the adsorption columns, a holding tank may need to be incorporated
into the design to ensure a continuous supply of liquid effluent into the ion exchange process, as the
previous filtration will also be batch and may cause delays to the ion exchange column loading time.
Upon having selected to use a batch process, the number of columns required must be determined.
Typically, the process will consist of two parallel columns, and at any point in time one column will
be adsorbing the required ions and the other regenerating. In this case the regeneration column
would be unloaded of used resin and loaded with fresh resin during this stage. The ion exchange
process will continue until just before the column reaches breakthrough time, at which point the feed
will be directed through the second ion exchange column, and the first column will be “regenerated”.
However, the capital cost of two columns as opposed to one would have to be considered following
more detailed design and costing of the column.
As the feed is a liquid, it is appropriate that it should be fed into the column from its base, this
will ensure that it flows upwards uniformly, preventing channelling. The velocity must however be
low enough such that the adsorbent will not be lifted, causing attrition. The column will need to be
designed to have a specific residence time, which will be determined through further analysis of the
clinoptilolite adsorption isotherm.
2.3.2 Carbonation Tower
Process Description
Liquid effluent leaving the ion exchange column is alkaline, and needs to be neutralized before it
can be discharged into the sea. The proposed process of neutralizing the alkaline liquid effluent in
the liquid effluent treatment process is known as carbonation. Carbon dioxide bubbles through and
dissolves in the liquid effluent, forming carbonic acid:19
H2O(l)+CO2(g) −−−− H2CO3(aq) (Reaction 2.16)
The carbonic acid exists in equilibrium with carbon dioxide when dissolved in water, with a
hydration equilibrium constant of 1.7×10-3. This indicates that most of the carbon dioxide dissolved
in the liquid remains as molecules and does not become carbonic acid. The use of carbon dioxide for
neutralization will prevent over-acidification of the liquid effluent owing to its buffering capacity;
over-acidification is a possible occurrence when mineral acids are used.20 The carbonic acid further
dissociates in two steps:
H2CO3(aq)+H2O(l) −−−− H3O+
(aq)+HCO−
3 (aq) (Reaction 2.17)
20

23. HCO−
3 (aq)+H2O(l) −−−− H3O+
(aq)+CO2−
3 (aq) (Reaction 2.18)
The established equilibrium is dependent on the pH level of the water; hydrogen carbonate ions
(HCO3
-) are primarily present when the pH level is between 6.33 and 10.33, while carbonate ions
(CO3
2-) are principally presently when the pH level exceeds 10.33. Both HCO3
- and CO3
2- can react
to form salts, while the hydroxide ions react with hydronium ions (H3O+) to form water.
Design Considerations
A possible design for the carbonation process is via bubbling carbon dioxide into a tank containing
the liquid effluent. The carbonation process will be done in an open tank containing the liquid
effluent at atmospheric pressure and temperature. Concerns regarding exposing radioactive
substances to the environment when an open tank is utilized should be negated by the fact that
the carbonation process is downstream of the ion exchange column; most of the radionuclides will
have been removed from the liquid effluent.
The carbon dioxide required for the process is typically stored as a liquid under a high pressure
of 20.5 bar for ease of storage and transport. Before it is bubbled through the liquid effluent, it is
converted to a gas at ambient temperature. A gas diffuser is installed at the bottom of the tank,
injecting carbon dioxide homogeneously into the tank of liquid effluent without additional energy
requirements. The pH level of the tank is measured, which in turn controls the amount of carbon
dioxide bubbled through the liquid effluent.20 Hence, the residence time of the liquid effluent in the
tank will vary according to its level of alkalinity.
Figure 2.5: Carbonation Process via Bubbling Carbon Dioxide in a Tank (Linde Group SOLVOCARB R -B
Process)20
21

24. 2.4 Ancillary Equipments
2.4.1 Pumps and Compressors
There are large amounts of radioactivity present in the process units, therefore pumping can be seen
as a major issue because maintenance cannot be performed on the pumps without large doses of
radiation being received.
On the PFD, currently it is necessary to have a pump or compressor on the recycle stream of CO2 over
the dissolution column. Although the CO2 is pressurised when first entering the column, the exit
stream is at atmospheric pressure and therefore a pump is required to increase the pressure on the
discharge side to the operating pressure of 5 bar. Generally, it is preferred to use a compressor over
a pump for the increase in pressure of a gas. Due to the nature of the stream, it would be necessary
to have a reciprocating compressor with a failure rate of zero for the first five years; the dissolution
column will be operated continuously. An appropriate reciprocating compressor would be a rotary
screw compressor because of the low maintenance required and strong efficiency.21 The problem
however, is that it does not have a failure rate of zero, and therefore if the flow of CO2 were to stop,
this would cause operability problems, and pose the problem of how to safety replace and dispose
of the contaminated pumping system. In terms of the operating range, a rotary screw compressor
can accommodate pressure differences of up to 12 bar and flow rates up to 60,000 m3 h-1,22 which is
well above what is required. Further detail of the compressor will be determined later.
A rough estimate of the power can be calculated using the following equation. This can then be used
to calculate electricity requirements on the plant. The compressor was assumed to be adiabatic for
the simplicity of the power equation, however this may not be realistically possible and should be
investigated further at a later stage.
P =
γQ1P1
γ−1
P2
P1
γ−1
γ
−1 (Equation 2.4.1)
P = Power required (at 100% efficiency)
γ = Ratio of specific heats = 1.2941 (at standard conditions)
Q1 = Volumetric flowrate inlet = 0.01738 m3
(calculated from mass balance with density of CO2 = 1.87 kg m-3)23
P1 = Inlet pressure = 101,300 Pa
P2 = Outlet pressure = 500,000 Pa
The power was found to be at 3388 W. This was based on the volumetric flow originally going into
the pump, but the power amounts will be smaller once the recycle is flowing due to a smaller flow
rate of CO2 . The value of power can be used later to estimate the cost of electricity required, and
then a trade-off can be developed between sending the CO2 that is purged off for further treatment
(PUREX), and recycling with the compressor.
The CO2 entering the carbonation tower is assumed to be pressurised and therefore does not require
a pump or compressor to raise the pressure.
Pumps may be required to physically move the sludge from its holding tank. To do this, it may
be necessary to use a pump with no working parts such as a reverse osmosis pump or a liquid jet
solids pump. This would have to accommodate the large solids content that is present in the feed
stream.24 It may be necessary to use a pump more suitable for sludge such as a rotary lobe pump
and although the initial capital cost is high, it has a high tolerance for rags, large solids and viscous
media (useful if pumping is required before filtration). The pump is also self-priming which makes
22

25. the overall life cycle economically attractive.25 If the pump cannot guarantee a failure rate of zero
over the five years that the plant is planning to be operated for, the risk of a malfunction would have
to be calculated in another cost trade-off: loss of production versus alternative pump designs. The
safety risk however is also very large; an automated robot may be necessary for the replacement of
the pump.
2.4.2 Heat Exchangers
Stream 16 which leaves the membrane filter F-201 is a concentrated slurry of water, magnesium
hydroxide, magnesium carbonate and magnesium bicarbonate. The evaporation unit is used to
recover the water solvent which has become contaminated with radioactive material for further
processing in the LEFT process, decompose the magnesium bicarbonate producing magnesium
carbonate, carbon dioxide and water, and thirdly produce dry magnesium hydroxide and
magnesium carbonate which will form the feed for the kiln.
Steam heated rotary drum dryers26 will be used to evaporate the water from the slurry via conduction
in a continuous process. In Figure 2.6 it can be seen that steam is fed into the centre of each drum,
heating their metal surfaces. Each drum is design to withstand steam pressures of up to 7 bar and
will have a diameter within the range of 1 to 1.8 m.27 The slurry to be dried is fed into the nip formed
between the drums,28 as the drums rotate (in opposite directions) a thin film of the liquid slurry
becomes attached to the metal surface, the heat provided by the steam is sufficient to flash evaporate
the water leaving a thin cake of dry magnesium hydroxide and magnesium carbonate. This cake is
removed by a set of knives and collected in a scroll conveyor. The steam produced is collected by a
fan assisted extraction hood. Typically the mass ratio of steam consumed to evaporation of water can
be as low as 1.3:1.29 This method of supplying the feed from the top of the drums increases the units
capacity as a heavier cake layer can be obtained.
Figure 2.6: Rotary Drum Dryer29
The low pressure steam which leaves the evaporator is of little value for process heating, it is therefore
considered appropriate to condense it to its liquid state and pump it to the ion exchange column
R-301 where it can be treated for radionuclides, such as strontium, as a precaution before disposal.
This condensation shall take place in a vertical shell and tube heat exchanger with condensate on the
shell side and cooling water on the tube side.30
23

26. 3. Material, Energy and Radioactivity Balances
3.1 Material Balance
3.1.1 Material Balance Calculation
The material balance for the process was computed using Microsoft Excel. The starting point for the
balance was at the feed which consisted of both the liquid effluent and magnox sludge. This was
sensible due to the fact that associated numerical quantities for this combined feed were available
from literature datasheets. The datasheets contained volumes, densities and mass fractions. This
was enough data to work out flow rates per hour assuming 300 full working days annually and
a total of 5 years. The remaining 65 days of the year would be downtime and take into account
safety checks and cleaning. A table of moles was produced for calculating conversion of reactants
to products based upon conversion targets. The corresponding mass table was also created using
corresponding molecular weights.
It was assumed that the first filter was 100% efficient, transporting all of the undesirable solids to the
cementation storage vessel. The second filter will be designed to allow complete separation of the
magnesium hydroxide, leaving stream 5 to be purely water.
For the dissolution of magnesium hydroxide, a conversion of 75% was set as a realistic target. There
will be carbonic acid formed as an intermediate, however this amount can be considered negligible
in the outlet for both batch and CSTR arrangements. For the batch reactor, only traces of carbonic
acid will be fed out due to the backwards reaction of the acid occurring, forming carbon dioxide
and water again. If the CSTR arrangement is chosen, the reaction kinetics would cause negligible
carbonic acid formation as it will be reacted at a greater rate than it is produced.
In the plug flow reactor, targets of 88% decomposition ratio (conversion) and 90% selectivity with
respect to magnesium carbonate have been set using conditions given in literature.12 In literature,
the magnesium bicarbonate feed was produced by reacting a mixture of 85% magnesium oxide and
15% of various metal oxides with carbon dioxide. In the proposed process, approximately 76% water
is present which would modify the maximum possible conversion attained.
The membrane filter separates the soluble and insoluble material. The water containing the
radioactive material and dissolved magnesium bicarbonate is sent through the LEFT process. The
magnesium carbonate and magnesium hydroxide are in the solid phase due to their low water
solubility values of 0.0106 g per 100 ml and 0.0014 g per 100 ml respectively.31,32
Both the magnesium hydroxide and magnesium carbonate can decompose to form desired products
and their respective decomposition temperatures are 330 ◦C and 660 ◦C. As the kiln will be operating
at the high temperature condition, a 100% conversion for the magnesium hydroxide has been
assumed. A high conversion target of 99% for the magnesium carbonate was also set in place for
the material balance. The decomposition reaction will be carried out as a continuous process and so
residence times will have to be studied in order to confirm the viability of the targets.
In the liquid effluent sub-process of the plant, the initial arrangement was to have a carbonation
tower placed before the removal of radionuclides in the ion exchange column. Further investigation
led to the rearrangement of order of these two units as it was discovered that the zeolites adsorb the
radioactive elements to greater effect in more alkaline conditions, as shown in Figure 2.4.
24

27. The mass balance across the carbonation process was performed by analysing the neutralization
reaction occurring in the process. The amount of carbon dioxide required to achieve a target pH
level of 8 was calculated, with the assumption that the feed entered the process with a pH level of
10 according to literature values.17 The neutralization is achieved when carbonic acid reacts with the
hydroxide present in the alkaline wastewater. Since carbon dioxide gas only weakly dissociates into
carbonic acid when dissolved in water, the dissociation constant was factored into the calculations.
As a result, the calculated solubility of carbon dioxide in the alkaline wastewater was higher than the
solubility of carbon dioxide in water, which is an expected result.33 It is assumed that the basicity of
the liquid effluent was provided by a common hydroxide such as sodium hydroxide (NaOH), and
hence the salt produced as a result of neutralization was sodium bicarbonate (NaHCO3).
Throughout the entire material balance, the masses of the important radionuclides; strontium-90,
caesium-137, cobalt-60, americium-241 and plutonium-241 have not been considered as they would
be negligible compared to the mass of the associated streams. This simplified the mass balance over
the ion exchanger.
3.1.2 Limitations of Simulation Packages
Simulation packages such as Aspen Plus v8.2 and Aspen HYSYS v8.2 were considered to confirm
the mass and energy balances carried out on the system. The simulation would also provide useful
information regarding the reaction kinetics and operating conditions, which could then be used to
design the equipment. Both simulation packages were tested comprehensively before being rejected
as a valid tool to model the process being designed for the treatment of nuclear waste.
The component packages available in HYSYS did not include multiple components that are part
of the process. The various magnesium salts present in the process were unavailable in HYSYS.
An alternative method of using calcium salts for the model was also tested, considering that both
calcium and magnesium are group 2 elements and would have a number of similar chemical
properties. However, this method of modelling was also rejected as calcium bicarbonate was
unavailable in the database. Therefore, it was decided to not use HYSYS as a simulation package for
this process.
Aspen Plus v8.2 was found to be considerably more sophisticated than HYSYS. The magnesium salts
found in the system were available on the database and the magnesium bicarbonate was assumed to
have dissociated into ions so as to model the process under consideration. Based on Aspen Plus’s
method selection assistant, UNIQUAC was selected as the preferred thermodynamic model for the
dissolution column. A reactor with the associated feeds and the two reactions was set up at the
assumed operating conditions with the desired conversion set. On running the simulation, Aspen
Plus returned with a failure message and it was due to the wrong selection of thermodynamic model.
The errors were read through and another thermodynamic model was tested. The Wilson model,
NRTL, UNIFAC, PRSV and the RK-S models all returned with the same error message indicating
that the parameters were set up wrong. The error message also asked for the molecular structures
of all the components in the process as it could not find them on the database. After the molecular
structures were input using the structure drawing tool available on Aspen Plus, the error message
appeared again.
The error message asked for new Antoine parameters and activity coefficients to be input by the
user, however, these values were unavailable. On further investigation it was found that the use
of magnesium bicarbonate in the dissociated ionic form was not accepted by Aspen Plus. Hence,
a reference thermodynamic model used for electrolytes, as per the model selection assistant, was
added. Carbon dioxide was set up as a component that relies on Henry’s law. It was assumed that
these changes would aid in modelling, however, this was not the case.
The reason behind the constant failure in modelling this process was assumed to be the presence of
three phases in the dissolution column and multiple reactions. It was found that the Gibbs reactor is
25

28. the most ideal reactor model on Aspen Plus for such situations.34 A Gibbs reactor was then set up
to model the vapour-liquid-solid equilibrium reaction. However, that model did not produce any
positive results. Further reading on thermodynamic models was carried out and it was found that
the most ideal thermodynamic model for a reaction with vapour-liquid-solid equilibrium is BARIN
equations for Gibbs energy, enthalpy, entropy and heat capacity. However, this thermodynamic
model is unavailable on Aspen Plus. It was then decided that modelling the dissolution column on
Aspen Plus is only possible if multiple assumptions were made, which would provide results that
might not be reliable.35
The plug flow reactor, responsible for the decomposition of magnesium bicarbonate was chosen next
to be modelled using Aspen Plus. The same error messages continued to occur due to the presence
of magnesium hydroxide in the reactor. This made the modelling of the two reactors very difficult
and hence accurate simulation results would not be obtained. Thus, it was decided that modelling
of processes using Aspen HYSYS or Aspen Plus would not be carried out for this process, due to
the complications raised due to working with three phases and unconventional components, such
as magnesium bicarbonate and magnesium hydroxide in slurry or solid form.
26

29. 3.2 Process Flow Diagram
Figure 3.1: Overall Process Flow Diagram
27

30. 3.2.1 Material Balance Table
Table 3.1: Mole and Mass Balance for Streams 1-10
Stream No Molecular Mass (kg kmol-1) 1 2 3 4 5 6 7 8 9 10
Temp (◦C) 15 15 15 15 15 15 15 15 10 10
Pressure (kPa) 100 100 100 100 100 100 100 500 100 100
Vapour Fraction 0 0 0 0 0 0 0 1 1 0
N2 (kmol h-1) 28
O2 (kmol h-1) 32
CO2 (kmol h-1) 44 3.435 0.859
Mg(OH)2 (kmol h-1) 58.3 1.717 1.717 1.717 1.717 0.429
MgO (kmol h-1) 40.3
Mg(HCO3)2 (kmol h-1) 146.3 1.288
MgCO3 (kmol h-1) 84.3
H2O (kmol h-1) 18 2.631 2.631 5.562 2.631 5.562 5.562 5.562
Solids (kmol h-1)
CH4 (kmol h-1) 16
H2CO3 (kmol h-1) 62
Na(HCO3) (kmol h-1) 84
Total Molar Flow (kmol h-1) 4.348 4.348 0 5.562 2.631 7.280 7.280 3.435 0.859 7.279
N2 (tonne h-1) 28
O2 (tonne h-1) 32
CO2 (tonne h-1) 44 0.151 0.038
Mg(OH)2 (tonne h-1) 58.3 0.100 0.100 0.100 0.100 0.025
MgO (tonne h-1) 40.3
Mg(HCO3)2 (tonne h-1) 146.3 0.188
MgCO3 (tonne h-1) 84.3
H2O (tonne h-1) 18 0.047 0.047 0.100 0.047 0.100 0.100 0.100
Solids (tonne h-1) 0.088 0.088
CH4 (tonne h-1) 16
H2CO3 (tonne h-1) 62
Na(HCO3) (tonne h-1) 84
Total Mass Flow (tonne h-1) 0.235 0.147 0.088 0.100 0.047 0.200 0.200 0.151 0.038 0.314
28

31. Table 3.2: Mole and Mass Balance for Streams 11-20
Stream No Molecular Mass (kg kmol-1) 11 12 13 14 15 16 17 18 19 20
Temp (◦C) 50 50 50 50 50 50 109 109 25 1500
Pressure (kPa) 100 100 100 100 100 100 100 100 100 100
Vapour Fraction 1 0 0 0 0 0 1 0 1 1
N2 (kmol h-1) 28 13.613 13.613
O2 (kmol h-1) 32 3.619 0.329
CO2 (kmol h-1) 44 1.247 0.077 2.731
Mg(OH)2 (kmol h-1) 58.3 0.543 0.543 0.543 0.543
MgO (kmol h-1) 40.3
Mg(HCO3)2 (kmol h-1) 146.3 0.155 0.155 0.077 0.077 0.077
MgCO3 (kmol h-1) 84.3 1.020 1.020 1.020 1.097
H2O (kmol h-1) 18 6.582 6.582 3.291 3.291 3.291 3.368 0.171 4.003
Solids (kmol h-1)
CH4 (kmol h-1) 16 1.645
H2CO3 (kmol h-1) 62
Na(HCO3) (kmol h-1) 84
Total Molar Flow (kmol h-1) 1.247 8.300 6.737 3.368 4.931 4.931 3.446 1.640 19.048 20.677
N2 (tonne h-1) 28 0.381 0.381
O2 (tonne h-1) 32 0.116 0.011
CO2 (tonne h-1) 44 0.055 0.003 0.120
Mg(OH)2 (tonne h-1) 58.3 0.032 0.032 0.032 0.032
MgO (tonne h-1) 40.3
Mg(HCO3)2 (tonne h-1) 146.3 0.023 0.023 0.011 0.011 0.011
MgCO3 (tonne h-1) 84.3 0.086 0.086 0.086 0.093
H2O (tonne h-1) 18 0.118 0.118 0.059 0.059 0.059 0.061 0.003 0.072
Solids (tonne h-1)
CH4 (tonne h-1) 16 0.026
H2CO3 (tonne h-1) 62
Na(HCO3) (tonne h-1) 84
Total Mass Flow (tonne h-1) 0.055 0.259 0.141 0.071 0.188 0.188 0.064 0.124 0.526 0.584
29

32. Table 3.3: Mole and Mass Balance for Streams 21-30
Stream No Molecular Mass (kg kmol-1) 21 22 23 24 25 26 27 28 29 30
Temp (◦C) 660 100 100 50 75 25 15 15 15 15
Pressure (kPa) 100 100 100 100 100 100 100 100 2050 100
Vapour Fraction 0 0.02243 0 0 0 0 0 0 1 -
N2 (kmol h-1) 28
O2 (kmol h-1) 32
CO2 (kmol h-1) 44 0.077 0.010
Mg(OH)2 (kmol h-1) 58.3
MgO (kmol h-1) 40.3 1.629
Mg(HCO3)2 (kmol h-1) 146.3 0.077 0.077 0.077 0.077
MgCO3 (kmol h-1) 84.3 0.011
H2O (kmol h-1) 18 3.368 3.368 3.291 6.660 6.660 2.631 9.290
Solids (kmol h-1)
CH4 (kmol h-1) 16
H2CO3 (kmol h-1) 62
Na(HCO3) (kmol h-1) 84
Total Molar Flow (kmol h-1) 1.640 3.446 3.368 3.368 6.737 6.737 2.631 9.367 0.010 0
N2 (tonne h-1) 28
O2 (tonne h-1) 32
CO2 (tonne h-1) 44 0.003 0.0004
Mg(OH)2 (tonne h-1) 58.3
MgO (tonne h-1) 40.3 0.066
Mg(HCO3)2 (tonne h-1) 146.3 0.011 0.011 0.011 0.011
MgCO3 (tonne h-1) 84.3 0.001
H2O (tonne h-1) 18 0.061 0.061 0.059 0.120 0.120 0.047 0.167
Solids (tonne h-1)
CH4 (tonne h-1) 16
H2CO3 (tonne h-1) 62
Na(HCO3) (tonne h-1) 84
Total Mass Flow (tonne h-1) 0.067 0.064 0.061 0.071 0.131 0.131 0.047 0.179 0.004 0
30

33. Table 3.4: Mole and Mass Balance for Streams 31-35
Stream No Molecular Mass (kg kmol-1) 31 32 33 34 35
Temp (◦C) 15 15 10 10 100
Pressure (kPa) 100 100 100 100 100
Vapour Fraction 0 1 1 1 1
N2 (kmol h-1) 28
O2 (kmol h-1) 32
CO2 (kmol h-1) 44 0.010 2.662 0.773 0.086 0.077
Mg(OH)2 (kmol h-1) 58.3
MgO (kmol h-1) 40.3
Mg(HCO3)2 (kmol h-1) 146.3 0.077
MgCO3 (kmol h-1) 84.3
H2O (kmol h-1) 18 9.290
Solids (kmol h-1)
CH4 (kmol h-1) 16
H2CO3 (kmol h-1) 62
Na(HCO3) (kmol h-1) 84 0.00002
Total Molar Flow (kmol h-1) 9.377 2.662 0.773 0.086 0.077
N2 (tonne h-1) 28
O2 (tonne h-1) 32
CO2 (tonne h-1) 44 0.0004 0.117 0.034 0.004 0.003
Mg(OH)2 (tonne h-1) 58.3
MgO (tonne h-1) 40.3
Mg(HCO3)2 (tonne h-1) 146.3 0.011
MgCO3 (tonne h-1) 84.3
H2O (tonne h-1) 18 0.167
Solids (tonne h-1)
CH4 (tonne h-1) 16
H2CO3 (tonne h-1) 62
Na(HCO3) (tonne h-1) 84 0.000001
Total Mass Flow (tonne h-1) 0.179 0.117 0.034 0.004 0.003
31

34. 3.2.2 Equipment List
Table 3.5: Equipment List
Equipment No. (PFD) Description Notes
F-101 Filtration system Bar, Vibrating, Coarse and Micro screens
F-102 Ultrafiltration membrane Mg(OH)2 removal
TK-101 Magnox feed pond Radioactive magnox sludge
TK-102 Liquid effluent pond Radioactive liquid effluent
E-201 Evaporator Steam heated
E-202 Heat exchanger Steam condensed
E-203 Heat exchanger Water cooled
F-201 Ultrafiltration membrane MgCO3 and Mg(OH)2 removal
P-201 CO2 compressor Fresh and recycled CO2
R-201 Dissolution column Jacketed 3-phase bubble reactor
R-202 Thermal decomposition reactor Jacketed plug flow reactor
R-203 Kiln MgO production
TK-201 Buffer tank Concentrated Mg(OH)2 slurry
TK-202 CO2 storage tank Pressurized
TK-203 Buffer tank Concentrated Mg(OH)2 and MgCO3 slurry
TK-204 Separation tank Removes CO2
R-301 A/B Ion exchange columns Radionuclide removal unit
R-302 Carbonation tower pH control
TK-301 Buffer tank Water permeate
TK-401 Storage tank CO2 storage (radioactive traces)
TK-402 Storage tank Radioactive solids
3.3 Energy Balance
An initial energy balance was carried out for the key units using Microsoft Excel. The balance was
conducted based on the general energy equation as shown below:
EnergyIn–EnergyOut +Generation = Accumulation (Equation 3.3.1)
The total energy in and out can be worked out from enthalpies of formation and specific heat
correlations as shown in equations Equation 3.3.2 and Equation 3.3.3. The plug flow reactor example
calculations have been shown.
Ein = nH2Oh0
f,H2O +nH2O
Tin
T0
Cp,H2OdT
+nMg(OH)2
h0
f,Mg(OH)2
+nMg(OH)2
Tin
T0
Cp,Mg(OH)2
dT (Equation 3.3.2)
+nMg(HCO3)2
h0
f,Mg(HCO3)2
+nMg(HCO3)2
Tin
T0
Cp,Mg(HCO3)2
dT
32

35. Eout = nH2Oh0
f,H2O +nH2O
Tout
T0
Cp,H2OdT
+nMg(OH)2
h0
f,Mg(OH)2
+nMg(OH)2
Tout
T0
Cp,Mg(OH)2
dT
+nMg(HCO3)2
h0
f,Mg(HCO3)2
+nMg(HCO3)2
Tout
T0
Cp,Mg(HCO3)2
dT (Equation 3.3.3)
+nCO2 h0
f,CO2
+nCO2
Tout
T0
Cp,CO2 dT
3.3.1 Dissolution Column
The dissolution column is a reactor operating at atmospheric pressure, with a feed temperature of
288 K, producing products at a temperature of 283 K.
Energy enters the dissolution column through streams 7 and 8, which contain Mg(OH)2, H2O and
CO2. Energy leaves the reactor through streams 9 and 10, which comprise of Mg(HCO3)2, Mg(OH)2,
H2O and CO2. Data was obtained for the enthalpy of formation at standard conditions using the
NIST Webbook, and the molar flow rates were taken from the PFD (Figure 3.1).23 The NIST Webbook
data is unfortunately only valid in the range above 298 K. This data can be extrapolated outside the
given range to 273 K, however this will produce an inaccuracy. It can be assumed that this difference
will not be significant, but some contingency should be incorporated to all further calculations. The
alternative would be to carry out the energy balance at the lowest possible temperature, 298 K,
however this would assume isothermal operation, something found not to be possible without an
excessively large heat requirement. Hence, extrapolation outside the range was deemed to be the
most suitable method.
To calculate the enthalpy of formation of Mg(HCO3)2, Hess’ law was used. However, as the enthalpy
of vaporisation was also unknown, this value was assumed to be negligible to allow the enthalpy
of formation of Mg(HCO3)2 to be estimated. Comparing the calculated value to the enthalpy of
formation of sodium bicarbonate, -950.9 kJ mol-1,36 a similar compound to Mg(HCO3)2, the value
appears to be a sensible estimation. In addition, no correlation could be found for the specific heat
capacity of Mg(HCO3)2, and so it was approximated using the specific heat capacity of sodium
bicarbonate. Comparing the specific heat capacities of both sodium bicarbonate and magnesium
carbonate, it was predicted that the specific heat capacity of magnesium bicarbonate is two-thirds
of that for sodium bicarbonate. It was also assumed that the specific heat capacity of sodium
bicarbonate was an average over the temperature range of the reaction 283 K to the standard
temperature, 298 K as this is a relatively narrow temperature range.
33

36. Table 3.6: Overall Energy Balance for the Dissolution Column
Energy In:
Component Molar Flow/ kmol h-1 CpdT/ kJ kmol-1 ∆f H0
298K/ kJ kmol-1 Energy/ kJ h-1
H2O 5.56 -6243 -285830 -1620000
Mg(OH)2 1.72 -1386 -924660 -1590000
CO2 3.43 -3688 -393510 -1350000
Total: / kJ h-1 -4570000
Energy Out:
Component Molar Flow/ kmol h-1 CpdT/ kJ kmol-1 ∆f H0
298K/ kJ kmol-1 Energy/ kJ h-1
H2O 5.56 -9227 -285830 -1640000
Mg(OH)2 0.43 -2081 -924660 -398000
Mg(HCO3)2 1.29 -876 -999680 -1290000
CO2 0.86 -551 -393510 -338000
Total: / kJ h-1 -3670000
The heat of reaction was calculated using the stoichiometric coefficients and enthalpy of formation
values obtained from the NIST Webbook. The chemical reaction taking place in the dissolution is as
follows:
Mg(OH)2(s)+2CO2(g)
H2O
−−−− Mg(HCO3)2(aq) (Reaction 3.1)
Table 3.7: Heat of Reaction Calculation for the Dissolution Column
In:
Component Stoichiometric Coefficient ∆f H0
298K/ kJ kmol-1 Enthalpy/ kJ h-1
CO2 2 -393510 -787020
Mg(OH)2 1 -924660 -924660
H2O 0 -285830 0
Total: / kJ kmol-1 -1711680
Out:
Component Stoichiometric Coefficient ∆f H0
298K/ kJ kmol-1 Enthalpy/ kJ h-1
Mg(OH)2 0 -924660 0
Mg(HCO3)2 1 -999860 -999860
H2O 0 -285830 0
Total: / kJ kmol-1 -999860
Generation = Products–Reactants (Equation 3.3.4)
Generation = −999860−(−1711680) = 711280
kJ
kmol
= 254.7kW (Equation 3.3.5)
Based on the heat of reaction calculated above, a positive value was obtained, demonstrating that the
reaction is endothermic and requires energy to proceed. Following this, the above energy equation,
and the values obtained from the mass balance and NIST Webbook were used to calculate the energy
in and out:
Ein = -4570 MJ h-1
Eout = -3670 MJ h-1
Ein - Eout = -9020000 kJ h-1
Accumulation = -250 + 255 = 4.22 kW
34

37. 3.3.2 Thermal Decomposition Reactor
An initial energy balance for the plug flow reactor was carried out on Microsoft Excel. The reactor
input includes stream 10 which consists of unreacted magnesium hydroxide and water as well as
the magnesium bicarbonate product from the dissolution column. The reactor feed temperature
(Tin) is at 10 ◦C. The reactor output includes stream 12 which comprises of unreacted magnesium
hydroxide, water and magnesium bicarbonate as well as magnesium carbonate product. Stream 11
which contains carbon dioxide is released from the reaction. The reactor outlet temperature (Tout) is
at 50 ◦C.
According to Equation 3.3.2 and Equation 3.3.3, the enthalpies of formation used for all inlet and
outlet components are based on the standard conditions (25 ◦C and 1 atm). These values can be
obtained from NIST Webbook. To calculate the heat of formation of magnesium bicarbonate, a similar
method was used as with the dissolution column.
Specific heat capacities were integrated between the inlet condition of 10 ◦C or the outlet condition
of 50 ◦C and the standard condition of 25 ◦C for all components except magnesium bicarbonate. The
specific heat capacity of magnesium bicarbonate used is the same as that for the dissolution column.
The results for total energy in and total energy out were tabulated in Table 1 and Table 2 respectively.
Q = Total EnergyIn–Total EnergyOut (Equation 3.3.6)
= −3280000–(−4150000)
= 864000kJ h−1
Table 3.8: Overall Energy Balance for the Thermal Decomposition Reactor
Energy In:
Component Molar Flow/ kmol h-1 CpdT/ kJ kmol-1 ∆f H0
298K/ kJ kmol-1 Energy/ kJ h-1
H2O 5.56 - 1133 -285830 -1600000
Mg(OH)2 0.43 -1134 -924660 -397000
Mg(HCO3)2 1.29 -876 -999680 -1290000
Total: / kJ h-1 -3280000
Energy Out:
Component Molar Flow/ kmol h-1 CpdT/ kJ kmol-1 ∆f H0
298K/ kJ kmol-1 Energy/ kJ h-1
H2O 6.58 1882 -285830 -1870000
Mg(OH)2 0.54 1989 -924660 -501000
Mg(HCO3)2 0.16 1460 -999680 -154000
MgCO3 1.02 1952 -1111900 -1130000
CO2 1.25 943 -393510 -490000
Total: / kJ h-1 -4150000
The chemical reactions taking place in the PFR are shown as follows:
Mg(HCO3)2(aq) −−−− MgCO3(s)↓ +H2O(l)+CO2(g)↑ (Reaction 3.2)
Mg(HCO3)2(aq) −−−− Mg(OH)2(s)↓ +2CO2(g)↑ (Reaction 3.3)
35

38. Table 3.9: Heat of Reaction Calculation for the Thermal Decomposition Reactor
In:
Component Stoichiometric Coefficient ∆f H0
298K/ kJ kmol-1 Enthalpy/ kJ h-1
Mg(HCO3)2 1 -999680 -999680
MgCO3 1 -1111900 -1111900
H2O 1 -285830 -285830
CO2 1 -393510 -393510
Out:
Component Stoichiometric Coefficient ∆f H0
298K/ kJ kmol-1 Enthalpy/ kJ h-1
Mg(HCO3)2 1 -999680 -999680
CO2 2 -393510 -787020
Mg(OH)2 1 -924660 -924660
Enthalpyof Reaction1 = EnthalpyProducts–EnthalpyReactants
= (−1111900+(−285230)+(−393510))−(−999680))
= −791560kJ kmol−1
Enthalpyof Reaction2 = EnthalpyProducts–EnthalpyReactants
= (−787020+(−924660)+(−393510))−(−999680))
= −712000kJ kmol−1
Heat of Reaction = (1.020×(−791560))+((0.543−0.429)×(−712000))
= −888192kJ h−1
Accumulation = 864000 + (-888000) = -24400 kJ h−1 = −6.78kW
Based on the heat of reaction calculated, a negative value was obtained. This demonstrates that the
reaction is exothermic which means that energy will be released during the reaction.
3.3.3 Kiln
There is no accumulation of energy in the system as it operates at steady state so the energy balance
reduces to:
EnergyGeneration = EnergyOut − EnergyIn (Equation 3.3.7)
The inlet stream to the kiln includes the magnesium carbonate and magnesium hydroxide mixture
coming from stream 18. The outlet streams are the flue gas stream 20 and the magnesium oxide
product stream 21. The heat capacities of the magnesium compounds and heats of formation of all
components were obtained from the NIST Webbook and the flow rates have been taken from the
PFD. The heat capacities of the components of the flue gases have been taken from literature. The
heat capacities taken from NIST were integrated over the desired temperature range to give the
desired heat flow.
36

39. The values for energy flow into and out of the kiln are shown below.
Table 3.10: Overall Energy Balance for the Kiln
Energy In at 109◦C:
Component Molar Flow/ kmol h-1 ∆f H0
298K/ kJ kmol-1 Energy/ kJ h-1
Mg(OH)2 0.543 -925 -502000
MgCO3 1.10 -1110 -1220000
Total: / kJ h-1 -1720000
Energy Out at 660◦C:
Component Molar Flow/ kmol h-1 ∆f H0
298K/ kJ kmol-1 Energy/ kJ h-1
H2O 0.543 -242 37500
MgCO3 0.011 -1110 -12200
MgO 1.630 -601 -979000
CO2 1.090 -394 -427000
Total: / kJ h-1 -1380000
Ein = -1720 MJ h-1
Eout = -1380 MJ h-1
Ereaction = 172 MJ h-1
Substituting these values into the energy equation gives the total amount of heating required by the
kiln as 46.9 kW.
It was then calculated how much fuel would be required for thermal decomposition. North Sea gas
was chosen as the fuel. This feedstock consists of 95% methane so for simplicity it was assumed that
the feedstock was entirely methane.
Calculation of the theoretical flame temperature worked on the assumption of 10 ◦C ambient air
temperature and a humidity of 80% which is about average for Cumbria. The theoretical flame
temperature was calculated to be 1820 ◦C. It was taken that the flue gases would leave the kiln
at 1500 ◦C and would then be used to generate steam which would heat other processes. Using
enthalpy of combustion data along with the heat capacities it was calculated that the kiln would
require 0.80 kmol h-1 of methane using a 10% excess of air to ensure complete combustion.
In the evaporator before the kiln, a mixture is being separated which contains suspended solids of
magnesium hydroxide and magnesium bicarbonate; the rest of the magnesium bicarbonate is being
decomposed in this unit. Due to impurities present in the water its boiling point will increase. The
small amount of aqueous magnesium carbonate and hydroxide are assumed to be negligible and
only the amount of solids will contribute to the boiling point rise.
Using equation Equation 3.3.8 the elevation can be calculated.
∆Tb =
RT2
b x2
∆hvap
(Equation 3.3.8)
=
8.314×3732 ×0.317
40.68×103
= 9◦
C
Therefore the boiling point of the solution and the outlet temperature of the products from the
evaporator will be 109 ◦C.
This has been taken into account in the kiln energy balance as this temperature corresponds to that
of the feed.
37

40. 3.3.4 Ion Exchange Column
The ion exchange column will be two batch columns operating between 15 - 25 ◦C at atmospheric
pressure.
Energy entering the ion exchange column is from streams 26 and 27 as shown on the PFD, which
both contain H2O, with the addition of Mg(HCO3)2 in stream 26. Energy leaving the ion exchange
column is via stream 28, which contains Mg(HCO3)2 and H2O.
As no reaction takes place within the ion exchange column, generation of energy through heat of
reaction is considered negligible.
For the purpose of this energy balance, liquid effluent was modelled as water as it would have been
previously filtered, so contain water and radionuclides, with only traces of other contaminants. Due
to unavailability of data below 298 K, two options were available; either data could be extrapolated
outside the given range to 288 K, giving an unknown difference between the calculated and true
value, or to carry out the energy balance at the lowest possible temperature within the range, 298 K.
However this would assume isothermal operation and would not be an accurate representation of
the energy requirements. Hence, extrapolation outside the validity range was deemed to be the
more suitable method. Data was obtained for the enthalpy of formation at standard conditions using
the NIST Webbook, and the mole flow rates taken from the PFD mass balance (Table 3.3).
Table 3.11: Overall Energy Balance for the Ion Exchange Column
Energy In:
From Stream 26
Component Molar Flow ∆f H0
298K/ CpT1 / CpT2 CpdT / Energy
/ kmol h-1 kJ kmol-1 kJ kmol-1 kJ kmol-1 kJ kmol-1 / kJ h-1
H2O 0.047 -285830 27100 20857 -6243 -13830
From Stream 27
Component Molar Flow / kmol h-1 ∆f H0
298K/ kJ kmol-1 Energy/kJ h-1
H2O 0.12 -285830 -34263
Mg(HCO3)2 0.011 -999860 -11304
Total: / kJ h-1 -45567
Energy Out:
Component Molar Flow ∆f H0
298K/ CpT1 / CpT2 CpdT / Energy
/ kmol h-1 kJ kmol-1 kJ kmol-1 kJ kmol-1 kJ kmol-1 / kJ h-1
H2O 0.1647 -285830 27100 20857 -6243 -48842
Mg(HCO3)2 0.011 -999860 N/A N/A -5834 -11309
Total: / kJ h-1 -45567
Ein = -54.9 MJ h-1
Eout = -60.2 MJ h-1
Accumulation = Ein - Eout = 753 kJ h-1 = 0.21 kW
3.3.5 Carbonation Tower
An energy balance was carried across the carbonation process, with similar assumptions made with
regards to using the enthalpy of formation at 298 K as in other energy balance sections. Although
the estimated enthalpy of formation for magnesium bicarbonate was used in the calculations, it
did not affect the accuracy of the solution, as the composition of magnesium bicarbonate remained
unchanged in the process; the enthalpy contribution from magnesium bicarbonate in and out of the
38

41. process was the same. It is targeted for the stream’s exit temperature to be the same as the inlet
stream temperature.
Table 3.12: Overall Energy Balance for the Carbonation Tower
Component ∆f H0
298K/ kJ kmol-1 In / kmol h-1 Out / kmol h-1 In / kJ h-1 Out / kJ h-1
Liquid Effluent -469.15 9.290208 9.290192 -4358501 -4358493
Mg(HCO3)2 -999.68 0.077279 0.077279 -77254.2 -77254.2
CO2 -393.51 0.009738 0.009722 -3832.13 -3825.61
Bicarbonate Salt -950.9 - 1.66×10-05 0 -15.7423
Total 10.0609 10.0609 4439587.541 -4439589.002
It can be noticed that the enthalpy of the stream leaving the process is slightly larger than the enthalpy
of the stream entering, but only by 1.46 kJ h-1. This can be accounted for by the exothermic enthalpy
of the neutralization reaction. Assuming a common base such as sodium hydroxide gives the liquid
effluent its basicity, carbonate salts are formed during neutralization with the net reaction represented
by,
NaOH(aq)+CO2
−−−− NaHCO3 (Reaction 3.4)
Due to a lack of literature values for the enthalpy of neutralization between sodium hydroxide and
carbon dioxide, it was calculated to be -88.24 kJ mol-1 in accordance with Hess’ law. Using this
value, the heat produced during neutralization was 1.46 kJ h-1, accounting for the enthalpy difference
between the inlet and outlet streams. The energy is therefore balanced across the carbonation process.
Table 3.13: Heat of Neutralisation Reaction
Amount of Na(HCO3) produced / kmol h-1 ∆f H0
298K/ kJ kmol-1 Heat produced/required / kJ h-1
0.00001656 -88.24 -1.46083
Theoretically, the cooling duty required by the carbonation process would then be 1.46 kJ h-1. This
is a very small cooling requirement with respect to the overall process and is therefore unlikely to
cause a change in the liquid effluent temperature. It was thus decided that the cooling requirement
for the carbonation process is negligible.
3.4 Radioactivity Balance
Radioactivity control is the main objective within this nuclear waste treatment plant. It is
therefore essential to monitor and assess the distribution of radionuclides within the process. The
radionuclides that are being considered are those that are decaying at a rate much greater than
the acceptable limit for disposal. These isotopes include: caesium-137, strontium-90, cobalt-60,
americium-241 and plutonium-241.
Limits of disposal for each of these radionuclides can be found in literature from environmental
agencies.37 The limits depend on the location of the site, the form of the waste stream, e.g. liquid
discharge or gaseous waste, and the radionuclide itself. The datasheets representative of the feed
for the plant contain radioactivity values associated with various radioactive isotopes. These values
shown are in units of radioactivity per volume. Since limits are given in units of activity per year, the
radioactivity in our streams were altered to these same units by using known quantities of volume
from the datasheet and a five year timeframe.
Table 3.14 compares the radioactivity in the feed to the process with the acceptable limits of disposal
for each radionuclide at sea through the pipeline at Sellafield.
Caesium-134 was originally considered as an important isotope although after performing
calculations as aforementioned, the activity was found to be below the permitted level.
39

42. Table 3.14: Comparison between Feed Radioactivity and Permissible Limits in Tera Becquerels per annum
Radioisotope Co-60 Sr-90 Cs-137 Am-241 Pu-241
Radioactivity limit (TBq year-1) 3.6 45 34 0.3 25
Radioactivity in feed (TBq year-1) 91 10300 10800 911 7420
The zeolite clinoptilolite has been chosen to adsorb some of the harmful nuclides. Its structure
allows adsorption of caesium-137, strontium-90 and cobalt-60. The other two nuclides of importance
will not be treated in this process but will be sent for further treatment elsewhere.
Gaseous discharge of radioactivity is also present within the carbon dioxide outlet streams. It has
been assumed that they are only present in small traces and insufficient to affect the radioactivity
balance due to the vast majority of radioisotopes remaining in aqueous solution. As the carbon
dioxide is still harmful, all of the carbon dioxide outlet streams are sent to a storage tank for further
treatmentl.
After the dissolution column unit, the radioactivity spread is calculated based on the split of the
moles of water. For example, when stream 14 splits into streams 15 and 25, the water is split evenly
and therefore the radioactivity is split evenly as well.
When solids were present within the system, mole fractions could not be calculated due to an
unknown makeup of the solids, therefore, radioactivity was split according to mass fractions.
This is not ideal, although there is not enough data on the solids present in the feed streams and
consequently their interactions with radionuclides cannot be understood. After the separation of the
solids, radioactivity was balanced by using mole fractions instead as this would be more accurate.
Stream 19, the inlet to the rotary kiln contains traces of activity. It has been assumed that the
radioisotopes have all been transported by the flue gas, based once again on the presence of water.
In reality, it is quite likely that the product will contain traces of the nuclides. For this reason it
has been decided to reuse the magnesium oxide elsewhere within the Sellafield site for packaging
transuranic waste.
40

43. Table 3.15: Radioactivity Balance Table
Radioisotope Co-60 Sr-90 Cs-137 Am-241 Pu-241
(TBq year-1) (TBq year-1) (TBq year-1) (TBq year-1) (TBq year-1)
Stream 1 91 10300 10800 911 7420
Stream 2 57 6480 6770 571 4650
Stream 3 34 3860 4040 340 2770
Stream 4 0 0 0 0 0
Stream 5 34.5 3920 4090 345 2810
Stream 6 22.5 2560 2670 225 1840
Stream 7 22.5 2560 2670 225 1840
Stream 8 TR TR TR TR TR
Stream 9 TR TR TR TR TR
Stream 10 22.5 2560 2670 225 1840
Stream 11 TR TR TR TR TR
Stream 12 22.5 2560 2670 225 1840
Stream 13 22.5 2560 2670 225 1840
Stream 14 11.3 1280 1340 113 918
Stream 15 11.3 1280 1340 113 918
Stream 16 11.3 1280 1340 113 918
Stream 17 11.3 1280 1340 113 918
Stream 18 TR TR TR TR TR
Stream 19 0 0 0 0 0
Stream 20 TR TR TR TR TR
Stream 21 0 0 0 0 0
Stream 22 11.3 1280 1340 113 918
Stream 23 11.3 1280 1340 113 918
Stream 24 11.3 1280 1340 113 918
Stream 25 22.5 2560 2670 225 1840
Stream 26 22.5 2560 2670 225 1840
Stream 27 34.5 3920 4090 345 2810
Stream 28 3.6 45 34 0.3 25
Stream 29 0 0 0 0 0
Stream 30 TR TR TR TR TR
Stream 31 3.6 45 34 0.3 25
Stream 32 0 0 0 0 0
Stream 33 TR TR TR TR TR
Stream 34 TR TR TR TR TR
Stream 35 TR TR TR TR TR
41

44. 4. Health and Safety Assessment
4.1 Preliminary Health and Safety Assessment
“The safe design and operation of facilities is of paramount importance to every company that is
involved in the manufacture of fuels, chemicals and pharmaceutical products.”38
Hazard and Safety assessments are performed to ensure that processes are designed in a manner
which minimises the possibility of accidents which could incur economic losses to the company
or the injury and possible fatality of personnel. These costs can manifest themselves as equipment
damage or loss, fines, injury claims or the loss of production.
Regulations regarding process safety and hazard reduction are specified by government and
regularly revised. In the UK the construction and safe operation of nuclear plants is governed by
the Nuclear Installations Act of 1965 which is administered by the Health and Safety Executive
(HSE) and carried out by the Office for Nuclear Regulation (ONR). A clear example of the UK
government’s policies to protect people and the environment from the potential hazards of nuclear
plants was demonstrated in May of 2012, when they increased the third party liabilities of operators
in the event of a nuclear incident from £140m to £1.2bn.39
Whilst this legislation does not directly impact upon the operation or design of the facility proposed
in this project it does highlight the very real concerns of the public and government regarding the
safety of the industry as a whole. More relevant to our facility is the Radioactive Substances Act 1993
which governs nuclear waste management and discharges to the environment.40
The liquid effluent that our plant disposes to the sea and the magnesium oxide produced will need
to be sufficiently non-radioactive to adhere to this regulation. After further investigation, it has been
found that there are large amounts of americium-241 and plutonium-241 in the liquid effluent exit
stream that will now require post treatment because it will be unsafe to dispose of these isotopes
to the sea. From a radioactivity balance, it was found that Pu-241 has an activity greater then
52,000 TBq yr-1 therefore it must be post-treated before being discharged because the activity limit is
25 TBq yr-1.37 Furthermore, americium has an upper limit of 0.3 TBq yr-1, but from the radioactivity
balance it was found to have an activity of 604 TBq yr-1.
Our facility will have to demonstrate the intrinsic safety of its equipment and components, with
specific detail being paid to the radioactive nature of the feed and the safety of its products.
4.2 Material Health and Safety Assessment
4.2.1 Carbon Dioxide Gas
CAS NUMBER: 124-38-941,42
Chemical Formula: CO2
Carbon dioxide is an inorganic gas which is used to regulate the pH of the liquid effluent and magnox
waste feeds in our process. At standard atmospheric conditions it is a colourless, odourless gas.
42

45. Potential Health Effects
At standard atmospheric conditions the main hazards of carbon dioxide are associated with its
inhalation. Symptoms include nausea, breathing difficulty, and changes in blood pressure. Exposure
to higher levels can result in headaches, dizziness, suffocation, unconsciousness and eventually
coma and death. Eye protection does not need to be worn for gaseous carbon dioxide although it
is recommended and protective clothing is not required. If air concentrations exceed 40,000 ppm
respirators should be used. Carbon dioxide for the plant will be stored in compressed canisters;
these represent a hazard as a release either through a puncture or explosion of the compressed gas
can cause severe damage to buildings, infrastructure and personnel. Small leaks may also introduce
the risk of frostbite.
First Aid Measures
In the event of over exposure to the eyes one should rinse with clean water for approximately 15
minutes and seek medical attention. Similar action should be taken in the case of contact with the
skin, with care being taken to ensure that their clothes and shoes have been properly decontaminated
before reuse. In the case of frostbite one should try to warm up the affected area slowly without
using water that is too hot. In the case of inhalation of moderate levels of carbon dioxide move
into an area of fresh air, while for higher levels, one should use artificial respiration equipment and
if respiratory arrest occurs, immediate medical attention should be sought and cardiopulmonary
resuscitation performed.
Flammability
Carbon dioxide is inflammable and is regularly used to extinguish fires; pressurised canisters may
explode if exposed to a heat source or punctured.
Environmental Effects
Carbon dioxide exhausts should be well ventilated and measures should be taken to ensure that the
gas (which is denser than air) does not accumulate in poorly ventilated areas or holes in the ground
which could become void of oxygen and hence very hazardous to life.
Hazards Associated with Disposal
This process will not dispose of CO2, it will be stored and treated for its radioactive content.
Storage Conditions
Store in pressurised canisters away from potential sources of heat and damage.
4.2.2 Magnesium Hydroxide
CAS NUMBER: 1309-42-843,44
Chemical Formula: Mg(OH)2
Magnesium hydroxide forms a large proportion of the magnox feed. The hazards of magnesium
hydroxide are associated primarily with the substance in its powdered form, which are not being
dealt with directly. However, an assessment of its safety is still prudent.
Potential Health Effects
Magnesium hydroxide is only slightly hazardous; it can cause mild irritation if contacted with the
skin or eyes and irritation of the upper respiratory tract if it is inhaled. Personnel suffering from
asthma, chronic lung conditions, dermatitis or skin rashes may experience aggravated conditions.
The main threat posed by magnesium hydroxide in the process is its radioactivelevel in the feed.
First Aid Measures
In the case of direct contact with the eyes one should rinse with clean water for at least 15 minutes.
For contact with the skin one should wash thoroughly with a non-abrasive soap paying particular
43