In this report, the objective has been set to provide intense view of boiling heat transfer and core hydraulics. In details the aim was to describe the importance and Fundamentals of Nuclear Power Reactors and thermal hydraulic processes involved in the transfer of power from the core to the secondary systems of a nuclear reactor plant and produce competence in the fundamentals of the calculations associated with these processes. By this process, here it has been included with a short overview over nuclear power plant definition and types, fission reaction, basic hydraulics, over view over nuclear reactor core and their inter relation.

1. Boiling Heat Transfer and Core Hydraulics
Md. Asif Imrul
Student, Bangladesh University of Engineering and Technology
Email: asifimrul1996@gmail.com
Phone: +8801770594774
Maria Rafique
Student, Bangladesh University of Engineering and Technology
Email: mariarafique13@gmail.com
Phone: +8801793204520

2. Introduction
In this report, the objective has been set to provide intense view of boiling
heat transfer and core hydraulics. In details the aim was to describe the
importance and Fundamentals of Nuclear Power Reactors and thermal
hydraulic processes involved in the transfer of power from the core to the
secondary systems of a nuclear reactor plant and produce competence in the
fundamentals of the calculations associated with these processes.
By this process, here it has been included with a short overview over nuclear
power plant definition and types, fission reaction, basic hydraulics, over view
over nuclear reactor core and their inter relation.
INPE Nuclear Engineering Course | Page No. 1

3. Chapter 1
Nuclear Power Plant

4. What is a Nuclear Power Plant?
A Nuclear Power Plant (NPP) or Nuclear Power Station is a standard
thermal power station that has Nuclear Reactor as the heat source for
generating energy and as is typical in all conventional thermal power
stations the heat is used to generate steam which drives a steam turbine
connected to a generator which produces electricity.
Nuclear plants are usually considered to be base load stations. In this
process fuel is a small part of the cost of production. They cannot be
easily or quickly dispatched. Their operations and maintenance (O&M)
and fuel costs are, along with hydropower stations, at the low end of the
spectrum and make them suitable as base-load power suppliers.
INPE Nuclear Engineering Course | Page No. 3

5. Fig. Nuclear Power Plant- Vatenfall, Sweden
Fig. Fukushima Daiichi Units 1-6
Fig. Rooppur Nuclear Power Plant (Projected)
INPE Nuclear Engineering Course | Page No. 4

6. Nuclear Reactor
A nuclear reactor is a key device of nuclear power plants. Main purpose of
the nuclear reactor is to initiate and control a sustained nuclear chain
reaction.
Steam Generators
Steam generators are heat exchangers used to convert feed water into
steam from heat produced in a nuclear reactor core. They are used in
pressurized water reactors (PWR) between the primary and secondary
coolant loops.
Pressurizer
Pressure in the primary circuit is maintained by a pressurizer, a separate
vessel that is connected to the primary circuit (hot leg) and partially filled
with water which is heated to the saturation temperature (boiling point) for
the desired pressure by submerged electrical heaters. Temperature in the
pressurizer can be maintained at 345 °C (653 °F), which gives a sub
cooling margin (the difference between the pressurizer temperature and the
highest temperature in the reactor core) of 30 °C.
Reactor Coolant Pumps
Reactor coolant pumps are used to pump primary coolant around the
primary circuit. These pumps are powerful, they can consume up to 6 MW
each and they can be used for heating the primary coolant before a reactor
start-up.
INPE Nuclear Engineering Course | Page No. 5

7. Safety Systems
According to the U.S. Nuclear Regulatory Commission primary
objectives of nuclear reactor safety systems are to shut down the
reactor, maintain it in a shutdown condition and prevent the release of
radioactive material.
Reactor safety systems consist of:
1. Reactor Protection Systems
2. Essential service water system
3. Emergency core cooling systems
4. Emergency power systems
5. Containment systems
Steam Turbine
A steam turbine is a device that extracts thermal energy from
pressurized steam and uses it to do mechanical work on a rotating
output shaft.
Generator
A generator is a device that converts mechanical energy of the steam
turbine to electrical energy.
Condenser
A condenser is a heat exchanger used to condense steam from last
stage of turbine.
Condensate-Feed water System
Condensate-Feed water Systems have two major functions. To supply
adequate high quality water (condensate) to the steam generator and
to heat the water (condensate) to a temperature close to saturation.
INPE Nuclear Engineering Course | Page No. 6

8. Fig. Fuel (Uranium Palette)
Fig. Turbine Hall Fig. Steam Turbine
Fig. Wet Spent Fuels Storage
Facility adjacent to Reactor Pool
Fig. Steam Spins Blades on
Rotating Turbine
Fig. Control Room
INPE Nuclear Engineering Course | Page No. 7

9. Power Control Modes
There are mainly 2 types of Power Control Modes. They are:
1. Reactor Lead >Turbine Offline
2. Turbine Lead >Turbine Online
Fig. Reactor Lead Fig. Turbine Lead
Reactor Types
There are many type of reactors. But 2 types of reactors are majorly in
use. They are:
1. Boiling Water Reactor (BWR)
2. Pressurized Water Reactor (PWR)
Fig. Boiling Water Reactor (BWR)
Fig. Pressurized Water
Reactor (PWR)
INPE Nuclear Engineering Course | Page No. 8

10. The Core of the Reactor
INPE Nuclear Engineering Course | Page No. 9

11. NPP’s Effect on Environment
Considering the high output energy of NPP, it shows that NPP creates less bad
impact on environment but greater impact over mankind. Here follows the impact
of NPP vs. Conventional Thermal Power Plant:
Name of Variables NPP Thermal power plant
Area 500 Acres 250 Acres
Fuel requirements 27 tons UO2 /year 1000 tons/year
Construction time 5-10 years 4-5 years
Manpower
requirements
Technical:200-300 Technical: Same
Engineers &
professionals:300 -
400
Engineers & professionals:
Same
Wastes Very scant Huge amount of waste as fly
ashes, COx, NOx, SOx
Water requirements Once-through-cycle: Once-through-cycle:
Closed-cycle: Closed-cycle:
Operating life 60 years 20 years
Thermal efficiency 34 % 30-35 %
The negative impact can be denoted as the annual exposure of radiation in
world. In this case, where the total value is just 3 millisievert,
Medical
Test
20%
Natural Resource
20%
Weaponry
<2%
Accidents
<1% NPP
<0.01%
INPE Nuclear Engineering Course | Page No. 10

12. Chapter 2
Fission : A Type of Nuclear Reaction

13. Nuclear Fission
A uranium-235 atom absorbs a neutron and eventually splits into two roughly
equal parts. This produces free neutrons and photons, releasing a large
amount of energy.
The fission may produce two or three more neutrons, triggering further
fission and a chain reaction can take place. The energy release in a
fission is quite large, so continuous chain reactions generate huge
amount of energy. As there are more neutrons released in this fission
than absorbed ones, the chain reaction becomes self-sustaining or
“Critical Mass”.
INPE Nuclear Engineering Course | Page No. 12

14. Nuclear Reactor Fission
In order to use fission for power generation in a nuclear reactor, it is
essential to fulfill the basic requirements-
1. Self-sustaining chain reaction
2. Controlled chain reaction to avoid hazardous situation
To fulfill these requirements, reactors need-
1. Moderators &
2. Controlling Rods
Moderators
Moderators are needed to reduce the pace of the neutrons to sustain the
chain reaction. Normally, the probability of neutrons’ interaction is too
small. Moreover, the required fraction of fissionable nuclei in natural
uranium for fission is 2-3%. Whereas, practically there is only 0.7% of it.
Unless moderators such as, heavy water or graphite is used, this cannot
be enriched to the desired percentage.
Control Rods
Control rods are used to control the uncontrollable chain reactions to
keep it just barely critical. The control rods absorb neutrons and are
made of cadmium or boron.
INPE Nuclear Engineering Course | Page No. 13

15. Chapter 3
Thermal Hydraulics

16. What is Thermal Hydraulics
Thermal Hydraulics is the study of the thermodynamics, fluid mechanics
and heat transfer processes involved in the removal of heat from nuclear
reactors. A common example is steam generation in power plants and the
associated energy transfer to mechanical motion and the change of states
of the water while undergoing this process.
It is obvious that the thermal hydraulics and the dynamics of a nuclear
reactor can be very different according to it’s reactor type, fuel design,
power requirement, etc. Within this process, heat must be removed from
the rector core in the same rate as it is generated to avoid core damage.
Usually the cooling of the reactor core is provided by forcing a working
fluid, that can denoted as coolant fluid, through it. Heat accumulated in the
coolant is used for variety of purposes according to the aim of the nuclear
reactor.
INPE Nuclear Engineering Course | Page No. 15

17. Thermal Hydraulics as a Part of the
Nuclear Reactor Analysis Chain
Thermal Hydraulics analysis provides input data to the reactor-physics
analysis, whereas the latter gives information about the distribution of
heat sources, which is needed to perform the thermal-hydraulic
analysis. (Notice the figure below)
The strong coupling between the two types of analyses including
Reactor Physics Analysis and Thermal Hydraulics Analysis causes that
iterative approaches have to be used. There is also a coupling
between the thermal-hydraulic and structure-mechanics analyses.
The kinetic energy of fission products in nuclear reactors is eventually
transformed into an enthalpy increase of the coolant. This
transformation is realized in several. steps, as shown in FIG. below.
The figure depicts thermal processes inside a nuclear reactor pressure
vessel and is valid for a Boiling Water Reactor (BWR).
INPE Nuclear Engineering Course | Page No. 16

18. Core Hydraulics of Nuclear Reactor
The thermal energy is generated in the reactor core, where the
coolant enthalpy increases due to heat transfer from the surface of
fuel rods.
In BWRs, the two-phase mixture that exits the core is separated into
water and wet steam. The steam is further dried in steam dryers and
finally exits through steam lines to turbines. After passing through
turbines and condensing in a condenser, it turns into feed water which
is pumped by the feed water pumps back to reactor pressure vessel.
In a PWR, water is heated in the core and converted to superheated
steam. This is a closed system and is called the primary loop. This
contaminated water/steam does not exit the containment. The heat from
the steam in the primary loop is transferred to a separate water supply
(the secondary loop) causing it to boil and turn to steam. This is done by
using “steam generators”. The steam from the primary loop travels
through the tubes giving up heat to the water surrounding the tubes.
INPE Nuclear Engineering Course | Page No. 17

19. Chapter 4
Heat Management

20. Heat Generation and Thermal
Efficiency
In thermodynamics, the thermal efficiency is a dimensionless
performance measure of a device that uses thermal energy, such as
an internal combustion engine, a steam turbine or a steam engine, a
boiler, a furnace, or a refrigerator for example.
There are two types of Reactor power cycles:
1. Carnot cycle
2. Rankine cycle
Carnot Engine is an ideal reversible closed thermodynamic cycle in
which the working substance goes through the four successive
operations of isothermal expansion to a desired point, adiabatic
expansion to a desired point, isothermal compression, and adiabatic
compression back to its initial state.
Whereas, The Rankine cycle is a model used to predict the performance
of steam turbine systems. It was also used to study the performance of
reciprocating steam engines. The Rankine cycle is an idealized
thermodynamic cycle of a heat engine that converts heat into
mechanical work while undergoing phase change.
Schematic of a heat engine.
Where
Again
INPE Nuclear Engineering Course | Page No. 19

21. Carnot Cycle
Execution of the Carnot cycle in a closed system is as follows:
Reversible Isothermal Expansion (process 1-2, TH = constant)
Reversible Adiabatic Expansion (process 2-3, temperature drops from TH to TL)
Reversible Isothermal Compression (process 3-4, TL = constant)
Reversible Adiabatic Compression (process 4-1, temperature rises from TL to TH)
The Carnot cycle consists of the following processes:
1. Delivering heat to the cycle by a reversible isothermal expansion
process (process from points 1 to 2);
2. Delivering work output by isentropic expansion of gas (process from
points 2 to 3);
3. Rejecting waste heat from the cycle by reversible isothermal
compression of gas (process from points 3 to 4);
4. Inputting work into the cycle by isentropic compression of gas
(process from points 4 to 1).
INPE Nuclear Engineering Course | Page No. 20

22. Reactor Power Cycle: Rankine Cycle
The ideal Rankine cycle consists of the following four processes:
1-2 Isentropic compression in a pump
2-3 Constant pressure heat addition in a boiler
3-4 Isentropic expansion in a turbine
4-1 Constant pressure heat rejection in a condenser
In details, the processes is as follows:
1. Water enters the pump at state 1 as saturated liquid and is
compressed isentropically to the operating pressure of the boiler.
2. Water enters the boiler as a compressed liquid at state 2 and leaves
as a superheated vapor at state 3. The boiler is basically a large heat
exchanger where the heat originating from nuclear reactors is
transferred to the water essentially at constant pressure. The boiler,
together with the section where the steam is superheated, is often
called the steam generator.
The superheated vapor at
state 3 enters the turbine,
where it expands isentropically
and produces work by rotating
the shaft connected to an
electric generator.
The pressure and the
temperature of steam drop
during this process to the
values at state 4, where steam
enters the condenser. At this
state, steam is usually a
saturated liquid–vapor mixture
with a high quality. Steam is
condensed at constant
pressure in the condenser.
3.
4.
INPE Nuclear Engineering Course | Page No. 21

23. Heat Conduction
Heat conduction is the process of internal energy exchange from one part
of a body to another. It can be via exchange of kinetic energy of motion of
molecules in direct communication or by the drift of free electrons in the
case of heat conduction in metals. In general, internal energy flows from
high energy regions to the low energy regions.
In nuclear reactor, heat conduction is done following the application of
Fourier’s law.
Fourier’s law states that the heat transfer q, is proportional to the area
and the temperature difference and inversely proportional to the
thickness. The equation can be generalized by considering the heat flow
at any point x through any material by introducing the differential
notation.
This equation describes one-dimensional heat conduction without heat
generation. However in the fuel itself the heat transfer should include a
term to deal with the fact that heat is being generated. The conductivity
has the units W/mK. Generally metals have high values for conductivity
of around 10-400 (4000 in natural copper at low temp) whereas gases
and other solids have values 0.004-0.5.
dx
dT
kA
a
)TT(kA
q 21
=
-
=
INPE Nuclear Engineering Course | Page No. 22

24. Temperature Distribution
The fuel pellets are in the fuel element and are surrounded by a small gap
between the fuel and the cladding, which offers substantial resistance to
heat transfer. The flowing coolant surrounds the pin.
The fuel and coolant temperature distributions depend on heat transfer
in the radial direction at each position in the channel axial direction. The
amount of heat generated along the fuel channel follows the neutron flux
and fission reaction distribution along the fuel channel.
Fig. Temperature distribution in the fuel cladding and fuel
element
INPE Nuclear Engineering Course | Page No. 23

25. Chapter 5
Comparison Chart Between BWR and
PWR

26. Items BWR PWR Remark
-Facility Size
-BWR Cooling System is more simple than
LWR.
-PWR provides S/Gs and Pressurizers.
-Power Efficiency
Slightly -BWR:33.4%
-PWR:32.2%
-Thermal Efficiency
-BWR:24.5%
-PWR:19.5%
-Power Density
PWR Core Volume is smaller than BWR.
-RPV Size
-Neutron Flux
Slightly -BWR:~4.2x1013n/cm2・s
-PWR:~4.7x1013n/cm2・s
-Max. Fuel Burn-up
Slightly -BWR:50,000MWD/t
-PWR:48,000MWR/t
-Doppler Effect
-BWR:~-1.2- -1.4%∆k/k
-PWR::~-3.0- -5%∆k/k
-Temperature Effect
In BWR, if RPV inside pressure rises,
Positive Reactivity insert into a core.
: Superior
-Radiation Control
Area
-In BWR, almost of the facilities including
a turbine build are radiation control area.
-While in PWR, the primary cooling system
located in PCV is only adopted.
-Installed Location
of CRDM
Own-weight dropping method of PWR is
more safety compared with push upping
method of BWR.
-Confining Function
of PCV
During a severe accident, BWR has a risk
of leaking from PCV caused deterioration
of a seal of the top flange and penetration
parts of PCV.
-Cooling Function
under Loss of All
Powers
PWR can removal heat from a core via S/G
with natural convection even fall into loss
of all powers.
-Easy of Power
Control
Slightly BWR can easily regulate power output by
only controlling pump speed (core flow
rate).
INPE Nuclear Engineering Course | Page No. 25

27. Conclusion of Report

28. Conclusion
In this report, intense view of boiling heat transfer and core hydraulics are
overviewed. In chapter one, the detailed description on the definition and
types of modern nuclear plants have been discussed. There is also
discussion on the fundamental components of nuclear power plant (NPP) or
nuclear power station. The impact assessment over environment has been
added at the last page of the chapter.
In second chapter, fission reaction has thoroughly been revised, as it is the
core reaction method operated in nuclear reactor. In chapter three, basics of
hydraulics has been taken under light along with the core hydraulics system
that is implemented within the nuclear reactors.
In fourth chapter, heat management procedure has been gone through. There
is also discussion over the types of heat engine cycles, Carnot cycle and
Rankine Cycle. The chapter includes heat condensation procedure and
temperature distribution process.
In fifth chapter, it has been analyzed over the comparison between two major
types of nuclear reactor:- Boiling Water Reactor (BWR) and Pressurized
Water Reactor (PWR).
INPE Nuclear Engineering Course | Page No. 27

29. Acknowledgement
1. www.nuclear-power.net
2. www.electrical4u.com
3. Thermal management systems for cooling nuclear reactor
during emergency [ Singh, Randeep , Mochizuki,
Masataka, Nguyen, Thang, Saito, Yuji, Matsuda,
Masahiro)
4. www.qats.com
5. www.world-nuclear.org
6. www.britannica.com
7. www.flaticon.com
8. whatisnuclear.com/
9. Importance of Nuclear Thermal Hydraulics [Md. Altab
Hossain, PhD, CEng (UK), MIMechE (UK), FIEB]
10. www.euronuclear.org