The document discusses sodium and its use as a coolant in fast breeder reactors. Sodium is chosen as the coolant due to its high heat transfer properties, low neutron moderation, and compatibility with steel. It describes the production of reactor-grade sodium through electrolysis and its transportation and storage. Sodium must be kept free of oxygen and other impurities to minimize corrosion. The document outlines the chemistry of sodium interactions with oxygen, nitrogen, and structural materials used in fast reactors like stainless steel and discusses measures to control corrosion like cold traps and maintaining low oxygen levels. Phase diagrams of sodium with various metals and oxygen are also presented.

1. Chemical Properties of Sodium – An Overview
T. Gnanasekaran
Raja Ramanna Fellow,
IGCAR, Kalpakkam

2.  Choice of sodium as fast reactor coolant
 Reactivity of sodium with ambient gases
 Manufacturing of sodium and its uses in other industries
 Handling and quality control of sodium
 Purification of sodium in coolant circuits
 Chemistry of sodium-steel system
 Chemistry of carbon in sodium
 Regeneration of cold traps
 Chemical interaction of sodium with nuclear fuels
 Sodium fires
Topics to be covered

3. Na IHX
Steam Generator
Na pump Na pump
H2O pump
Schematics of a Fast Breeder Reactor

4. Fast Breeder Reactor and its coolant
 When neutron induced fission of 235U and 239Pu occurs, 2 to 3 neutrons of
high energy generated.
 In thermal reactor, energy of fission neutrons are thermalised. Thermal
neutrons propagate the controlled chain reaction.
 In a fast reactor, high energy neutrons are used in fission process. No
moderation of neutron energy is allowed Hence no hydrogen bearing
materials (eg H2O) allowed.
Coolant of a FBR should be poor moderator.
 Fast reactor fuel contains large quantities of either 235U and/or 239Pu
Expensive fuel.
Hence power rating (i.e. extraction of power) should be high.
So a coolant of extraordinary
heat transfer characteristics
needed.

5. Heat transfer at the interface
with water as coolant
Thermal conductivity
Water: 0.0067 J/cm/s/oC at 50oC
Sodium: 0.84 J/cm/s/oC at 200oC
Fuel
pellets
Cladding

6. Choice of Sodium as Fast Breeder Reactor Coolant
 Sodium has excellent heat transfer properties
(two orders higher than water)
 Comparatively higher mass and hence low moderation
 Neutron activation product, 24Na is short lived (half life = 15h)
 Though chemically reactive, pure sodium is compatible with steels.
 Sodium is inexpensive (Several tonnes of coolant are required)
Amount of Sodium Used in FBTR : ~ 150 Tons
Amount of Sodium for PFBR : ~ 1200 Tons
23Na - Fast neutron absorption cross section ~0.003 barn
Thermal neutron cross section ~0.5 barn
~ 150 Rs/kg

7.  Most abundant alkali group metal
 Present 2.3 % in the earth’s crust.
 Very reactive and never found free in nature.
Sodium metal
Down’s Cell
Production of sodium
Metallic sodium finds extensive
technical and industrial
applications including
manufacture of drugs &
pharmaceuticals and nuclear
reactors .
Sodium for PFBR is imported from France

8. M. Pt of NaCl is very high (801oC) .
Binary eutectic of NaCl-CaCl2 with 58 mass % of CaCl2
or
Ternary electrolyte with 46 to 53 mass % BaCl2 – 23 to 26
% mass % CaCl2 – 24 to 28 mass % NaCl used
Electrolysis temperature
580 to 610oC
Production of sodium
Ca (and Ba) impurity
in the product

10. MSSA Métaux Spéciaux
Sodium for PFBR is imported from France

11. Sodium Bricks
Sodium bricks,
packed in polythene
bags and sealed in
steel drums, are
transported.

12. Molten sodium loaded in tankers
(provided with heaters), cooled
and transported in solid form. At
the user site, sodium would be
melted and transferred.

13. Electronic configuration of sodium atom
Silvery shining metal
Na sealed in glass
ampoule
1s2, 2s2 2p6, 3s1
In lab. always stored
under kerosene

14. Reactivity of sodium metal
 Instantaneously reacts with moisture in air. Forms NaOH and
hydrogen gas.
NaOH absorbs CO2 in air and forms NaHCO3 and Na2CO3.
Reaction is highly
exothermic. Unattended
exposure to humid air
can lead to fire.
NaOH(s) + CO2(g)  NaHCO3(s)
Na(s) + H2O(g)  NaOH(s) + 1/2 H2(g)

15.  Instantaneously reacts with
moisture in air. Forms NaOH and
hydrogen gas.
NaOH absorbs CO2 in air and
forms NaHCO3 and Na2CO3.
If left in air with low humidity, a thick
layer of NaHCO3, NaOH and Na2CO3 is
formed

16. Reaction with oxygen  Highly exothermic
Reaction with oxygen in air
4Na(l) + O2(g)  2Na2O(s) H = - 9459 kJ per kg of Na
2Na(l) + O2(g)  Na2O2(s) H = -11279 kJ per kg of Na
Subsequently products react with water vapour and CO2 in air
Na2O(s) + H2O(g)  2NaOH(s/l)
2Na2O2(s) + 2H2O(g)  4NaOH(s/l) + O2(g)
Na2O(s) + CO2(g)  Na2CO3(s)

17. Nitrogen does not readily react with sodium and its solubility in sodium
is extremely low (ppb levels) – solubility similar to that of argon in
sodium and is pressure dependent (directly proportional).
(Though sodium azide Na3N is known, it decomposes on its contact with
liquid sodium)
 Why nitrogen is not preferred as cover gas ?
Chromium, manganese and molybdenum can form very stable nitrides.
Though nitrogen is almost insoluble in sodium, nitridation of steel can
occur at the sodium-nitrogen interfaces.
(Sodium removes the passive Cr2O3 layer and exposes active metal surface)
Interaction of nitrogen with sodium
SS
Passive
“Cr2O3” layer
Cover Gas
Na
2Cr2O3 + 3 Na  3NaCrO2 + Cr

18.  For preparation of tetra ethyl lead and tetra methyl lead (antiknocking
agents for gasoline) and many other organo-metallic compounds.
 In manufacture of refractory metal such as titanium, zirconium, hafnium
and tantalum by reaction of their halides with sodium.
 Also used for manufacture silicon and potassium-sodium alloys.
 Calcium metal and calcium hydride, sodium hydride.
 As reducing agents in many preparation, manufacture of dyes, herbicides,
pharmaceuticals etc. The reaction products of sodium metal with a suitable
alcohol are the starting material for the production of large number of drugs and
pharmaceuticals.
 Descaling of metals such as stainless steel and titanium.
Uses of metallic sodium:
MXn + nNa  M + n NaX

19. Sodium box
Liquid Sodium

20. Specifications of reactor grade sodium (partial list)
Element Level (ppm)
O < 2
C < 20
Ca 2
B < 5
Cl + Br 5
Ba, Pb, Li < 1
Quality control of sodium

21. Chemical analysis of sodium for metallic and non-metallic impurities
VACUUM DISTILLATION
Carbon: Residue burnt in oxygen and CO2 produced is measured manometrically.
Oxygen: Residue (Na2O) dissolved in dil HCl and. analysed for Na and back calculated for oxygen
Trace metals: Residue dissolved in dil. HCl and analysed for the elements by AAS / ICPMS / ICPOES
Vacuum distillation of matrix
sodium at 350o C
(vapour pressure of sodium at
350oC ~ 0.1 mm Hg)
Residue free from sodium
taken for analysis
Quality control of entire sodium supply for PFBR was carried out at RCL

23. Purification of sodium in coolant circuits
Impurities have temperature
dependent solubility
By cooling and providing sites
for nucleation, these impurities
can be removed.
Cold traps:
Generally SS mesh packed.
Sodium cooled and made to
pass through.
Would need several passes to
achieve levels dictated by
solubility (even if there is no
steady leak into the system)
Kinetics of precipitation
Carbon cannot be removed by
this method.
0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
0.1
1
10
100
1000
10000
12001000 800 600 400
C H
O
Solubility
/
wppm
1000 K / T
T / K

24. Choice of Structural Materials in Fast Reactors
Sodium, when pure, highly
compatible with steels
 Austenitic Stainless Steels (304 / 316) grades
with 17-18% Cr, 8-12% Ni) used in the primary
circuit
 Austenitic Stainless Steels prone to chloride
stress corrosion and caustic cracking
Ferritic Steels (2.25 Cr- 1Mo or 9 Cr – 1Mo steels) used in steam generator section
 Coolant circuit characterised by large temperature gradients (T ~ 150 - 200oC)
 Use of different materials
Corrosion and Mass Transfer
Data on solubilities of
alloying elements of steels in
sodium required

25. Measured at IGCAR
Solubilities of Fe, Cr, Ni, Mn and Mo in liquid sodium
Iron solubility strongly
dependent on oxygen
concentration in sodium
Austenite bulk
Ferritic layer
(NaFexOy)* complex
dissolved in sodium
are involved
Ferrite layer formation in
Na-Austenitic steel interface

26. Corrosion and Mass transfer in Sodium Circuits
At low oxygen levels
[O]Na < 5ppm
Metal dissolution and
transfer due to
temperature gradient
At higher oxygen
levels, [O]Na > 5ppm
Enhanced corrosion and rate
dependent on oxygen conc  Control of oxygen level by
cold trapping & its continuous
monitoring desired
~ 540oC
400oC 355oC 215oC
2o Na loop
165 bar
~ 620oC ~ 580oC
1o Na loop
Turbine
generator
Condenser
Reactor core
 Thermodynamic data on
Na-M-O systems required.
Sources of oxygen  Argon cover gas circuit (Trace level oxygen impurity)
 Steam leak into sodium
Log R = 2.85 + 1.5 log [O] – 3.9 x 103 T-1 where R is in m/year.

27. Corrosion in aqueous systems – unwanted electrochemical cells
In neutral solution:

28. Corrosion Protection
Enhanced corrosion of iron Reduced corrosion of iron

30. 1/2(O )
3
Na Ni O
Na NiO
Na O
Na O
Na
NaNiO
2
2
2
2 2
2
Ni
2
NiO
Na NiO
2 2
3 5
PHASE DIAGRAM OF Na-Ni-O SYSTEM
PHASE DIAGRAM OF Na-Ni-O SYSTEM
(623 < T < 923K)
PHASE DIAGRAM OF Na-Fe-O SYSTEM
Na FeO
Na
4 3
Na FeO3
3
Na O
2
Na O
2
2
NaFeO
NaO2
2
Fe
Fe O
Fe O
FeO
3
1/2(O )2
3 4
2
PHASE DIAGRAM OF Na-Fe-O SYSTEM
Phase Diagrams of Na-M-O systems established at IGCAR
 Na4FeO3 stable in Sodium
 Oxygen dependent corrosion of steels
in sodium systems
R. Sridharan, T. Gnanasekaran and C.K.Mathews,
J. Alloys and Compounds, 191 (1993) 9-13.
V.Ganesan, Ph.D. Thesis, University of Madras (1989)
 No ternary compound of nickel is
stable in sodium
 Oxygen independent corrosion of
nickel and nickel based alloys in sodium

31. PHASE DIAGRAM OF Na-Mo-O SYSTEM
1/2(O )2
Mo
Na
NaO2
Na MoO
2 4
Na O
2 2
Na O
2
MoO 3
2
Na Mo O
2 7
MoO 2
2
Na Mo O
3 6
Na MoO
4 5
at Temp > 681.1K
Phase Diagrams of Na-M-O systems established at IGCAR
PHASE DIAGRAM OF Na-Mo-O SYSTEM
Na
Na O
2
1/2(O )2
Na Mo O
2 7
2
MoO3
MoO2
Na Mo O
2 3 6
Na MoO
4 5
Mo
NaO2
Na MoO4
2
Na O
2 2
at Temp < 681.1K
T. Gnanasekaran, K. H. Mahendran, K. V. G. Kutty and C. K. Mathews, J.Nucl.Mater. 165 (1989) p.210..
T.Gnanasekaran, K.H.Mahendran, G.Periaswami, C.K.Mathews and H.U.Borgstedt, J.Nucl.Mater. 150 (1987) p.113.
 At high temperatures, Na4MoO5 is stable in liquid sodium

32. PHASE DIAGRAM OF Na-Mo-O SYSTEM
1/2(O )2
Mo
Na
NaO2
Na MoO
2 4
Na O
2 2
Na O
2
MoO 3
2
Na Mo O
2 7
MoO 2
2
Na Mo O
3 6
Na MoO
4 5
at Temp > 681.1K
fGo
m(Na4MoO5, s, T) determined by measuring equilibrium oxygen potentials
using a high temperature emf cell
T.Gnanasekaran et al, JNM 150 (1987)113
Measurement of Thermochemical Properties
Pt,Ar + CO2 + O2, Na2CO3 // NASICON // Na2Mo2O7 +
Na2MoO4,
O2+CO2 + Ar, Pt
Na(l),Ma4MoO5(s), Mo(s) // YDT// In(l),In2O3(s)

33. 400 600 800 1000 1200 1400
-750
-700
-650
-600
-550
-500
Na - [Fe]SS
- Na4
FeO3
Na - [Mo]SS
- Na4
MoO5
Na - [Mn] SS
- NaMnO 2
N
a
- N
a 2
O
Na - [Cr] SS
- NaCrO 2
100 wppm
10 wppm
1 wppm
0.1 wppm
RT
lnpO
2
/
(kJ.mol
-1
)
Temperature / K
Note: (NaFexOy)* complex dissolved in sodium are involved in corrosion process
Influence of carbon in sodium in formation of NaCrO2 in low temperature section of
sodium systems:
[Cr]Na + [C]Na  CrCx
CrCx + [Na2O]Na  NaCrO2(s) + x [C]Na + Na(l)
10 to 25 ppm [O]Na needed for
NaCrO2 precipitation

34. Chemistry of carbon in sodium
 Carbon is an unavoidable impurity in sodium
(Electrodes of carbon are used as anode in Down’s cell)
 Carbon is an interstitial alloying element in structural
steels.
300 series SS have a carbon content range of 0.03 to
0.08 wt%. Change in this conc. Would severely affect
the mechanical properties.
NON REVERSE
RATCHET
CONNECTION
OD1850
3279
FLEXIBLE
COUPLING
BEARING & MECH.
IMPELLER
DIFFUSER
MOTOR
PUMP - PIPE
ID 900
OD 2350
Ø630/590
10960
Ø1850
SHAFT
14239
5955
SEALS ASSEMBLY
NON REVERSE
RATCHET
3279
FLEXIBLE
COUPLING
BEARING & MECH.
SEALS ASSEMBLY
 Shaft cooling by
hydrocarbon oils
 Oil leak can lead to
cracking and increase of
carbon activity in
sodium. Hence
carburisation of steels.
Centrifugal
pump for sodium
systems

35. CnH2n+2  CH4 + C2H6 + … + C + H2(g)
Oil leaks can be detected carbon meter and/or by
monitoring for CH4 in argon cover gas
Carbon in sodium
Carbon in sodium has active form (dissolved) and inactive form (suspended
carbonaceous material)
Sodium from Indian sources have around 15 to 20 ppm of total carbon
(solubility at around 500oC ~ 1ppm)
Inactive form  Active (dissolved) form
Kinetics of the process unknown

36. Carbon in steels

37. Influence of carbon in sodium in formation of
NaCrO2 in low temperature section of sodium
systems:
[Cr]Na + [C]Na  CrCx
CrCx + [Na2O]Na  NaCrO2(s) + x [C]Na + Na(l)

38. Steam leak into sodium releases
hydrogen
2Na + H2O  2 NaOH + H2
H2 + 2 Na  2NaH
NaOH + 2 Na Na2O + NaH
Steam Leak and need for Hydrogen Sensors
 Leak self- propagating
in nature
 Detection of leak at its
inception essential
2Na + H2O  2 NaOH + H2
Exothermic rx & highly
corrosive product
Dissolution kinetics of products into sodium:
TNa > 450oC : Fast Instantaneous increase of H
level in sodium
Leak detection by In-sodium Hydrogen Sensor
TNa < 250oC : Slow Gaseous H2 escapes into cover gas
Leak detection by Cover Gas Hydrogen Sensor

39. Na IHX
Steam Generator
Na pump Na pump
H2O pump
Schematics of a Fast Breeder Reactor

40. Regeneration of cold traps:
Oxygen impurity (Constant source: Impurities in argon cover gas)
Hydrogen impurity ( steam side corrosion, steam leak and oil leak)
Purification of cold trap should remove oxygen and hydrogen impurities.
Saturation of the cold trap, reduces its efficiency.
Can lead to flow blockage also.
Contains Na2O, NaH and Na.
Why not NaOH ?
Na2O(s) + NaH(s)  NaOH(s/l) + Na(l)
NaOH unstable in sodium at low temperatures –
by calculation and experimentally studied.
3Fe + 4H2O  Fe3O4 + 4H2

41. GO2 = RT lnPO2
G
H2
=
RT
lnP
H2

42. G
H2
=
RT
lnP
H2
GO2 = RT lnPO2

43. 683 K
G
H2
=
RT
lnP
H2
GO2 = RT lnPO2
Similarly Na2CO3 is also unstable in
liquid sodium up to about 600oC.
Na2CO3(s)+ 4 Na(l)  3 Na2O(s) + C

44. Na(l) - Na2O(s) – ‘NaOH’(l)

45. NaH(s)  Na(l) + H2(g)
Hydride loaded cold trap
regeneration:
 Heat under vacuum
 Temperature to be
chosen so that sodium
vapour does not interfere
the process

46. Regeneration of cold trap loaded with mainly
sodium oxide and less of hydride
 After draining the residual sodium, heat to a
chosen temperature (above 723K) under pH2 above
that in phase field 16
 Temperature to be chosen so that sodium vapour
does not interfere the process
Continuously drain the liquid

47. Fuel-coolant chemical interaction
In case of a breach of the clad, sodium would come into contact with fuel.
 Carbide and metallic fuels are quite compatible with sodium.
 In fact sodium bonded carbide fuels have been tested in
Germany.
Oxide fuel, (U,PuO2) with O/M ratio < 2 (~ 1.97 – 1.98) is used.
In the reactor, O/M redistributes.
O/M at the clad boundary is 2 and is lower at the centre.
Sodium reacts with the fuel to yield Na3MO4(s) [ M= U,Pu]
Low density material.
In case of a pin hole, this product can block the pin hole.
In case of a large breach of clad, this can escalate the extent of reaction.

48. Na-U-O Phase diagram

49. Sodium Fire
Ignition Temperature of Na
 Varies as a function of oxygen
and moisture content
205oC in dry air
320oC in damp air
 Ignition temp. reduction under
stirred condition
Reaction is highly
exothermic. Unattended
exposure to humid air can
lead to fire.

50. Sodium fire is different from oil fire
Burns only on the surface
Produces dense white smoke (Na2O, Na2O2, NaOH and Na2CO3)

51. Spray fires
Sodium leaks out as jet & hits the ceiling/walls of the containment cell
Shower of sodium droplets falling downwards
Small droplets and hence the burning area high
Burning rate higher than in pool fires
Types of Sodium Fires
Pool fires
Leaked sodium collects as a pool and leads to burning
Burning rate in pool fires is low
Burning area is confined to the pool surface

52. Spray Fire
Sodium leaks out as jet & hits the ceiling of the
containment cell
Shower of sodium droplets falling downwards
Small droplets and hence the burning area high
Burning rate higher than in pool fires

53. Sodium
Conduction
O2
Na
Oxide
Na(g)
Convection
Combustion Plane
Pool Surface
Flame combustion model for sodium pool fire
Combustion plane – pool distance (l)
(l)
Sodium temperature
Estimated distance (l) at 823 K ~ 50 

54. Spray Fire
Sodium leaks out as jet & hits the ceiling of the
containment cell
Shower of sodium droplets falling downwards
Small droplets and hence the burning area high
Burning rate higher than in pool fires

55. combustion model for sodium spray fire
Oxygen supply diffusion controlled
 Na2O formed as major fraction

56. Thank you