19-04-17 sodium as a coolant - P Selvaraj.pptx

sodium

1. Department of Atomic Energy Indira Gandhi Centre for Atomic Research Kalpakkam Rationale of sodium coolant for Fast reactors P. Selvaraj Director Fast Reactor Technology Group

2. Considerations for Coolant of FBRs Coolant should transport heat in useful form and at as high a temperature as possible High Thermal efficiency Coolant should preserve the hardness of the neutron spectrum in a fast reactor This criteria precludes the use of moderating materials such as water as coolant 2

3. Selection criteria of coolant for Fast Breeder Reactors Thermo-physical Properties Excellent heat transfer Low vapor pressure High boiling point Low melting point Low pumping power Transparency Material properties Thermal stability Radiation stability Material compatibility Neutronic properties Low neutron absorption Minimal induced radioactivity Negligible moderation Support passive safety Cost Initial inventory Make up inventory Low pumping power Hazards Non-toxic Non- reactive 3

4. Considerations for Coolant of FBRs Neutronic Properties Coolant normally occupies more than 30% of the core volume, hence its nuclear properties are very important Its moderating power should be low to preserve spectral hardness of neutrons It is desirable that the temperature coefficient of reactivity shall be negative The coolant should have low cross-sections of absorption and inelastic scattering 4

5. Considerations for Coolant of FBRs Neutronic Properties It should have minimal induced radioactivity Ideally a coolant should develop no gamma activities from in-vessel radiation If it does, these activities should preferably have short half-lives and low energies Alpha and beta activities can easily be shielded, but must be contained 5

6. Thermo-physical Properties High heat transfer coefficients To limit the clad temperature and achieve high linear power ratings High Boiling Point We can have Low pressure system High heat capacity For a given power and ∆T, high heat capacity – lesser will be the flow & lesser will be the Pumping power Considerations for Coolant of FBRs 6

7. Thermo-physical Properties Low melting point To reduce the requirements of preheating Low vapor pressure To prevent the aerosol deposition in the low temperature regions of material handling system, roof slabs etc Pumping power - Pumping power requirement of the coolant should be low for high cycle efficiency Transparency Easiness in the in - service inspection Considerations for Coolant of FBRs 7

8. Thermophysical properties of coolants Na NaK Pb Pb -B Hg He H 2 O Melting Point (°C) 98 18 328 327 -38 - Boiling Point (°C) 880 826 1743 1670 357 -267 100 Density (g/cm 3 ) 0.88 0.81 10.5 10.3 12.9 0.0002 1 Cp (kJ/kg/K) 1.3 1.2 0.16 0.146 0.14 5.2 4.2 K (W/m/k) 76 26 16 11 12 0.2 0.7 h (kW/m 2 /K) 36 20 23 17 32 10 17 Relative Pumping power required 0.93 0.93 11.5 11.5 13.1 6.0 1.0 8

9. Material Properties The coolant should be stable in a high temperature and radiation environment The coolant should have low chemical affinity for the container walls, fuel cladding and all solid surfaces It should be chemically inert and compatible with the working fluids in the case of leaks Insoluble compounds or reaction products that might cause plugging should be kept to a minimum. Considerations for Coolant of FBRs 9

10. Support Passive Safety Liquid-metal coolants have high heat capacity and high thermal conductivity It Provides large thermal inertia against system heating during loss-of-flow accidents Combined with favorable reactivity feedback (core design/fuel choice) Sufficient cooling is available to support passive shutdown Considerations for Coolant of FBRs 10

11. Cost Abundant supply at low cost Easy transportation and amenable to easy method of purification Considerations for Coolant of FBRs 11

12. Non Corrosive corrosion reduces the strength of material Corrosion products impede heat transfer and can plug flow passages Non- toxic Compatibility with secondary fluid in steam generator and with materials of steam generator Chemical reactivity Considerations for Coolant of FBRs 12

13. Which coolant? High heat capacity and low moderation is required Liquid water or organic coolants are excluded Liquid metals, gases or molten salts are possible options It is difficult to satisfy all the requirements Based on quantification of these factors, the coolants can be evaluated 13

14. The most common choices for gas-cooled fast reactors are helium and CO 2 Advantages: Good chemical compatibility with water, obviating the need of an intermediate coolant loop Good chemical compatibility with structural materials. Negligible activation of coolant. Optically transparent, simplifying fuel shuffling operations and inspection Gas coolants 14

15. Advantages: Gas coolants cannot change phase in the core, reducing the potential of reactivity swings under accidental conditions. Reduction of the positive void effect typically associated with sodium Harder neutron spectrum, which increases the breeding potential of the reactor. Low density allow “open" core arrangement increases neutron leakage into the breeding blankets, improving the breeding gain. Gas coolants 15

16. Disadvantages: Higher pumping power compared to liquid coolants. Need to maintain high pressure in the system High coolant flow velocity can lead to significant vibrations of the fuel pins . Decay heat extraction from the high power density core is difficult particularly following a depressurization event Loss of pressure accident are difficult to survive in a fast neutron reactor Rejected due to need of engineered safety precaution for depressurization and high operating cost Gas coolants 16

17. Molten Salt Advantages: It has excellent heat transfer properties Good neutronic properties High boiling point High plant efficiency and thus low operating cost   Smaller containment   Since the system pressure is low and the heat capacity is high, the containments can be smaller and thinner Negative temperature coefficient of reactivity No chemical reactivity with air or water 17

18. Molten Salt Disadvantages: Material Degradation there are lots of corrosion related concerns Induced radioactivity If lithium is used in the salt, tritium will be produced, which is radioactive and extremely mobile - large amount will escape to the environment High melting point Rejected due to corrosive nature and complex plant operation 18

19. Good thermal properties Low pumping power Low operating pressure due to high boiling point Better material compatibility Best suited for natural convection in order to remove decay heat Costly Induced activity is high Liquid Metals 19

20. Liquid metals Liquid metals have high thermal conductivity Heat conduction in solid metals is due to two processes: Orderly crystal lattice vibration (as with non-metals) Electron motion, where the electrons of the atoms migrate in the same manner as electrical conduction These electrons carry with them the heat energy and transport it to low-temperature regions The electron migration process results in greater heat conduction (10 to 1000 times as much) than the lattice vibration process 20

21. Liquid metals Due to high thermal conductivity liquid metals are low Pr number liquids   T Pr >>1  >>  e.g., oils  T  Pr <<1  <<  e.g., liquid metals ,  T Pr = 1  =  e.g., air and gases have Pr ~ 1 (0.7 - 0.9 ) 21

22. Liquid metals Heat Transfer Heat transfer correlations for ordinary fluids do not apply to liquid metals With ordinary fluids the molecular conductivity of heat is negligible compared with the eddy conductivity Liquid metals have large molecular conductivities As a means of transporting heat in the turbulent core 22

23. Liquid metals Heat Transfer Heat transfer coefficient for liquid metals is very high The behavior of Nusselt number for liquid metals generally follows the relations: Nu = A + B . Pe c “ Pe ” is the Peclet number “A” represents effect of molecular conduction on convective heat transfer The constants (A, B, C) depend on the geometry and the boundary conditions 23

24. Liquid metals Heat Transfer Correlation Heat transfer of liquid metals has been studied by many researchers using different liquid metals Lyon derived the following correlation; Nu = 7 + 0.025Pe 0.8 for a constant heat flux condition Seban and Shimazaki ; Nu = 5 + 0.025Pe 0.8 for constant wall temperature 24

25. Liquid metals Heat Transfer Correlation Lubarsky and Kaufman; Nu = 0.625Pe 0.4 Reed ; Nu = 3.3 + 0.02 Pe 0.8 for uniform wall temperature conditions Kottowiski ; Nu = 0.75(r2/r1) 0.3 (7 + 0.025Pe 0.8 ) flows in annuli with radii r2 and r1 25

26. Which liquid metal? Liquid metal coolants for Fast Reactors Mercury Lead Lead-Bismuth Eutectic (LBE) NaK Sodium 26

27. Mercury Advantages: Very low melting point – Liquid at room temperature Disadvantages: High cross section for capture Very low boiling temperature – high vapor pressure Higher densities – greater pumping power requirement Toxic Less operating experience – Clementine & BR-2 Rejected due to its low boiling point, toxicity and high pumping power 27

28. Lead and LBE Advantages: Suitable from core boiling point of view due to high BP They have got superior neutronic properties They do not have any reaction with water 28

29. Lead and LBE Disadvantages: Higher densities – greater pumping power requirement Toxic Po hazard during repair and maintenance operation, fuel reloading 210 Po is produced in the lead-bismuth coolant by neutron activation of 209 Bi according to the following reaction chain: Bi 209 + n Bi 210 Po 210 Pb 206 Not much of the R& D has been carried with regard to their corrosion related behaviour β - t 1/2 = 5 days α t 1/2 = 138 days 29

30. Lead and LBE Lead and lead-bismuth are very dense, increasing the weight of the system therefore requiring more structural support and seismic protection which increases building cost . While lead is cheap and abundant, bismuth is expensive and quite rare. A lead-bismuth reactor will require hundreds to thousands of tonnes of bismuth depending on reactor size . Lead has a higher melting point of 327.5 °С. However ,  lead-bismuth eutectic  has a comparatively low melting temperature of 123.5 ° C. 30

31. Lead and LBE Lead-bismuth produces a considerable amount of  polonium , a highly radioactive and quite mobile element. This can complicate maintenance and pose a plant contamination problem. Lead produces orders of magnitudes less polonium, and so has an advantage over lead-bismuth in this regard . Rejected due to high melting point, low K and high pumping power 31

32. NaK Advantages: NaK has the advantage of being in liquid state at room temperature Disadvantages: Even a small leak may lead to rapid fire Rejected due to its high reactivity and cost 32

33. Sodium Advantages: It has excellent heat transfer Since it has high thermal conductivity Its neutronic properties are good Relatively less neutron absorption cross section High boiling point Low vapour pressure High plant efficiency and thus low operating cost Lower density comparative with other liquid metals Low pumping power Non toxic Support passive safety Availability Proven to work under industrial conditions Sodium is chosen as coolant for fast reactors 33

34. Sodium Disadvantages: Purification systems is required Cover Gas Purification System for keeping the cover gas free of impurities has to be designed Comparatively high melting point. It melts at 97.8  C Heaters have to be provided for keeping the sodium above its melting point Sodium reacts with water and air Special provisions have to be provided for detecting sodium leaks to prevent sodium fires 34

35. Comparison of Liquid Metal Coolant 35 Properties/Liquid metal Na NaK Pb Pb -B Hg Melting Point (°C) 98 18 328 327 -38 Boiling Point (°C) 880 826 1743 1670 357 K (W/m/k) 76 26 16 11 12 h (kW/m 2 /K) 36 20 23 17 32 Relative Pumping power required with respect to Na 1 1 12.5 12.5 14 Chemical Activity High High Low Low Low Corrosivity Low Low High High High Toxicity Low Low High High High Occurrence % of weight 2.4 0.016 5x10 -4 Cost with respect to Pb /kg 1-3 3-5 1 25-30

36. Historical perspective of coolant in Fast Breeder Reactors 36 Facility Country First Critical Coolant Clementine USA 1946 Mercury EBR-1 USA 1951 NaK BR-2 Russia 1956 Mercury BR-5/BR-10 Russia 1958 Sodium DFR UK 1959 NaK Fermi USA 1963 Sodium EBR-II USA 1963 Sodium Rapsodie France 1967 Sodium BOR-60 Russia 1968 Sodium SEFOR USA 1969 Sodium KNK-II Germany 1972 Sodium BN-350 Kazakhstan 1972 Sodium Phenix France 1973 Sodium PFR UK 1974 Sodium FFTF USA 1980 Sodium BN-600 Russia 1980 Sodium JOYO Japan 1982 Sodium FBTR India 1985 Sodium Super- Phenix France 1985 Sodium MONJU Japan 1995 Sodium

37. Properties of Sodium Sodium is the most common and readily available of all alkali metals Sixth most abundant element and constitutes 2.83 % of the earth's crust Sodium is very reactive, it does not occur in nature in the elemental form Sodium is a soft, silver-white metal, with a faint pinkish colour when it is freshly cut It melts at 97.8  C and boils at 882  C 37

38. Solid sodium when exposed to air, forms a film of oxide Molten sodium burns readily to form dense fumes of sodium oxide Molten sodium burns with a yellow flame, forming a mixture of sodium monoxide and sodium peroxide The reaction of sodium with water is energetic and exothermic The reaction of sodium with alcohols is less vigorous The viscosity of liquid sodium is very low, being slightly less than that of water at 100 °C Sodium metal has a very low vapor pressure only 3 mm of mercury at 500°C Properties of Sodium 38

39. Sodium properties 39

40. Heat Transfer Properties Being liquid metal, sodium has the properties of an excellent heat transfer medium Specific heat: The specific heat (~1340 J/kg-K) Nearly one third of that of water (~4200 J/kg-K) Low melting point and high boiling point Melting point of 97.8°C Boiling point of 883 °C Wide liquid range- around 800 °C Hence the coolant is a low-pressure fluid in the operating temperature range 40

41. Reactions Reaction of sodium with air 2Na(s) + O 2 (g)-  2Na 2 O 2 (s) ; ΔH = -435 kJ/mol Sodium peroxide is yellowish-white powder 4Na(s) + O 2 (g)  2Na 2 O(s) ; ΔH = -518 kJ/mol Sodium oxide is white powder Reaction of sodium with water The primary reaction taking place is very instantaneous Na + H 2 O → NaOH + ½ H 2 ; ΔH = -185 kJ/mol (1) This is followed by Reaction 2 and 3 2Na + NaOH → Na 2 O + NaH ; ΔH = -45.7 kJ/mol (2) Na + ½ H 2 → NaH; ΔH = -56.4 kJ/mol (3) Reactions 2 and 3 depends on conditions of pressure and temperature 41

42. Reactions Reaction of sodium with the halogens Sodium metal reacts vigorously with all the halogens to form sodium halides 2Na(s) + F 2 (g)  2NaF(s) ΔH = -569 kJ/mol 2Na(s) + Cl 2 (g)  2NaCl(s) ΔH = -411 kJ/mol 2Na(s) + Br 2 (g)  2NaBr(s) ΔH = -361 kJ/mol 2Na(s) + I 2 (g)  2NaI(s) ΔH = -288 kJ/mol 42

43. Handling of sodium Material for sodium handling Stainless steel Nickel and chromium alloy Nickel Inconel Sodium dissolves oxygen and solubility increases with temperature. Oxygen reacts with sodium to form Na 2 O It is also relatively insoluble in sodium, especially at low temperatures, causing deposition in cooler passages Because of this narrow passages should be avoided in sodium system. 43

44. Cover Gas Possible cover gas: Helium Argon nitrogen Helium -no induced radioactivity Argon- heavier than air, solving blanketing problem Nitrogen- soluble, mass transport of N 2 through the system Nitrogen causes nitriding at high temperatures Nitriding damages the thin walled components such as cladding, valve bellows, etc. 44

45. Solubility of Impurities in Sodium 45 Solubility of O2 in liquid sodium J.D.Nodens equation Log (O) w ppm = 6.2571-2444.5/T , (Tin K) Solubility of H2 in liquid sodium   Whittinghams equation log (H) w ppm = 6.467-3023/T, (T in K) Solubility of Carbon in liquid sodium It is very low, but it can be present as fine dispersed particulates, thus degree of removal is unpredictable

46. Solubility of Hydrogen and Oxygen 46

47. Permissible Impurities in Nuclear Grade Sodium Commercial Sodium After coarse filtration After micro filtration Nuclear grade Oxygen > 200 < 20 < 8 < 10 Hydrogen > 200 < 5 < 1 < 2 Calcium 40 to 200 < 100 < 10 < 10 Magnesium 40 to 100 < 50 < 2 < 2 Carbon < 30 < 30 < 30 < 30 Sodium 98.5% 99.5% 47

48. 48 Thank You