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Hydrogen-tritium transfer in SFR Concepts K. LIGER, T. GILARDI Tél : 33 (0)

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2 OUTLINES Theory of diffusion and mass transfer phenomena –Fick’s law, parameters, steady state... –Data’s for liquid Na and stainless steel: Sievert constants, permeation, diffusion –Permeation Na/Metal/Na and Na/Metal/gas –Equilibrium between Na and cover gas –Cold trap and cristalisation –Links between H and T transfers Mass transfer in a reactor System definition Pollution sources Modeling Estimation of the fluxes of Hydrogen and tritium

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3 General goal for tritium transfer estimation Estimate : –The distribution of H and T in the circuits and then the gaseous and liquid release of T as well as the accumulation of T in the cold traps SO THAT: During operation –The release does not exceed release authorisation During conception –A suitable release limit authorisation could be asked

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4 Theory: Mass transfer through a wall Hydrogen permeation includes severall phenomena –Molecule dissociation at the interphase between metal and medium –Adsorption, Absorption –Diffusion in the metal –De-absorption, De-adsorption –Atoms combination In general, mass transfer is controlled by diffusion (combination is the second predominant phenomena) Hence, permeation can be represented by Fick’s law

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5 Theory of Diffusion : Fick ’s law Équations de Fick - Fick’s law - Mass conservation’s law For a simple geometry E.g.: Evolution of concentration in a plan wall after a step of concentration from C = C2 to C1 o x j : flux D : diffusivity C : concentration e : thickness e

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6 Steady state vs transient state ? When steady state and transient meet each other… –Assumption : plan wall –Time to reach 99,99% of the steady state flow depends on: D, diffusivity of material (function of temperature and nature of the material) e, thickness t p does not depends on the concentration gradient Time to reach 98,5% of the steady state flow: t p /2 Over the lifespan of a reactor, steady state can be assumed!

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7 Theory: Diffusion depends on… Nature of material: Austenic steel versurs ferritic steel,.... –factor 100 for D at 250°C, and only 10 at 500°C Temperature: –D = A exp( -E / T(K) ), m² /s –SS316 : factor 10 5 between room temperature and 500°C Surface state : Oxidised layer is a permeation barrier Hydrogen trapped in the metallic structure

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8 Diffusion : Hydrogen/tritium trapped in metallic structure Gaseous adsorption on metallic surface –external on surface –internal on small fissuration and defect structure In the matrix –Impurities –Grain boundaries –dislocations... Some of these mechanisms are irreversibles –E.g.: during heating of metal in a vacuum oven, hydrogen release is observed up to melting temperature Behaviour of T similar to 1 H, but isotopic exchange may modify macroscopic behaviour of T –In presence of hydrogen trapped in the structure: Shorter transient state for T diffusion Lower diffusion flux under steady state

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9 Theory: H/T equilibrium between cover gaz and Na Sievert constant Hydrogen equilibrium between Na (liquid or solid) and the cover gas

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10 Theory: equilibrium between gas and metal Sievert constant Hydrogen equilibrium between metal and the cover gas Similar solubility of H and T in steel Diffusion depends on atomic mass Hence, diffusion is « easier » for H

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11 Solubility in metal : Sievert constant E.g.: SS316, mol(H)/m 3 (acier)/pa 1/2 –K TISON (1983) = 0,9123 exp( -1352,1 / T(K) ) –K FORCEY (1988) = 0,9424 exp( / T(K) ) –K GRANT (1988) = 2,2191 exp( / T(K) ) D FORCEY (1988) = 3, exp( -5472,4 / T(K) ), m² /s

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12 Theory: Diffusion through a wall immersed in Na Plan wall Similar equations for T

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13 Theory: Diffusion through a wall immersed in Na and gas Similar equations for T

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14 Theory: Diffusion through pipes In that case, diffusion flux through the surface is:

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15 Flux of hydrogen to the cold trap: Flux of Tritium to the cold trap: –Co-cristallisation of tritium with H –Isotopic exchange and T decay neglected Cold traps : Cold trap efficiency: C*: Solubility of H in Na

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16 Theory: Isotopic exchange in gas phase hydrogen - tritium Isotopic exchange reaction: Equilibrium constant is:

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17 Tritium transfer in a Reactor 1.Steady state calculation 2.Homogeneity of concentrations in the circuits 3.Isotopic exchange in cold traps neglected as well as T decay 4.Source of T: –In primary circuit: Ternary fission reactions Control rod reactions Activation of impurities: B, Li Estimation of the source on the base of Superphenix and Phenix past experience 5.Source of H: –In primary circuit: fission reactions. –In secondary circuit: Gaz in the ternary circuit: source = 0 Water in the ternary circuit –Aqueous corrosion of GV –Thermal decomposition of N2H4 used in water to limit presence of O : 3 N 2 H 4 = 2 NH N H 2 for T>250°C Estimation of the source on the base of Superphenix and Phenix past experience Assumptions:

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18 RUR Na/Na Ar GVTurbine I III II RUR Na/Air PF I BPR ~ Schematic view of the reactors Y- H 2 O - He-N 2 - SCO 2 SPX:reference case Improvement of the models for Tritium transfer in other SFR concepts And for other fission reactors (EPR, HTR, VHTR…) PF II

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19 SFR: Mass balance Diffusion through heat exchangers Diffusion through GV Diffusion through pipes and volumes Trapping in cold traps (for H in Na) / Sources in the circuits H exchange with covering gas for Hydrogen: Diffusion through heat exchangers Diffusion through GV Diffusion through pipes and volumes Trapping in cold traps (for T in Na) / Sources in the circuits H/T exchange with covering gas for Tritium:

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20 Localisation of exchange in the different concepts SFR Na/Na/H2O SFR Na/Na/SCO2 SFR Na/Na/He-N2

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21 Concepts comparison SFR Na/Na/H2O, Na/Na/SCO2, Na/Na/He-N2 Presence of H2O in the ternary circuit leads to a source of H, which is benefit to reduce gaseous leakage: Release of T for Na/Na/H2O: 65 kBq/s Release of T for other concepts: nearly 1200 kBq/s Presence of: secondary cold traps of great importance for Na/Na/H2O concept primary cold traps of great importance for other concepts Permeation through GV: is of great importance for Na/Na/H20 concept. Great PE lowers gaseous release has no effect for other concepts Addition of secondary hydrogen source minimises T release

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22 Conclusion... –Diffusion –T release depends on the concept –Importance of cold traps –Importance of Hydrogen source –Ways of limitation of diffusion: nature of metal, oxydised layer, thickness, temperatures, aeras –Modeling partially validated on Phenix and Superphenix former results –Modeling Improvement needed: Colds traps modeling should be improved Transient state should be implemented Measurement of H/T diffusivity through metals

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23 References [1]Paul TISON Influence de l’hydrogène sur le comportement des métaux. Rapport CEA-R-5240 ; Thèse présentée à l’université Paris 6 le 9 Juin 1983 [2]K.S. FORCEY ; D.K. ROSS ; J.C.B. SIMPSON ;D.S. EVANS Hydrogen transport and solubility in 316L and steels for fusion reactor applications. Journal of Nuclear Materials 160 (1988), North Holland, Amsterdam. [3]D.M.GRANT ;D.L. CUMMINGS and D.A. BLACKBURN Hydrogen in 316 steel ; diffusion, permeation and surface reaction. Journal of Nuclear Materials 152 (1988), North Holland, Amsterdam.

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