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**Hydrogen-tritium transfer in SFR Concepts**

K. LIGER, T. GILARDI Tél : 33 (0)

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**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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**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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**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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**Theory of Diffusion : Fick ’s law**

j : flux D : diffusivity C : concentration e : thickness 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 x e o

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**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 tp does not depends on the concentration gradient Time to reach 98,5% of the steady state flow: tp /2 Over the lifespan of a reactor, steady state can be assumed!

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**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 105 between room temperature and 500°C Surface state : Oxidised layer is a permeation barrier Hydrogen trapped in the metallic structure

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**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 1H, 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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**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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**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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**Solubility in metal : Sievert constant**

E.g.: SS316, mol(H)/m3(acier)/pa1/2 KTISON (1983) = 0,9123 exp( -1352,1 / T(K) ) KFORCEY (1988) = 0,9424 exp( / T(K) ) KGRANT (1988) = 2,2191 exp( / T(K) ) DFORCEY (1988) = 3, exp( -5472,4 / T(K) ) , m² /s

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**Theory: Diffusion through a wall immersed in Na**

Plan wall Similar equations for T

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**Theory: Diffusion through a wall immersed in Na and gas**

Similar equations for T

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**Theory: Diffusion through pipes**

In that case, diffusion flux through the surface is:

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**Cold traps : C*: Solubility of H in Na**

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 trap efficiency: C*: Solubility of H in Na

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**Theory: Isotopic exchange in gas phase hydrogen - tritium**

Isotopic exchange reaction: Equilibrium constant is:

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**Tritium transfer in a Reactor**

Assumptions: Steady state calculation Homogeneity of concentrations in the circuits Isotopic exchange in cold traps neglected as well as T decay 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 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 : N2H4 = 2 NH3 + 2 N2 + 3 H2 for T>250°C

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**Schematic view of the reactors**

RUR Na/Na Ar GV Turbine I III II Na/Air PF I BPR ~ PF II SPX:reference case Improvement of the models for Tritium transfer in other SFR concepts And for other fission reactors (EPR, HTR, VHTR…) Y - H2O - He-N2 - SCO2

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**SFR: Mass balance for Hydrogen: for Tritium:**

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 Tritium: 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

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**Localisation of exchange in the different concepts**

SFR Na/Na/H2O SFR Na/Na/SCO2 SFR Na/Na/He-N2

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**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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**Conclusion ... Diffusion Modeling Improvement needed:**

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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**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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