CD Technical Meeting (RES3): Diffusion and retention of multiple H isotopes in irradiated Tungsten and Vanadium

Thursday 5 Mar 2026, 11:00 → 12:00 Europe/London

Sanjeet Kaur (Computational Materials)

Fusion Science via Computer Simulation

 

Abstract

A fundamental description of gas transport and retention in plasma-facing materials is crucial for tritium inventory modelling. During future reactor operations, the material microstructure is expected to evolve and reveal defects which act as “traps” for diffusing gas atoms, thereby compromising material performance. The classical formalism for gas diffusion and trapping is given by the McNabb-Foster equation [1], where the rates of trapping and detrapping are determined with transition state theory. To describe the reduction in diffusivity in the presence of traps, the “effective” diffusivity was initially proposed by Oriani [2] under a trapping-detrapping equilibrium.

In recent years, the McNabb-Foster equation has been extended for multiple gas atoms of a single isotope in a single trap, where the incremental binding energy of each gas atom is uniquely defined [3,4]. This presentation outlines the trapping equations for multiple isotopes in multi-occupancy traps, applicable for point defects such as vacancies, vacancy clusters and voids. A spectral analysis of the trapping system reveals how quickly the trapping-detrapping equilibrium arises across gas concentrations and temperatures. Under this equilibrium, a generalized effective diffusivity is derived. For a homogeneous distribution of vacancies in tungsten, the diffusivity monotonically increases with increasing hydrogen concentration. However in vanadium, a drop in diffusivity is predicted as the hydrogen concentration approaches the vacancy concentration. The difference in behaviour between the metals is traced back to the difference in incremental binding energy of the nth atom to a (n-1)-vacancy complex [5]. We validate these results by comparison with effective diffusivities obtained from molecular dynamics as well as kinetic Monte Carlo simulations.

 

Acknowledgments

This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No. 101052200 – EUROfusion) and by the EPSRC Energy Programme [Grant No. EP/W006839/1].

[1] A. McNabb, P. K. Foster, "A new analysis of the diffusion of hydrogen in iron and ferritic steels", Trans. Metall. Soc. AIME, 227: 618-627 (1963)

[2] R. A. Oriani, "The diffusion and trapping of hydrogen in steel", Acta Metall., 18 (1): 147-157 (1970)

[3] E. A. Hodille, et.al., "Study of hydrogen isotopes behaviour in tungsten by a multi trapping macroscopic rate equation model", Phys. Scr., 2016 (T167): 014011 (2016)

[4] K. Schmid, et.al., "Transport of hydrogen in metals with occupancy dependent trap energies", Metall. Mater. Trans. B, 116: 134901 (2014)

[5] S. Kaur, et.al., "Effect of multi-occupancy traps on the diffusion and retention of multiple hydrogen isotopes in irradiated tungsten and vanadium", Phys. Rev. Mat., 9 (12): 125404 (2025)

 

11:00 → 11:45 Talk

Speaker: Sanjeet Kaur

11:45 → 11:55 Discussion