Reaction–diffusion simulations of hydrogen isotope trapping and release from cavities in tungsten, I: Single cavity

Abstract

We present simulations of deuterium thermal desorption spectra (TDS) from a single nm-sized spherical cavity in tungsten. We study both D2 gas-filled cavities and cavities with trapping only at surface sites. The simulations are based on the diffusion theory and the kinetic model of deuterium interaction with metal surfaces. We show that the previously used approaches based on local thermodynamic equilibrium between D2 gas and the subsurface solute deuterium are inadequate for simulating TDS spectra. We demonstrate the influence of cavity radius, D2 gas pressure, the parameters of the surface and the bulk. The deuterium release rate from a gas-filled cavity at a fixed temperature (below the TDS peak) stays constant until the D2 gas is depleted, in contrast to exponential decay observed for conventional deuterium trapping sites. Using the simulations and a simplified analytic model we demonstrate that for cavities the dependence of the TDS peak position on the heating rate does not follow exactly the Kissinger equation.