Thermal gradient effect on hydrogen transport in tungsten

TL;DR

This study investigates how thermal gradients influence hydrogen transport in tungsten, a key material in fusion reactors, by developing an analytical model based on molecular dynamics simulations to understand tritium retention issues.

Contribution

The paper introduces an analytical approach to compute the heat of transport ($Q^*$) for hydrogen in tungsten, incorporating thermal gradients and validated by molecular dynamics simulations.

Findings

$Q^*$ can be expressed as a function of temperature and thermal gradient.

Dependence of $Q^*$ on thermal gradient is negligible at first order.

Average $Q^*$ value for hydrogen in tungsten is -5.41×10^{-3}kT^2 eV.

Abstract

One key challenge for efficiency and safety in fusion devices is the retention of tritium (T) in plasma-facing components. Tritium retention generates radioactive concerns and decreases the amount of fuel available to generate power. Hence, understanding the behavior of T in tungsten (W), as the main candidate as armor material, is critical to the deployment of fusion as a reliable energy source. In this work, we have studied the effect of a thermal gradient in the transport properties of hydrogen (as a T surrogate) in pure W. Strong thermal gradients develop in the divertor as a result of the intense energy fluxes arriving at the material. We have developed an analytical approach to compute the heat of transport ($Q^*$) that is parameterized from molecular dynamics (MD) simulations. $Q^*$ is a parameter needed in irreversible thermodynamics frameworks to understand mass transport in the presence of thermal gradients. We show that $Q^*$ can be written as a function of temperature, temperature gradient, a characteristic length and the ratio of the rates towards hot and cold regions. Furthermore, we describe how, to first order, the dependence of $Q^*$ on the thermal gradient vanishes, in agreement with MD results. On average, we find $Q^*=-5.41\times 10^{-3}kT^2~\text{eV}$ for H in pure W, with $k$ the Boltzmann constant and $T$ the temperature.