Radiation-Induced Evolution of Tungsten Carbide in Fusion Reactors: Accommodation of Defect Clusters and Transmutation Elements

The development of suitable shielding material is a key challenge in the advancement of spherical tokamak fusion reactors. Tungsten carbide (WC) is a promising candidate material owing to its low neutron and gamma attenuation lengths resulting from the combination of high-Z and low-Z elements, but its behavior under prolonged exposure to fusion neutrons is poorly understood. Here, we shed light on the microstructural evolution of WC under neutron irradiation by investigating the formation and clustering of defects using density functional theory atomic simulations. It is found that deviation from stoichiometry is accommodated entirely by C defects (vacancies and interstitials), while the disorder induced by radiation damage may be accommodated by three competing processes (C- Frenkel, antisite, and Schottky) with similar energetics. Vacancy clusters involving a combination of both tungsten and carbon vacancies show increasingly favorable binding energies with increasing cluster size and may lead to the formation of undesirable extended defects such as dislocation loops and voids. The accommodation of the three main transmutation elements Re, Os, and Ta was also considered, as well as their interaction with radiation-induced intrinsic defects. It was found that all three species are preferentially accommodated as substitution defects on the W sublattice, and they all exhibit a thermodynamic drive to bind with W vacancies, while Ta also binds with C vacancies and with Re and Os substitutions. This suggests that radiation-induced vacancy sinks (e.g., voids and dislocation loops) will likely be enriched in transmutation elements, and that these further stabilize the vacancy clusters, potentially aggravating swelling. However, it is predicted that under equilibrium conditions, all three species exhibit limited solubility in WC, assuming that the competing phases for precipitation are TaC(s), Os(s), and Re(s). Further investigation into the C-(Ta,Re,Os)-W phase diagram is needed to improve the accuracy of these predictions.