Oxidation resistance of WB and W2B-W neutron shields

Abstract

Tungsten borides are candidate radiation shielding materials for compact fusion reactors. However little is known about their performance when oxidised at high temperatures in the case of an accident combining loss of coolant and vacuum. Candidate materials containing 0, 16, 30, and 50 at.% B were exposed to air at 600–1100 °C in a thermogravimeter and the kinetic rate constants were evaluated. The corresponding oxide scales were characterised by X-ray diffraction and electron microscopy. The boron containing materials showed parabolic kinetics, more protective behaviour than tungsten, and an improvement in oxidation resistance with increasing boron content. For example, the rate constant was a factor of 600 lower for 50 at.% B vs. pure W at 1000 °C. In 16 and 30 at.% B materials the scale formed a microstructure of interpenetrating B2O3 and WO3. In the 50 at.% B material, two distinct layers formed, with a B2O3-rich layer on the surface, and fine WO3-based layer beneath. The protective scale was disrupted in all materials at 1000–1100 °C, depending on boron content. The disruption resulted in a transition to linear kinetics and can be explained by the B2O3 evaporation rate exceeding its formation rate. Encouragingly, all tungsten borides have superior oxidation resistance compared to pure tungsten, suggesting favourable accident tolerance.