U₃Si₂ behavior in H₂O environments: Part II, pressurized water with controlled redox chemistry

Recent interest in U₃Si₂ as an advanced light water reactor fuel has driven assessment of numerous properties, but characterization of its response to H₂O environments is sparse in available literature. The behavior of U₃Si₂ in H₂O containing atmospheres is investigated and presented in a two-part series of articles. This work examines the behavior of U₃Si₂ following exposure to pressurized H₂O at temperatures from 300 to 350 °C. Testing was performed using two autoclave configurations and multiple redox conditions. Use of solid state buffers to attain a controlled water chemistry is also presented as a means to test actinide-bearing systems. Buffers were used to vary the hydrogen concentration between 1 and 30 parts per million H₂. Testing included UN, U₃Si₅, and UO₂. Both UN and U₃Si₅ were found to rapidly pulverize in less than 5 h at 300 °C. Uranium dioxide was included as a control for the autoclave system, and was found to be minimally impacted by exposure to pressurized water at the conditions tested for extended time periods. Testing of U₃Si₂ at 300 °C found reasonable stability through 30 days in 1–5 ppm H₂. However, pulverization was observed following 35 days. The redox condition of testing strongly affected pulverization. Characterization of the resulting microstructures suggests that the mechanism responsible for pulverization under more strongly reducing conditions differs from that previously identified. Hydride formation is hypothesized to drive this transition. In conclusion, testing performed at 350 °C resulted in rapid pulverization of U₃Si₂ in under 50 h.