A study of reactions between simulated solid fission products and ThO2

Thoria specimens containing oxide and metallic components to simulate a thoria-based fuel element after high fission burnup were studied. Simulation techniques involving isothermal and non-isothermal heat treatment were employed. The results of this study were compared with those from post-irradiation examination studies on urania-based fuels. As with real UO2-based fuel elements after high burnup, barium zirconate-type phases were found to be stable in the thoria matrix at least up to 16000C; however, in the ThO2 system, a Nd2Zr2O7 synthetic pyrochlore-type phase, not found with UO2, was detected. It appeared that formation of this phase was favoured over the formation of barium zirconate under reducing conditions. Under the influence of a high thermal gradient the simulated fuel elements appeared to show centre core melting and equiaxed grains but failed to show columnar grain formation. (Author)