CHEMICAL FORM OF THE SOLID FISSION-PRODUCTS IN (TH,U) O2 SIMULATING HIGH BURNUP

The chemical form of the solid fission products has been studied for (Th0.81U0.19) O2 simulating 21.5% FIMA in an HTGR environment. Experiments have been performed with X-ray diffraction, electron-probe microanalysis, ceramography and hardness measurement. The results showed that fission-product phases of two types, Mo-Ru-Pd and (Ba, Sr)(Zr, Ce) O3, are present in the simulated fuel pellet. The fuel matrix comprises (Th1-xUx, Zr, Ce, RE) O2 with an x value of 0.067(RE = Nd, La, Pr, Y, Sm). Dissolution of the rare earths (RE) and the residual Zr plus Ce in (Th0.933U0.067) O2 was accompanied by contraction of the unit cell of the oxide matrix. Reaction behavior in the selected fission product system BaO-ZrO2-Nd2O3 was also investigated. The results showed that in the presence of BaO, Nd2Zr2O7 is converted to barium zirconate: at 1630°C, Nd2Zr2O7 + 2 BaO → 2 BaZrO3 + Nd2O3. This fact, combined with thermochemical assessment, confirms the relative stability of (Ba, Sr)(Zr, Ce) O3 against Nd2Zr2O7 in the simulated (Th0.81U0.19) O2. From these results and fission product inventories, it is inferred that the chemical state of high-burnup ThO2 is very similar to that of (Th0.81U0.19) O2. © 1979.