Equilibrium-Based Computation on High Temperature Reaction Behaviors of a Cs-U-O System with Oxidation and Reduction Gas Flows

Abstract

A high temperature reaction process was involved with the dry treatment of spent nuclear fuels (SNFs). SNFs include various kinds of fission products and one of the significant elements is cesium because its isotopes emit high radioactivity and heat load. Therefore, the knowledge of its behavior during the high temperature process is important to design a reactor and estimate material flows connecting with other processes. The main constituent of SNFs is uranium and, therefore, it was the first step to identify cesium compounds which was stable with uranium oxide. The Tpp analyses were performed for three temperatures of 500, 1000, and 1400 degrees C which represented relatively a low, a moderate, and a high temperature condition, respectively. Two types of cesium uranates (Cs2U2O7 and Cs2UO4) appeared to be stable phase domains with respect to partial pressures of Cs and O-2. The reaction behaviors of the two types of cesium uranates under a gas flow were calculated based on an equilibrium approach. Four different gas conditions were taken into consideration; (i) pure O-2, (ii) air, (iii) Ar with 0.001% H2O impurity, and (iv) Ar with 4% H-2. With the continuous introduction of gas flows, the reaction behavior of the uranates were simulated to find that the reducing atmosphere of Ar with 4% H-2 flow converted them into UO2 and resulted to complete removal of cesium elements from a reactor.