Consideration on formation of U3O8

According to ''The survey report on fuel rod pieces in Mihama No. 1 reactor'' drawn up by the Japan Atomic Energy Research Institute in October, 1977, the existence of U3O8 in white foreign phase was observed. In this study, the environment in which U3O8 can be formed in nuclear reactors and the attached facilities was investigated. The oxidation of UO2 to U3O8 is possible only when oxygen potential exceeds a certain value, depending on temperature. Therefore the oxygen potential in nuclear reactor environment was estimated, and compared with the value mentioned above. First, the range of oxygen potential in which the formation of U3O8 is possible is shown in relation to temperature. Next, the oxygen potential in three typical reactor environments, namely in primary coolant, in a fuel storage pool and when primary coolant entered into fuel rods, was estimated. The following conclusion was obtained. U3O8 is not formed in the primary coolant containing hydrogen of 30 cc per kg of water. There is some possibility to form U3O8 in a fuel storage pool. In case of the fuel into which primary coolant entered, it is possible to form U3O8 between 1080 and 1290 deg C according to the calculation, but the excess oxygen potential is only 2 kcal/mole O2 at most, therefore the realization of the formation of U3O8 is another question. (Kako, I.)