Preparation and characterization of dicesium tetravalent plutonium hexanitrate

In experiments on U crystallization in a dissolver solution containing Cs, there is concern that Cs and Pu(IV) nitrate complex are deposited on the UNH crystal in the dissolver solution at the time of the U crystallization. The characteristics of generation of Cs and Pu(IV) nitrate complex with dissolver solution of MOX fuel were examined. This complex was obtained as a precipitate by mixing dissolver solution of MOX fuel and CsNO₃ solution, and was identified as dicesium tetravalent plutonium hexanitrate, Cs₂Pu(NO₃)₆ by concentration analysis and XRD. The precipitate has a tendency to be generated at high HNO₃ concentrations. Thermal analysis shows that the precipitate is stable below 245 °C, and a weight loss of 10.29 ± 0.23% is observed between 245 and 297 °C. This result indicates the decomposition of Cs₂Pu(NO₃)₆ to Cs₂PuO₂(NO₃)₄. With these properties, the UNH crystal should melt at the condition between 60 and 100 °C and be separable from the Cs complex by filtration. This suggests a new method of crystal purification allowing higher decontamination of UNH crystal in the U crystallization process.