Plutonium is extracted from spent fuels in the repro­cessing plants in the form of plutonium nitrate. In order to utilize extracted plutonium for MOX fuel production, plutonium nitrate is converted to oxide powder by three methods: one is an oxalate pre­cipitation method; the other two methods involve coconversion with uranium, the ammonium uranyl plutonyl carbonate (AUPuC) conversion method, and the microwave heating denitration method (MH method). The AUPuC conversion method is described in Section 39.5.2.3 as part of the AUPuC fuel fabrication process.

2.15.5.1.1 Oxalate precipitation method

In the oxalate precipitation method, the plutonium oxide powder is prepared from plutonium nitrate by the following two reactions.50 Pu(NO3)4 + 2(COOH)2 ! Pu(COO)4 + 4HNO3

Pu(COO)4 ! PuO2 + 2CO2 + 2CO

Oxalate acid, H2(COOH)2, is added to plutonium nitrate solution at about 60 ° C, and the temperature maintained until the precipitation reaction (1) is completed. The plutonium oxalate precipitate is filtered and then dried in air. Dried plutonium oxalate is calcined in a furnace at temperatures from 350 to 650 °C. It has been reported that reac­tion (2) begins below 100 °C and is completed at around 350 °C.50 The characteristics of the obtained PuO2 powder vary depending upon the precipita­tion and calcination conditions, that is, the precipi­tation temperature, addition rate of oxalate acid to plutonium nitrate, oxalate acid concentration, and calcination temperature. This PuO2 powder is commonly utilized as a feed material for MOX fuel production in the world. The microstructure and characteristics of PuO2 powder prepared by the oxalate precipitation method have also been explained elsewhere.51