A Study on Precipitation Behavior of Plutonium and Other Transuranium Elements with N-Cyclohexyl-2-pyrrolidone for Development of a Simple Reprocessing Process

Precipitation behavior of Pu and other transuranium elements with a combustible organic reagent N-cyclohexyl-2-pyrrolidone (NCP) has been examined to develop a simple reprocessing process of spent nuclear fuel based only on precipitation method. Experiments on the precipitation behavior of Pu in HNO3 solution containing only Pu showed that both Pu(VI) and Pu(IV) were precipitated with NCP, but they required more NCP than in the U(VI) precipitation. Selective U(VI) precipitation from HNO3 solution containing U(VI) and Pu(IV) (U(VI)-Pu(IV) solution) was achieved by stirring the solution for sufficient time after addition of NCP with ratio of [NCP]/[U] = 1.4. Addition of an enough amount of NCP to U(VI)-Pu(VI) or U(VI)-Pu(IV) solutions gave a quantitative precipitation of both U and Pu. From a viewpoint of the physical property of the precipitate, Pu(VI) was found to be favorable for the U-Pu precipitation. Hence, we have studied the oxidation of Pu(IV) to Pu(VI). As a result, heating of the solution was found to be a promising method for the present process. Furthermore, it was clarified that neither Am(III) nor Np(V) was precipitated in the selective U precipitation and the simultaneous U-Pu precipitation. These results demonstrate the feasibility of the reprocessing by precipitation with NCP.