Basic study of photochemical valency adjustment of plutonium (Pu) and neptunium (Np) in nitric acid solution

A photochemically-induced valency adjustment method has been studied to remove Np from the mixed nitric acid solutions of Pu and Np in connection with the Purex reprocessing. The valencies of Pu and Np ions were adjusted to be Pu(III) and Np(V) under the initial conditions and their concentrations were 1x10-4 and 1x10-3 mol·dm-3 respectively. Experiments were carried out under various conditions changing irradiation intensities of the Hg lamp in various concentrations of HNO3. It was found that the rates of redox reactions of Pu ions were significantly affected by the irradiated light as well as the acid strength. Under the irradiation of a 0.015W Hg lamp in 3M HNO3 solution containing a tenfold excess of hydroxylamine and hydrazine, more than 95% Pu(III) was oxidized rapidly to Pu(IV) within 10 minutes irradiation and it remained at the same valency even after continuous further irradiation. On the other hand, the irradiation did not change the valency of Np(V) under the conditions studied. These valency conditions, i.e. Pu(IV) and Np(V), are appropriate for separating Np from Pu by solvent extraction with TBP-n-dodecane. The present results lead to the conclusion that the photochemical method has a high potential for removing Np from mixed solution of Pu and Np. (author)