The reduction of neptunium(VI) by formohydroxamic acid

The application of formohydroxamic acid (FHA) (HCONHOH) in an Advanced Purex process for the reprocessing of irradiated nuclear fuel is well suited to the separation of Np(IV) from U(VI) by the formation of a hydrophilic complex with Np(IV). U(VI) extraction in to 30% tributyl phosphate is unaffected. However FHA is also a very fast reducing agent for Np(VI) (NpO22+). The reduction of Np(VI) to Np(V) was studied by near infra red spectrophotometry. Measurements using both an external sample compartment and a dipping optrode (to follow the reaction in-situ) connected to a spectrophotometer by fibre optics, have been used. FHA did not reduce uranium (VI) although the slow reduction of Pu(IV) to Pu(III), probably through the hydrolysis product, hydroxylamine, has been observed. Therefore Np(VI) can be efficiently stripped by reduction to Np(V) by FHA when solvent phase (30% tributyl phosphate in odorless kerosene) solutions of Np(VI) and U(VI) are contacted with nitric acid and FHA. Distribution coefficients for Np are very low and similar to those predicted for Np(V). Spectrophotometric evidence indicated complete reduction. The experimental distribution coefficient for U(VI) is equivalent to the predicted distribution coefficient in the absence of FHA thus proving that the extractability of U(VI) is unaffected. Finally, the separation of Np(VI) and U(VI) has also been studied in a single centrifugal contactor stage to confirm that the reduction, under typical process conditions, is compatible with centrifugal contactors which have very low residence times