[en] In spent fuel reprocessing by Purex process uranium and plutonium are extracted together in the solvent 30% TBP-dodecane to effect decontamination from fission products and trivalent actinides. In a subsequent partition step, uranium and plutonium are separated from each other by selectively reducing Pu(IV) to Pu(III) in which valency it is least extractable in the above solvent. The reduction of Pu(IV) to Pu(III) is accomplished with the reducing agent uranous nitrate, which is produced conventionally by electrolytic reduction of U(VI) with hydrazine nitrate as uranous nitrate stabilizer. In this method of reduction the percentage conversion attainable is only 50 to 60%. The use of this solution as partitioning agent leads not only to the dilution of the plutonium product but also to increase in uranium processing load by each externally fed uranous nitrate batch. Also the oxide coating of the anode, TSIA (Titanium Substrate Insoluble Anode) wears out after a certain period of operation. This necessitates recoating which is quite cumbersome considering the amount of the decontamination involved. So an alternative to the conventional electrolytic method has been explored in the laboratory at FRD. It is observed that quantitative reduction of uranyl nitrate to uranous nitrate can be achieved with hydrazine nitrate as reductant in presence of PtO₂ catalyst. Extensive laboratory scale studies were carried out to optimize the various parameters for quantitative reduction. The optimum parameters for the reduction of 4 grams of uranyl ions as a single batch (20 ml) with a catalyst amount of 320 milligrams are H⁺ 1 M, N₂H₂⁺ = 1.0 M and temperature = 60 degC and the time for quantitative reduction under these optimized conditions was 60 minutes. These studies were then extended to pilot plant scale and the catalyst amount used in these in studies was 80 grams. When the column was operated under the optimized conditions, the uranous production per single batch (10 litres) was 1.75 kg in five hours. The amenability of the process for batch as well as continuous modes of operation was studied and the results of which are discussed in this report. The pilot plant scale studies revealed that the developed process could produce uranous nitrate at a significant rate for Purex process requirements. The near quantitative reduction and the redundancy of the supply of electrical energy render this process a more viable alternative to the existing electrolytic method of reduction. (author)