Study for reprocessing improving separation efficiency of Np

A study has been conducted to establish a flow sheet to extract Np effectively in the first cycle and separate it from the product stream before the purification cycles. Three different Np valency control techniques by organic reductants decomposable to gas and water, electrolysis, and photochemical reaction have been studied by laboratory-scale experiments. These techniques must have potential to reduce Np(VI) to Np(V) selectively in the coexistence of U(VI) and Pu(IV). Normal-butylaldehyde is found to be a favorable reductant for the selective Np(VI) reduction in the present study. A chemical flow sheet experiment using miniature mixer settlers showed that about 91 % of Np extracted by 30 % tri-n-butylphosphate/n-dodecane solvent along with U(VI) and Pu(IV) was reduced by n-butylaldehyde and separated from U and Pu. As for electrolysis, the selective reduction of Np(VI) by a potentiostatic method is confirmed in the coexistence of U(VI) and Pu(IV) under the cathodic potential of Pt electrode at 0.0 - 0.80 V vs. a saturated calomel electrode. The experimental results of photochemical reaction using a flow type cell with an UV lamp showed that 93 % of Np(VI) was reduced in the residence time of 30 minutes indirectly by nitrous acid produced photochemically in 3 M nitric acid solution. As a part of NUCEF (Nuclear Fuel Cycle Safety Engineering Research Facility) Project in JAERI, a flow sheet study under hot conditions is planned to demonstrate an improved reprocessing by applying the above-mentioned techniques. (author)