Numerical simulation of neptunium extraction behavior in co-decontamination stage for late split Purex system

Numerical simulation of neptunium concentration profiles is carried out in order to understand the extraction behavior of neptunium in the co-decontamination stage which decontaminate through two extraction banks before the uranium and plutonium partitioning steps. Simulation results show that leaking of neptunium to the waste stream at the second extraction bank is caused by reduction from Np(VI) to Np(V) unextracted by TBP in the presence of nitrous acid of 10-3 mol/l. It is found that the prevention of the reduction reaction from Np(VI) to Np(V) is effective for the recovery of neptunium with uranium and plutonium into the product stream. Two methods for the recovery of neptunium are suggested; i) to decrease residence time in the acid adjustment vessel located between first and second extraction banks, ii) to diminish nitrous acid in the aqueous phase by hydrazine. Simulation results show that the recovery of neptunium with uranium and plutonium is improved from 40 to 60% and from 40 to 70% by the first and the second methods, respectively. (author)