An advanced reprocessing system for spent FBR fuels based on two precipitation processes has been proposed. In the first process, only U(VI) species is precipitated using a pyrrolidone derivative (NRP) with lower hydrophobicity and donicity, which should yield a lower precipitation ability. In the second process, residual U(VI) and Pu(IV, VI) are precipitated simultaneously using an NRP with higher hydrophobicity and donicity, which should yield a higher precipitation ability. In order to select the precipitants for the first precipitation process, we have examined the precipitation behavior of U(VI), Pu(IV), and Pu(VI) species in HNO3 using N-n-propyl-2-pyrrolidone (NProP), N-n-butyl-2-pyrrolidone (NBP), and N-isobutyl-2-pyrrolidone (NiBP) with lower hydrophobicity and donicity than N-cyclohexyl-2-pyrrolidone (NCP) previously proposed as the precipitant. It was found that NRPs could precipitate U(VI) nearly stoichiometrically and that the decontamination factors for simulated fission products were higher than those in NCP systems. Furthermore, as seen in NCP, it was found that in the U(VI)-Pu(IV) mixtures, a small amount of Pu(IV) was temporarily coprecipitated with U(VI) by NRPs in spite of their lower precipitation ability and then the coprecipitated Pu(IV) component was redissolved with continuous stirring. From these results, NRPs can be proposed as candidate precipitants for the first precipitation process. In particular, NBP is considered to be the most promising precipitant, because of the relatively high solubility of the NProP precipitant, the increases in viscosity of NiBP slurry with stirring, and the relatively fast sedimentation rate of NBP precipitates.
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