[en] Critical control can be achieved and decontamination factor increased by adding metallic ions having a large neutron absorption cross section, such as Hf, Sc and In, to the dissolution step and/or the extraction and separation steps of a spent nuclear fuel reprocessing method. This method performs critical control by the addition of soluble neutron poison metallic ions, and restrains the decrease of the decontamination factor of fission products and the reduction of recovery of plutonium and/or super plutonium by masking acidic organic phosphorus compounds. As the result of experiments, it was found that the increase in distribution ratio of radioactive zirconium due to formation of dibutyl phosphate can be restrained by the addition of a variety of metallic ions. A nitric acid solution containing U, Pu and fission products were contacted with tributyl phosphate/dodecane solution. The extracted U and Pu were washed with nitric acid solution. After 7 to 8 hours of operation, the compositions of extraction waste and extracted media were analyzed. The plutonium recovery was 99% or more. The material balance was 97-99%. The distribution and the concentration were satisfactory. The addition of nonradioactive zirconium increased by four times the decontamination factor once lowered to 1/8. (Iwakiri, K.)