Critical safety maintaining method in nuclear fuel reprocessing step by using organic boron compound as soluble neutron poison or fixed neutron absorber

In a step of extracting nuclear fuels by using an organic solvent in a spent nuclear fuel reprocessing step, occurrence of uncontrollable fission chain reactions of fission products such as plutonium and uranium is prevented. Namely, chemically and physically stable organic boron compounds which are soluble only to an extracting organic solvent are added as a soluble neutron poison. In addition, organic boron compounds of high boron and hydrogen content are used, as a fixed neutron absorber, for constituent materials of equipments to be used. m-carbolan or a derivative thereof is used as the organic boron compound. Then, an extractor capable of ensuring critical safety can be designed more rationally without worsening performances of equipments to be used during a spent fuel reprocessing and extracting step. Space for installing a solution storage vessel can be saved, and equipments having complicated shape can be simplified. (N.H.)