PURPOSE: To shorten the isolation period of high level waste from living environment by removing a transuranium element from the extract and raffinate of a co-decontamination process and converting the transuranium element to a nuclide of a short half-life using a proper annihilation means.

CONSTITUTION: A solution of used fuel is applied to a co-decontamination process to be separated into an aqueous phase (raffinate, high level waste water) containing the greater part of a transuranium element and a fission product and an org. phase (extract) containing U, Pu and Np. Np is removed from the extract before U and Pu are distributed while Am, Cm and the transuranium element containing Np and Pu being extraction leak components are removed from the high level waste water and these transuranium elements are converted to a nuclide of a short half-life along with Np removed by main flow to be subjected to annihilation processing. The high level waste water from which the transuranium element is removed is solidified to the subjected to storage disposal. When the transuranium element is processed by this method, the storage disposal isolating the high level waste water from living environment may be performed over about 300 - 1,000 years and, therefore, the high level waste water can be stored on the ground or in a state shallowly buried in the ground.