[en] The Japan Atomic Energy Research Institute has been conducting a decommissioning program of the Japan Power Demonstration Reactor (JPDR) in order to provide technical data for future decommissioning of commercial reactors. The program involves a study of radioactive inventory estimation within reactor based on both calculations and measurements. For this purpose a calculational code system has been developed consisting mainly of neutron transport calculation codes. Its validity has been verified through comparison of the calculations with the systematically measured inventory in JPDR. The purpose of the present paper is to present the outline of the code system and to verify the accuracy of it. 11 refs, figs and tabs