Conservative Evaluation of Remaining Radioactivity on Decommissioned Nuclear Power Plant Sites

Nuclear facility sites after decommissioned are necessary to confirm to be decontaminated prior to be released from nuclear safety regulations. A conservative method to evaluate radioactivity remaining on the sites of decommissioned nuclear power plants was proposed in this study, and its validity was discussed. Cesium 137 was taken up to validate the evaluation method as one of the typical radionuclides for nuclear power plants. In the evaluation method concept, all of the Cs-137, which are in reality distributed across the area of interest, is assumed to be the single point source located at the furthest position on the ground surface of the area from a detector. In such a configuration, most conservative result was given by Monte Carlo calculations, and the detectable time (time need to detect enough amount of radioactivity) predicted by the calculation was approximately equal to the actual in-situ measurement time by the potable pure-germanium semiconductor detector. These results indicate that the proposed evaluation method was reasonable for the conservative evaluation of remaining radioactivity.