Conservative evaluation of remaining radioactivity on decommissioned nuclear power plant sites

Tanaka, Tadao ; Shimada, Taro ; Ito, Takeshi*; Hirano, Takahiro*; Sukegawa, Takenori

Nuclear power plant sites are allowed to be released from nuclear safety regulations after the plants are decommissioned in Japan. The regulatory compliance will require confirming that there is no significant radioactivity remaining on the sites for the site release. In the present study, we propose an evaluation method of Cs-137 remaining on sites of decommissioned nuclear power plants. The method is time-efficient and gives a conservative result. In the evaluation method concept, all of the Cs-137, which is in reality distributed across the area of interest, is assumed to be the single point source located at the furthest position on the surface of the area from a detector. For such a configuration, the counting time that the Cs-137 point source is detectable is predicted using Monte Carlo calculations. If radiation from the Cs-137 point source is not detected for longer than the predicted counting time, it can be proven that the radioactivity remaining on the surface of the area is lower than the radioactivity corresponding to the assumed Cs-137 point source. A Cs-137 radiation source was placed at a fixed distance from the Ge detector, and the peak counting rate of Cs-137 were measured. The detectable time predicted by Monte Carlo calculations was approximately equal to the actual measurement time by the Ge detector, which means that the proposed evaluation method was reasonable for the conservative evaluation of remaining radioactivity.