Study on residual radioactivity measurement method for site release of nuclear facilities; In-situ radioactivity measurement test with the use of portable Ge semiconductor detector (Contract research)

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

Nuclear facility sites after decommissioning are allowed to be released from nuclear safety regulations after confirming that sites have been decontaminated to acceptable levels. In-situ measurement with the use of a portable Ge detector is a suitable technology for confirmatory survey. A conservative method to evaluate residual radioactivity was proposed in this study. In the evaluation method concept, all of the radionuclide, which are in reality distributed across the area of interest, is assumed to be the single point source located at the furthest position of the area from a Ge detector. Based on this assumption, the detectable minimum time of the interest radionuclide were predicted by the calculation. If radiation from the point source is not detected for longer than the predicted detectable time, it can be proven that the radioactivity remaining in the interest area is lower than the radioactivity corresponding to the assumed point source. Results of the field test in JAEA site indicate that the proposed method was reasonable for the conservative evaluation of residual radioactivity.