Measurement of residual radioactivity in the nuclear facility being decommissioned

Before site radioactive-free release of a nuclear facility being decommissioned, a termination survey should be carried out on radioactivity, because the site is possibly deposited mainly by 60Co, 137Cs, etc. This requires to be confirmed by a radioactivity measurement. However, the site is characterized by a very large area and by a very low level or little residual radioactivity. Therefore, detection and measuring works are required to be very efficient. Further the detection of 60Co is disturbed by the background due to 40K and other natural radioactivities. In order to enhance the efficiencies, a measuring system is being developed, which consists of several Nal(Tl) detectors and electronic circuits mounted on a vehicle and further, the position of measuring points is be determined by an auto-positioning apparatus. As a result of field running test, the vehicle can be located within a short distance from the measuring goal and it was confirmed that measuring operation is organized in sequence