RADIOACTIVE CONTAMINATION INSPECTING SYSTEM FOR VEHICLE

抄録

We have developed an automatic radioactive contamination inspecting system for vehicle entering and exiting the radiation controlled area. At interim storage facilities that store radioactive wastes released by the accident at the Fukushima Daiichi Nuclear Power Station, contamination inspection of articles taken out of the controlled area is mandatory, based on the Ionizing Radiation Hazard Prevention Regulations. Contamination inspection is also required for large trucks, but it is usually carried out manually using a portable survey meter. In this case, the inspection time will be very long, so it is expected that traffic congestion will occur at the screening site. In order to solve these problems, we have developed a “radioactive contamination inspecting system for vehicle” that enables quick and reliable contamination inspection by joint development of Mitsubishi Electric Plant Engineering and Mitsubishi Electric Corporation. In order to detect slight contamination on the surface of vehicles in areas with relatively high background dose rates such as interim storage facilities, we have newly developed a large-area radiation detector.

The detector has high sensitivity to β-rays from radioactive cesium, but on the other hand, it has very little sensitivity to ambient γ-rays because of very little thickness of the detector. Furthermore, in order to efficiently inspect the entire vehicle, we have designed a movable inspection gate that installed five independent-moving radiation detector units. The detector units have several range finders to measure the distance to the surface of vehicles so that the inspection can be performed while keeping the distance from the surface of the vehicle constant. As a result, contamination on the surface of the vehicle can be detected quickly and reliably without radiation shield even in an area where the ambient dose rate is several μSv/h. For example, although conventional manual measurement method took about 10 minutes by 10 operators to inspect a 10-ton truck, the developed system can automatically inspect it within 4 minutes. The five systems were operated at the screening site of the interim storage facility from January 2019 to March 2022.