An In-situ Alpha Air Monitor for the Retrieval of Fuel Debris at the Fukushima Daiichi Nuclear Power Station

In the decommissioning of the Fukushima Daiichi Nuclear Power Station (FDNPS), the dismantling of nuclear fuel debris (NFD) in the damaged reactors is a significant issue with many technical difficulties. The dismantling is presumed to involve cutting of NFD, generating significant concentrations of particulates containing α−radionuclides (α−aerosols) that pose significant health risks upon inhalation. In order to minimize the radiation exposure to workers and general public, it is essential to monitor the concentration of a-aerosols inside the primary containment vessels (PCV). We have developed an in-situ α−air monitor (IAAM) that combines a flat-type flow path (FFP), an air heater, a ZnS scintillator, and a multi-anode photomultiplier tube (MA-PMT) to measure α−aerosols directly and on a real-time basis. The monitor operates under high humidity and high β/γ−ray background and is capable of measuring more than 10 times the expected concentration inside the FDNPS-PCV. In this study, we mainly describe the reliable detection operation under a high-humid environment and suppression of coarse particles detection by changing suction direction and the shape of the air inlet. Specifically, the optimum value of the heating temperature was determined considering the air drying and long-life use of the device, and suction direction was determined to suppress the influence of coarse particles; the coarse particles were reduced to approx. 1/2 to 1/3 in the best case. These results made it possible for the long-life use of the IAAM and selective measurement of fine α-particles inside the FDNPS-PCV.