DEVELOPMENT OF DOSE RATE MEASUREMENT TECHNIQUE USING A COMPACT HIGH-SENSITIVITY SELF-POWERED GAMMA-RAY DETECTOR FOR INVESTIGATIONS INSIDE THE PRIMARY CONTAINMENT VESSEL

抄録

For decommissioning of the Fukushima Daiichi Nuclear Power Station, it is important to clarify the dose rate distribution inside and outside the containment vessel and the point cloud information of the structures. We developed a self-powered gamma-ray detector (SPGD) that can measure dose rates under high dose conditions. We devised a detector structure that was both compact enough to be mounted on a remotely operated survey vehicle (survey ROV) and sensitive enough to measure dose rates in low-dose-rate areas. To achieve high sensitivity, a multilayer structure was devised to increase the surface area of the emitter, insulator, and collector layers. An internal collector was provided to collect the electrons emitted by the gamma-rays, and the thickness of each layer was designed based on the electron range. We fabricated a prototype SPGD, 35 mm in diameter, 100 mm long and 0.36 kg in weight, as a compact, high-sensitivity detector. To evaluate the stand-alone performance of the detector, we conducted an irradiation test using Co-60. The measurement lower limit dose rate value was 0.06 Gy/h. The detector was mounted on the survey ROV in a mock-up facility simulating the structures around the pedestal, and noise evaluation tests during various operations were conducted to identify the noise source and implement countermeasures. As a result, we obtained the prospect that measurement in the assumed dose rate range was possible.