Abstract
In-pile instrumentation systems in present-day light water reactors (LWR) are indispensable to monitor all situations during reactor operation and reactor shut down. However, the systems did not work sufficiently well in situations like the severe accident (SA) at the Fukushima Dai-ichi nuclear power station. Thus, it is necessary to develop monitoring systems for the prevention of injuries in the event of an SA. Mineral-insulated (MI) cables with radiation and heat resistance are exposed to a mixture gas including nitrogen, oxygen, hydrogen, water vapor, and fission products at high temperature under SA conditions. In this study, corrosion tests under the simulated SA conditions (air, air/H2O, and I2/CO/O2/H2O) for temperatures up to 1015°C were performed for candidate sheath materials of type 316 stainless steel (316SS) and nickel-based alloy (NCF600). As a result, a uniform oxide film was formed on the surface of both the 316SS and NCF600 specimens in the air or air/H2O environments from 720°C to 1015°C, and the fracture time of the MI cable was evaluated by the degree of corrosion and the parabolic law. Conversely, when the gas mixture environment contained I2, each surface of the specimen showed complicated corrosion behavior that caused not only local corrosion but also the formation of a uniform oxide film at 800°C.
