Cables used as severe accident management equipment in nuclear power plants are required to maintain their intended electrical insulating function under severe accidents (SAs) after having aged in normal operating conditions during their service life. To examine the electrical insulation behavior of the cables under SA environmental conditions, two types of cables were first pre-aged by simultaneous thermal and radiation exposure in accordance with the procedure prescribed in “The Guide for Ageing Evaluation of Cables for Nuclear Power Plants” (JNES-RE-2013-2049, 2014), irradiated to simulate radiation exposure during SA, and then subjected to two steam exposure conditions: 155°C and 0.444 MPaG for 14 days, and 200°C and 0.620 MPaG for 7 days. In addition, electrical insulation resistance was measured for the cables during the steam exposure. As a result, all the cables passed the voltage withstand test conducted in accordance with JIS C 3005, which is prescribed as a final cable evaluation test in the JNES Guide for the design basis accident. It was found that the minimum insulation resistance per meter of the cables during the steam exposure at 155°C is around 10 8 m, confirming that the cables have sufficient insulation performance. In contrast, the resistance falls to as low as 10 5 m during the test at 200°C due to the increase in leak current.
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