Insulation performance of safety-related electrical penetrations for pressurized water reactors under simulated severe accident conditions

Electrical penetrations (EPs) in nuclear power plants play an important role in keeping a primary containment vessel airtight and connecting electrical cables from inside to outside of the vessel. Safety-related EPs that are categorized as “severe-accident (SA) equipment” must maintain their intended functions under various SA environmental conditions even after aging under their normal operating conditions throughout their service lifetime. To investigate the insulation performance of EPs in a SA environment, we tested two EPs that were equivalent to those used in pressurized water reactors in Japan. One EP specimen was pre-aged in a concurrent thermal and radiation environment at 100 °C and 100 Gy/h for 100 days to simulate the degradation induced during the designated service period. The other EP specimen was used without pre-aging. To simulate the degradation induced by radiation during a SA, the EPs were irradiated with gamma rays to a total dose of 500 kGy at a dose rate of 7 kGy/h. Subsequently, the EP specimens were exposed to saturated steam at 155 °C and 0.45 MPaG for 168 h. The electrical insulation resistance between the insulated cores of cables in each EP specimen was monitored during the steam exposure. The minimum insulation resistances measured during the steam exposure for the EP specimens with and without pre-aging were about 10⁹ and 10⁸ Ω, respectively, confirming that they retained sufficiently high insulation performance.