Insulation Performance of Safety-related Cables under Simulated Severe Accident Conditions

Cables used in nuclear power plants have functions of supplying electric power or transmitting signals for monitoring or controlling equipment. Safety-related cables are required to maintain their intended function under all assumed environmental conditions including various design basis accidents, such as a loss-of-coolant accident, throughout their service life [1].

Figure 2 Cables set in test chamber before steam exposureThe regulatory requirements in Japan, which went into force in 2013, require measures for prevention of severe accident (SA) and mitigation of consequence from SA. When a SA occurs at a nuclear power plant, it is essential to monitor important parameters of conditions inside the primary containment vessel necessary. Certain safety-related cables in a category called “SA equipment” are used for this purpose. These cables are required to maintain their intended function under SA environmental conditions after having been aged in normal operating conditions in their service life [2].

The Regulatory Standard and Research Department, Secretariat of Nuclear Regulation Authority (S/NRA/R), conducted research on cable aging from 2015 to 2017, in which a series of tests were conducted to examine the electrical insulation behavior of the cables under accident conditions. Ethylene propylene diene rubber insulated cables and silicone rubber insulated cables were pre-aged by simultaneous thermal and radiation exposure, then exposed to high dose radiation and high temperature conditions to simulate an SA. Electrical insulation performance was monitored during and after the steam exposure. In addition, the voltage withstand test was conducted after the steam exposure as a final functional test.

As a result, all of the cables tested in this study passed the voltage withstand test. It has also been found that there is a case where the cable electrical insulation resistance decreases during steam exposure even if the cable passes the voltage withstand test [3, 4]. These observations indicate that it is important to measure the electrical insulation resistance of cables during steam exposure in a type test for evaluating cable integrity in the assumed SA conditions. Cables tested in this study will be analyzed in the future to clarify the mechanism of the decrease in electrical insulation resistance of the cables during the steam exposure.

 

[1] Nuclear Regulation Authority, Japan, “NRA ordinance prescribing standards for the location, structure, and equipment of commercial power reactors and their auxiliary facilities, Clause 3 of Article 12,” Nuclear Regulation Authority Ordinance No. 5 of 2013, 2013 (in Japanese).

 

[2] Nuclear Regulation Authority, Japan, “NRA ordinance prescribing standards for the location, structure, and equipment of commercial power reactors and their auxiliary facilities, Article 43” Nuclear Regulation Authority Ordinance No. 5 of 2013, 2013 (in Japanese).

[3] T. Minakawa, M. Ikeda, N. Hirai, and Y. Ohki, “Insulation performance of safety-related cables for nuclear power plants under simulated severe accident conditions,” Conf. Proc. Internat. Symp. Electr. Insul. Mater. 2017, pp.716-719, 2017.

[4] T. Minakawa, M. Ikeda, N. Hirai, and Y. Ohki, “Insulation performance of safety-related cables for boiling water reactors during and after high temperature steam exposure,” Technical papers of 15th annual meeting of Japan Society of Maintenology, Fukuoka, 2018 (in Japanese).

 

Contact
Mr. Takefumi Minakawa
Regulatory Standard and Research Department,
Secretariat of Nuclear Regulation Authority, Japan
takefumi_minakawa@nsr.go.jp