Optimal replacement and inspection periods of safety and control boards in Wolsung nuclear power plant unit 1

In nuclear power plants, the safety and control systems are important for operating and maintaining safety of nuclear power plants. Due to the failure of the instrument and control devices of nuclear power plants caused by aging, nuclear power plants occasionally trip. Since the start of first commercial operation of Kori nuclear power plant (NPP) unit 1, the trips caused by instrument and control systems account for 28% of total trips of NPPs in Korea. Even a single trip of a nuclear power plant causes an extravagant economical loss and deteriorates public acceptance of nuclear power plants. Therefore, the replacement of the instrument and control devices with proper consideration of the aging effect is necessary in order to prevent the inadvertent trip. In this work we investigated the optimal replacement periods of the digital control computer's (DCC) and the programmable digital comparator's (PDC) electronic circuit boards of Wolsung nuclear power plant Unit 1. We first derived mathematical models which calculate optimal replacement periods for electronic circuit boards of digital control computer (DCC) and for those of the programmable digital comparator (PDC) in Wolsung NPP unit 1. And we analytically obtained the optimal replacement periods of electronic circuit boards by using these models. We compared these periods with the replacement periods currently used at Wolsung NPP Unit. The periods used at Wolsung is not based on mathematical analysis, but on empirical knowledge. As a consequence, the optimal replacement periods analytically obtained for the electronic circuit boards of DCC and those used in the field shown small difference : the optimal replacement periods analytically obtained for the electronic circuit boards of PDC are shorter than those used in the field in general. The engineered safeguards of Wolsung nuclear power plant unit 1 contains redundant systems of 2-out-of-3 logic which are not operating under normal conditions but they are called upon to act when emergency conditions develop. To ensure their operability, the systems are periodically tested. In this paper, unavailability formulae for 2-out-of-3 logic configurations are developed, taking into account the probability of failure of channels tested due to human error in the simultaneous testing scheme. The probability of the channel being down due to human error is assumed to depend on the number of channel which have gone through the tests consecutively prior to the inspection of the channel under consideration. The system unavailability is the sum of unavailability due to hardware failure alone and the unavailability contribution due to human error. And we considered the probability that the reactor will be tripped during surveillance test. We determined the optimal inspection periods of safety system, taking into account both unavailability of safety system and the probability that the reactor will be tripped during surveillance test. We compared these periods with the inspection periods currently used at Wolsung NPP Unit 1. As a consequence, the inspection periods obtained using a minimum human error (8.24 x 10-6) are shorter than those currently used in Wolsung NPP unit 1 while inspection periods obtained using a maximum human error is (4.44 x10-4) longer than those used in Wolsung NPP unit 1