During the course of efforts in reexamining the technical specifications for nuclear power plants, a particular attention has been paid to the "additional test" requirement on the redundant components in a standby safety system. When a failure of one part is detected by the scheduled test, the following three test/repair policies are considered on the other redundant parts during the allowed outage time of the first failed component :
Policy 1 : An additional test is carried out promptly.
Policy 2 : No additional test on the other parts is performed. The test is carried out according to the periodic test scheme as originally scheduled.
Policy 3 : An additional test will be performed after the first failed part is repaired.
For the analysis of the three test/repair policies, a computer program MARADD (Markov Reliability Analysis for the Additional Test Requirement) was developed in the present study, based on semi-Markov reliability analysis. The allowed outage time (AOT) is usually larger than the calculational time step required by acceptable accuracy. A component can change its state from repair to operable state during the AOT according to the assumed Poisson repair process. However, some system states make transitions to the plant shutdown state on exceeding the AOT that is a deterministic parameter. Thus, the three test/repair policies have semi-Markovian characteristics.
The methodology was applied to the diesel generator system on which Policy 1 is currently performed. The following results are obtained in the present study.
Policy 1 may induce higher risks than Policy 2 or 3 in a nuclear power plant with relatively large shutdown risks and with a diesel generator system that has poor test overriding capability and high failure rate. Furthermore, the plant unavailability (i.e., plant downtime) in Policy 1 is much greater than that in Policy 2 or 3 since Policy 1 invokes too many inadvertent reactor trips due to the requirement of the plant shutdown in case that at least one of the two failed diesel generators cannot be repaired within the specified allowed outage time.
Since Policy 2 and Policy 3 put only one diesel generator out-of-service, the two policies show mixed results. Although Policy 2 gives slightly higher core damage frequency but lower plant unavailability than Policy 3, the differences are very small. However, the unquantified impacts such as manpower requirement and equipment wear would be higher in Policy 3 than in Policy 2 due to the additional tests.
Another notable result is that by extending the allowed outage time of the diesel generators, we can obtain significant benefits in terms of the plant availability at negligible expense of the system unavailability and the core damage frequency, when the shutdown risk initiated by the reactor trip is small. However, both the plant unavailability and the core damage frequency are reduced by extending the allowed outage time in a nuclear power plant with a large shutdown risk.
As the failure rate of the diesel generators increases, the surveillance test interval (STI) should be shortened in all policies to reduce the system unavailability and the core damage frequency. However, the reduced STI leads to the increased plant unavailability (i.e., plant down time).
정기적인 시험시 한 부품의 고장이 판별되었을 때 고장난 부품의 허용시험시간 (Allowed Outage Time, AOT) 동안 대기안전계통의 다른 부품에 대해 다음과 같은 세가지 시험보수 정책들이 본 연구에서 고려되었다. (1) 정책 1 : 추가시험이 즉각적으로 행해진다, (2) 정책 2 : 추가시험이 없이 시험은 원래의 정기적인 계획을 따른다, 그리고 (3) 정책 3 : 추가시험은 고장난 부품의 수리가 끝난 후에 실시된다.
위의 세 가지 시험보수 정책의 분석을 위해, 본 연구에서는 세마이-마코프 신뢰도 분석을 사용하여 컴퓨터 프로그램 MARADD (Markov Reliability Analysis for the Additional Test Requirement)를 개발하였다. 그리고 실제로 정책 1 이 스태거드 시험 (Staggered Test) 주기를 따라 수행되고 있는 두 대의 디젤발전기를 가진 비상디젤발전계통에 대해 세 가지 정책에 관하여 신뢰도분석을 수행하였다.
원자로정지당 조건부 노심손상 확률이 커지면 정책 1 이 특히 불리하고 대부분의 조건변화에 있어 정책 1 이 다른 정책들 보다 큰 발전소 이용 불능도를 초래하였다. 정책 2 와 3 는 비슷한 위험도를 보여주었다. 그러나 인력과 기기마모에 대해 분석이 수행되지는 않았지만 이 두 가지 원인을 고려할 때 정책 3 가 더 큰 위험도를 초래함을 예측할 수 있었다. 모든 정책에 있어 디젤발전기의 고장률이 커질수록 정기검사주기 (Surveillance Test Interval, STI)를 줄이는 것이 유리함을 알았다. 또한 허용시험시간을 늘리는 것이 계통이용불능도와 노심손상에 큰 변화 없이 또는 노심손상을 감소시킴과 동시에 모든 경우에 있어 발전소 이용도 측면에서 크게 유리하였다.
