Abstract
The aim of this study is to verify if the reliability of a digital four-channel RPS under the design phase satisfies the specified target and to identify the weakness of system design and potential solutions for system reliability improvement. The event-tree/fault-tree (ET/FT), which is the method used in the current probabilistic safety assessment (PSA) framework of nuclear power plants (NPPs), was adopted to developed reliability modeling for the RPS with the Top Events defined as the system failure to generate reactor trip signal and the system generating spurious trip signal. The evaluation results indicate that the probability of the system failure on demand and the frequency of spurious trip signal generation are 1.47 × 10−6 with a 95% upper bound of 4.63 × 10−6 and 7.94 × 10−4/year with a 95% upper bound of 2.50 × 10−3/year, respectively. The importance and sensitivity analyses were conducted and it was found that undetected unsafe common cause failures (CCFs) of signal conditioning modules (SCMs) dominate the system reliability. Two preliminary optimization schemes relative to reducing periodic test interval and adapting two kinds of diverse SCMs were proposed. Results of the quantitive evaluation of the schemes show that neither of them could determinedly improve the system reliability to the target level. In the future, more detailed optimization analysis shall be required to determine a feasible system design optimization scheme.
Highlights
E calculation results for three types of signals are shown in Tables 3 and 4. e results indicate that when the input scram parameters are thermocouple signals the probability of the reactor protection system (RPS) failing to generate a trip signal on demand is 1.47 × 10− 6 with a 95% upper bound of 4.63 × 10− 6 in case of considering common cause failures (CCFs), which is larger than the other two types of signals
Taking CCFs into account, the system reliability does not fulfill the specified reliability goal with regard to the probability of failure on demand of the system function. e results make it clear that CCFs of modules are the main contributors of the system failure; this is consistent with the consensus that the safety-critical protection system with redundancy multiple-channel is remarkably affected by CCFs [4, 11]
A safety-related digital four-channel RPS within design phase was assessed by ET/FT method to verify if the system reliability meets specified requirements regarding the function to generate reactor trip signal and to obtain important risk information for design feedback
Summary
(iii) LCS signals used for voting to generate trip signals are transmitted among channels of the RPS through the data communication network Such effect of its failure on the reliability of the RPS should not be ignored, which may result from failure of hardware or software of communication module or faults in transmission medium of communication cable and would lead to loss of communication of LCS signals. E reliability models used for basic events in the quantitative analysis include a repairable component for detected failure and a periodically tested component for undetected failure. E results indicate that when the input scram parameters are thermocouple signals the probability of the RPS failing to generate a trip signal on demand is 1.47 × 10− 6 with a 95% upper bound of 4.63 × 10− 6 in case of considering CCFs, which is larger than the other two types of signals. Taking CCFs into account, the system reliability does not fulfill the specified reliability goal (see section 2) with regard to the probability of failure on demand of the system function. e results make it clear that CCFs of modules are the main contributors of the system failure; this is consistent with the consensus that the safety-critical protection system with redundancy multiple-channel is remarkably affected by CCFs [4, 11]
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