Probabilistic safety assessment of unavailability of the manual shutdown system of Ling'ao phase II nuclear power plant

Distributed Control System (DCS) is implemented in Reactor Protection System (RPS) of Ling'ao Phase II. But manual reactor trip system, which is part of the RPS, is a conventional electric control system. Fault tree method is used in this article to evaluate failure rate of manual reactor trip system after the automatic means fail. Human reliability analysis is based on the Technique for Human Error Rate Prediction (THERP) methodology. For common cause failure analysis, Multiple Greek Letter (MGL) method is implemented. And general data of American NPP is used as data source. It shows that, the failure rate per demand of manual reactor trip system is 5.27 x 10-2. And operator failure to respond to the alarms, mis-reading neutron flux, omitting step to verify neutron flux increasing, selecting wrong control to scram the reactor, all these human errors contribute a lot to system failure. So operators should be well trained, thus can minimize the failure rate of manual reactor trip system. (authors)