Relay chatter and its effects on nuclear plant safety

This paper describes a limited scope probabilistic risk assessment (PRA) that was conducted for the U.S. Nuclear Regulatory Commission. The PRA addressed the effect of a strong-motion earthquake on the control and instrumentation systems at a nuclear power plant. Accident sequences were generated that incorporated seismically induced failure modes of relays, pressure switches, circuit breakers, motor operated valves, instrumentation, control systems and electrical systems. Specifically, the effects of chattering of relay contacts and pressure switch on contacts during a major earthquake were studied. Special emphasis was placed on identifying circuits that could seal-in due to the effect of relay contacts chatter. In this paper, the authors emphasize qualitative insights that were obtained from the PRA