Anticipated transient without scram events at Salem. Another lesson in operating experience

The two anticipated transient without scram (ATWS) events at the Salem nuclear power plant have been called the most significant operating events in terms of reactor safety since the accident at Three Mile Island. Simultaneous equipment failures at Salem in February 1983 resulted in the first time a U.S. commercial nuclear power plant failed to scram automatically on a valid reactor protection signal. The plant was operating at low power levels on both occasions and was promptly scrammed manually so there was no plant damage and no direct danger to public health and safety. However, the implications of the Salem events are of wide significance in terms of reactor trip system reliability in particular and utility management controls in general. The Salem event, by itself, was a mild transient, but the potential existed for a much more serious event. Together, the two Salem events provide a number of specific lessons for improving and assuring the safety, reliability and economics of commercial nuclear power plants. The paper gives the details of the ATWS events. The reactor trip system is described with emphasis on the reasons why the trip breakers failed to open on demand. The operating history of the circuit breakers used in the reactor trip system at Salem and other PWRs is analysed and evaluated. The implications of the Salem events were assessed by the United States Nuclear Regulatory Commission (NRC) and a number of proposed actions have been identified to ensure proper operation of all reactor trip breakers in the future. Further, as the result of the Salem events, the NRC continued to study the proposed ATWS regulation with regard to additional requirements such as whether a diverse scram system for Westinghouse-designed plants is necessary