Severity, causes, and frequencies of pressure vessel thermal shock at US PWR plants (1963-1981)

A systematic search for events and precursors to events of pressure vessel thermal shock (PVTS) was made. The data base included some 16,000 licensee event reports of 47 U.S. PWRs covering 329 reactor years (RY) of operation between 1963 and 1981. Ninety-nine events were found, of which 34 were considered significant. The analysis of the finds were based on five categories of severities, six modes of reactor operation, eight types of initiating sequences, and eight types of causes. After correcting for factors of completeness, the frequency of events and precursors to PVTS was found to be about 1 per RY. Two-thirds of the causes originated from equipment and system inadequacies. About a quarter of the causes were due to human errors. Repressurization of the reactor coolant system was found to take place in the majority of cases considered significant. (orig.)