Interim reliability evaluation program. Analysis of the Calvert Cliffs Unit 1 Nuclear Power Plant. Volume 1, main report

The analysis used fault tree and event tree models as the primary tools to evaluate the risk due to a core melt at Calvert Cliffs. Core melt sequences initiated by one of three break-size LOCAs or one of six categories of transients were evaluated, and the dominant (i.e., highest frequency) sequences were further analyzed to estimate the magnitude and frequency of radionuclide release. The accident sequences were then placed into the release categories defined in the Reactor Safety Study. The most significant sequences contributing to the core melt frequency are (1) Anticipated Transients Without Scram (ATWS) (33% of the total core melt frequency), (2) Small-small LOCAs 1.9 to 3 in. in diameter) with makeup system failure in the recirculation phase (20% of the total core melt frequency), (3) the loss of a DC bus followed by failure of secondary heat removal (16% of the total core melt frequency), and (4) Loss of Offsite Power following by failure of secondary heat removal or a stuck open relief valve (12% of the total core melt frequency. Insights were developed concerning the importance of plant design features. The analysis led to the identification of key components/events, that contribute most to the core melt frequency. Similar insights were developed concerning plant operations. Operator errors made during the course of accidents were found to be significant. Some changes to plant procedures could be made in order to take greater advantage of possible operator actions. Operator recovery actions were very important in preventing or mitigating accidents. Test and maintenance contributions were, in general, small; however, maintenance of several key components was found to be a significant contributor to particular sequences. The estimated core melt frequency is similar to the values predicted by probabilistic risk assessments of other pressurized water reactors