Safety assessment of Diablo Canyon risks during shutdown operations. EPRI Outage Risk Assessment and Management (ORAM) program. Final report, June 1993

For nuclear power plants, a safety assessment is an essential step toward developing a cost-effective approach to managing safety. This applies to shutdown operations as well as assuring safe operation while at power. This report is a probabilistic and deterministic assessment of the frequency of coolant boiling and core uncovery during shutdown operations of the Diablo Canyon PWRs. The results of the generic PWR shutdown risk assessment (NSAC-176L) are applied for the purposes of validating the generic report and determining results specific to Diablo Canyon. Both forced outages and scheduled outages (for refueling and maintenance) are considered. The emphasis is on loss of decay heat removal and loss of coolant while the fuel is in the reactor vessel, since NSAC-176L found those to be the dominant contributor to core uncovery and possible core damage. Deterministic analyses are performed to determine the expected system response and the time available for remedial action by the plant operators. Probabilistic assessment includes simple event trees and quantification of accident sequences. Vulnerabilities are identified and corrective actions were developed and implemented by Pacific Gas and Electric. (author)