7 Assessment of Risk of Severe Accidents for All Modes of Operation at Novovoronezh-5 Nuclear Power Plant (PSAM-0356)

A full-scope Probabilistic Safety Assessment (PSA) has been completed for the Novovoronezh Unit 5 Nuclear Power Plant (NVNPP-5), the first Russian WER-1000. The analysis has been carried out within the framework of the joint Swiss-Russian PSA project (i.e., the SWISRUS). In early 2002, the SWISRUS project entered its third and final phase, involving the extension of the PSA to non-full power modes of operation (i.e., shutdown, refueling, startup, and accidents involving the spent fuel pool) and development of various regulatory guidelines for risk-informed applications.

The paper provides an overall summary of the scope, methodology, results, findings and lessons learnt from the Novovoronezh full-scope PSA study. The paper addresses the major contributors to the core/fuel damage frequency, and the insights gained with focus on the assessment of risk of severe accidents for non-full power modes of operation. Different outage types such as annual refueling and maintenance and unplanned outages are considered. The actual outage schedules and the plant records have been investigated to identify the various plant operating states and their associated duration. To enhance the completeness of the information utilized in the Shutdown Probabilistic Safety Assessment (SPSA), unplanned outages were examined. This investigation resulted in a total of 18 groups of plant operating states that were defined. The set of plant operating states modeled in the SPSA include the transition between various modes of operation.

The study has covered risks originating from damage to the reactor core, fuel handling accidents and other excore accidents such as loss of spent fuel pool cooling. In evaluating risk contributors internal initiating events, internal and external hazards were considered.

The SPSA addresses 62 internal initiating event groups, including heavy load drops. The initiating event frequencies have mainly been derived from WER-1000 operational experience with plant specific updates. Event tree sequences have been developed and quantified using the SAPHTRE computer code. The existing at-power fault trees have been modified to include shutdown operational configurations. The human actions that have been modeled were identified by reviewing the various plant procedures, other industry SPSAs, and through operator interviews and surveys performed at the Novovoronezh plant. Plant-specific thermal hydraulic analyses to estimate the time to core/fuel damage have been performed to support accident modeling and human reliability analysis.

The full-scope PSA results are being actively used by the Novovoronezh unit 5 plant in their implementation of a risk management program to support the plant hardware and operational improvements.