Development of a Shutdown PSA Model for the TQNPC CANDU Reactor

Abstract

This paper presents the steps to update the existing Shutdown Probabilistic Safety Analysis (SPSA) for the CANDU™ reactors on the Third Qinshan Nuclear Power Company (TQNPC) site. The existing SPSA was developed as a design PSA in 2003. Subsequent PSA updates did not include an update of the SPSA models. A complete update of the SPSA was necessary to reflect current plant operation and PSA analysis, and to assess risk during outage for various plant configurations. The update of the SPSA included the initiating event identification and frequency derivation, event tree analysis, fault tree and failure database updates, and accident sequence quantification (ASQ). The updated models were developed for consistency with current plant design, operating practices (Emergency Operating Procedures—EOP) and plant-specific preventive maintenance activities during outages. Design changes since plant commissioning included the addition of a second auxiliary feedwater pump and the installation of a Backup Service Water system, used during outages when the service water system is under maintenance. The updated SPSA models were used to develop a single fault tree model to evaluate the Core Damage Frequency (CDF) for various plant configurations during outages. The SPSA results include the identification of contributing initiating events, failure types, and safety-related systems to CDF. Insights from the SPSA included EOP improvements and identification of higher risk plant states during outages. The development of the TQNPP SPSA provides CNNP with a modern tool to evaluate the annual shutdown risk and to assess risk for specific outage configurations.