Analysis of the Presence of Vapor in Residual Heat Removal System in Modes 3/4 Loss-of-Coolant Accident Conditions using RELAP5

The Westinghouse Nuclear Safety Advisory Letter NSAL-09-8 investigated the possibility of presence of vapor in Residual Heat Removal (RHR) System in Modes 3/4 Loss-of-Coolant Accident (LOCA) Conditions. This concerns the Westinghouse standard 3-loops plant for which the RHR is the low pressure part of the Safety Injection (SI). In some cases one or both RHR trains may become inoperable for Safety Injection (SI) function. As a response to this letter, Westinghouse Electric Belgium is providing RELAP5 analyzes for Westinghouse Nuclear Steam Supply System (NSSS) European plants to assess the thermal hydraulic behavior of the RHR suction piping system for Emergency Core Cooling System (ECCS) initiation events postulated to occur during startup/shutdown operations. Several concerns including condensation induced water hammer and voiding at the RHR pump have been investigated. As a conclusion, the analysis allowed to define the bounding Hot Leg temperature conditions under which both RHR Trains remain safely operable. These bounding conditions are then implemented by the customer in their Operating Procedures (OPs) to achieve safe operations and successful accident management.