Design of an Automatic Waterhammer Prevention System

Abstract

The Nuclear Regulatory Commission (NRC) issued Generic Letter (GL) 96-06 [1] which required utilities to evaluate the potential for waterhammers in cooling water systems serving containment following a Loss of Offsite Power (LOOP) concurrent with a Loss of Coolant Accident (LOCA) or Main Steam Line Break (MSLB). At Duke’s Oconee Nuclear Station, analysis and system testing in response to GL 96-06 concluded that waterhammers occur in the Low Pressure Service Water (LPSW) system during all LOOP events. Column Closure Waterhammers (CCWH) occur when the LPSW pumps restart following a LOOP and rapidly close vapor voids within the system, specifically, in the Reactor Building Cooling Unit (RBCU) and Reactor Coolant Pump (RCP) motor piping. Condensation Induced Waterhammers (CIWH) occur when heated steam voids interact with sub-cooled water in long horizontal piping sections, specifically in the RBCU and Reactor Building Auxiliary Coolers (RBAC) piping. These waterhammers were not expected to result in pipe failure, but resulted in piping code allowable stresses being exceeded. Piping code compliance was achieved by installing modifications that prevent all GL 96-06 related waterhammers inside containment. Two modifications were designed and implemented. These modifications were designed to isolate the piping inside containment, the high point in the open loop system, in order to maintain it in a water solid state. This was accomplished by a valve closure scheme that is actuated by low LPSW supply header pressure. Additionally, “controllable vacuum breakers” (pneumatic valves) open on low LPSW supply header pressure to eliminate void formation and collapse while the isolation valves are closing. The pneumatic isolation valve arrangement is single failure proof to open and to close. The Waterhammer Prevention System (WPS) circuitry closes the valves by one of two digital channels consisting of relays, which are triggered by two of four analog channels consisting of a pressure transmitter/current switch. The valves re-open on increasing supply header pressure. A “leakage accumulator” was provided in the supply header to make-up any boundary valve leakage that may occur when the system is isolated. This provides for a larger allowable aggregate boundary valve leakage rate. The system response was predicted by a model using the thermal-hydraulic code GOTHIC. Following installation, an integrated test was successfully conducted by inducing a LOOP into the LPSW system.