In a large commercial nuclear power plant, several valve failures had occurred in the heater drain system. These failures primarily included broken yokes in the level control valve between two consecutive stages of heater drains. This paper describes an analysis to evaluate the root cause of the failures. Threshold values of the loads required to cause the reported damage were determined from a stress analysis of the valve yoke. Water hammer was suspected to be the primary cause of the failures. The affected heater drain line was modeled by using the RELAP5 computer program. Several realistic operating transient scenarios and water hammer mechanisms were simulated. Only in one of the scenarios analyzed, the calculated force was sufficient in magnitude to be the possible cause of the observed damage. Having identified the mechanism responsible for the failure, it was possible to recommend modifications to prevent/mitigate the water hammer.
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