[en] Potential safety concerns associated with water-hammer occurrences in operating nuclear power plants are primarily related to the frequency and severity of the water-hammer occurrences, the level of damage incurred due to hydraulic loads, and the safety function of the system involved. These safety concerns were evaluated by the US Nuclear Regulatory Commission (NRC) in the course of resolving Unresolved Safety Issue (USI) A-1, Water Hammer. Since the resolution of USI A-1, an additional 40 water-hammer events have been reported (from 1981 through 1985). This study was initiated by the Office for Analysis and Evaluation of Operational Data to evaluate these recently reported water-hammer events to assess the underlying causes and adequacy of the corrective actions taken to prevent recurrence. The data compiled in this study show that 18 of the reported water-hammer events occurred at boiling water reactors (BWRs) and 22 of the events occurred at pressurized water reactors (PWRs). The results indicate that the BWR systems most susceptible to water hammers continue to be the residual heat removal (RHR) and high-pressure coolant injection systems. Flow into voided (or partially voided) lines and steam-water entrainment continue to be the most frequently cited underlying causes for BWR water-hammer events. The results show that the PWR systems most susceptible to water hammer are the feedwater system and the steam generators