 INES 2 |
Plant Design for Flooding Events May Not Mitigate the Consequences of Piping System Failures |
During a past inspection, Nuclear Regulatory Commission inspectors identified, and Kewaunee later reported, that the Kewaunee plant design for internal flooding events may not mitigate the consequences of non-safety related piping system failures in the turbine building. High water level in the turbine building would result in water flowing into certain Engineered Safety Features (ESF) equipment rooms because ESF rooms are separated from the turbine building by non-water-tight doors and the plant floor drain system. The ESF equipment rooms contain the auxiliary feedwater system (AFW), emergency diesel generators, and both 480 volt and 4160 volt ESF switchgear. The water could reach levels that may result in failure of certain ESF and plant safe shutdown equipment.
Documentation which considers specific flooding events from postulated failures of plant equipment exists; however, a complete internal plant flooding analysis was not developed during or subsequent to the plant's original design. In response to the inadequate plant design, physical changes were made to minimize challenges to plant equipment and personnel in combating potential flooding events. Although an event resulting in actual flooding of any portion of the plant has not occurred, the potential for certain piping and tank failures resulting in unacceptable flooding existed. The licensee completed a past operability evaluation to assess what equipment would have failed during postulated flooding events. The NRC Significance Determination Process assessed the safety consequences and implications for any equipment that would have failed and determined that the condition was characterized as Yellow (i.e., an issue with substantial importance to safety, that will result in additional NRC inspection and potentially other NRC action).
Licensee Event Cause Summary
Design basis documentation regarding flooding, high energy line break, seismic, and tornado protection lacked detail and was difficult to retrieve, and some processes related to maintaining the design basis were weak and were inconsistent with industry standards. The plant staff lacked a thorough knowledge of the design basis and fully effective processes for determining the significance of problems and prioritizing their resolution. The plant staff evaluation and resolution of identified and potential problems lacked rigor. The probabilistic risk assessment (PRA) submitted for Kewaunee's individual plant examination (IPE) was flawed with respect to flooding risk.
Licensee Corrective Actions
The design and licensing basis for internal flooding to support current and future flooding design was compiled, and seismic qualification of selected piping and components was completed. Design modifications to protect Class 1 plant systems and components as defined in the Kewaunee Updated Safety Analysis Report were completed. This included installation of flood barriers at the doors to rooms containing ESF equipment, installation of check valves in selected floor drain lines, and installation of circuitry to trip the circulating water pumps on high water level in the turbine building basement.
Event date: Tue, 15-03-2005
