NRC seeks comment on proposed guidance for examining plant response to updated flooding hazards

The Nuclear Regulatory Commission is seeking public comment on proposed Interim Staff Guidance to U.S. nuclear power plants for evaluating how re-analyzed flooding hazards could affect plant performance.

The need to re-analyze the hazard is one of the lessons learned from the Fukushima Dai-ichi nuclear accident. The new analysis will take advantage of recent advances in understanding flooding hazards.

The proposed guidance would provide a means for meeting the requirements in a request for information the staff issued on March 12. The guidance would not be mandatory, but should a plant decide to take a different approach, the NRC would review both the plant’s methodology and results when they submit their response.

The NRC will accept comments on the proposed guidance until Oct. 29; comments can be submitted via regulations.gov using Docket ID NRC–2012–0222.

The NRC began examining flooding issues, in the form of upstream dam failures, prior to the Fukushima Dai-ichi accident. That work was incorporated into the agency’s post-Fukushima efforts, which include requiring all U.S. plants to re-analyze potential flooding hazards at their sites using the latest available information. The plants will use present day guidance and analysis methods that have been used in new reactor applications to analyze hazards including stream and river flooding, hurricane storm surges, tsunami, and dam failures. In May, the NRC announced a schedule for all U.S. nuclear power plants to complete the hazard re-analysis by March of 2015.

If the re-analyzed flood hazards exceed the levels a plant was originally designed for, the plant will tell the NRC what interim measures it will use to safely deal with the new hazard. The plant will also perform an “integrated assessment” to identify specific vulnerabilities and examine how existing or planned systems or procedures will prevent or mitigate flood damage.

The staff’s draft guidance lays out several assumptions for the integrated assessment, such as taking into account available onsite resources and systems for responding to flooding. The assessment must also consider any mode of operation (at full power, for example, or during a refueling outage) that could be affected by a flood, as well as simultaneous events such as losing power from the electric grid. The assessment cannot exclude a flooding event based can solely on how rare that flood might be.