The NRC has issued its final interim staff guidance describing one acceptable method for conducting integrated assessments of external flooding hazards at nuclear energy facilities. The guidance is intended for use in cases where a re-evaluation of the flooding hazard exceeds levels defined by the utilities’ flooding design basis.
The new guidance, JLD-ISG-2012-05, is the latest in a large volume of work the NRC staff is developing to address lessons learned from the 2011 reactor accident in Japan. Interim staff guidance clarifies issues that are not addressed in the standard review plan for nuclear energy facilities.
Last March, the NRC issued three orders and a detailed request for information based on issues or questions raised by the accident, which occurred after a tsunami struck the Fukushima Daiichi nuclear energy facility. The orders required mitigation strategies for severe events that exceed design parameters, enhanced instrumentation for used fuel pools, and reliable hardened containment vents for some reactors. The NRC also required companies to provide detailed information on seismic and flooding hazards and emergency communications systems.
The flood-related portion of the NRC’s request calls for licensees to use the latest available information and methodologies to analyze site-specific hazards, including stream and river flooding, hurricane storm surges, tsunamis, and dam failures. This re-evaluation will determine whether the hazard exceeds the facility’s flooding design basis. If it does, the licensee must perform an integrated assessment for external flooding.
The integrated assessment must consider all modes of plant operation that could be affected by a flood, including shutdown, and take into account other events that could reasonably be expected to occur at the same time as a flood. When licensees report the results of these assessments to the NRC, they must outline the measures that have been taken or are planned to deal with the re-evaluated flooding hazard.
For licensees that must perform an integrated assessment, their report is due to the NRC within two years after submittal of their flooding re-evaluations.
The final guidance can be found on the NRC’s ADAMS document tracking database under ML12311A214.