A July 2011 report by Nuclear Regulatory Commission (NRC) staff states that its analysis ‘suggests that external flooding due to upstream dam failure poses a larger than expected risk to plants and public safety’ and that the probability and consequence of those events require NRC attention. The report gives details of 34 reactors, one-third of the US nuclear fleet, which are located across 15 states. In the event of dam failure upstream the reactors may face flooding hazards greater than their design specifications can handle. NRC has known about some of these problems for more than ten years and has not effectively addressed them, the scientists claim.

A redacted version of the NRC report was posted to its website in March 2012. However an unredacted version was recently obtained by Greenpeace and shared with The Huffington Post in the US. It is claimed that failure of Jocassee dam upriver from the Oconee nuclear station in South Carolina would cause floodwaters to overwhelm the plant’s three reactors and their cooling equipment – not unlike what happened in Japan at Fukushima after an earthquake and tsunami struck last year.

The potential higher-than-assumed flooding risk from upstream dam failures was recognised as updated flooding analyses of Oconee and Fort Calhoun nuclear stations in Nebraska went beyond studies typically done for reactors. Based on those studies, the July 2011 NRC staff report concludes the NRC should systematically investigate other sites using more detailed analysis and more accurate data than used in previous studies.

In particular, the report found that past analyses of flood levels at Oconee and Fort Calhoun were ‘based on relatively outdated flood estimation methods and/or probable precipitation estimates’. It also stated that new flood estimates should include more accurate and up-to-date analyses of dam failure. Changes in land use since the plants were built should be taken into consideration as these can have a significant impact on local watersheds and flooding. Such factors are likely to produce flooding estimates that are different than previous studies’ estimates, including those used for reactor licensing and safety regulations. In the case of Oconee, the NRC report notes that the licensing process did not consider the possible consequence of a failure of the nearby Jocassee dam. The Union of Concerned Scientists claims that this could be true for other plants as well.

NRC says it began examining flooding issues, in the form of upstream dam failures, prior to the Fukushima accident. That work was incorporated into the agency’s post-Fukushima efforts which include requiring all US plants to re-analyse potential flooding hazards at their sites using the latest available information. NRC stated that it was prompted to start a formal evaluation of potential generic safety implications for upstream dam failures after inspection findings at two plants.

‘While this screening did not identify any immediate safety concerns,’ NRC said in a press release, ‘inspections or other reviews at individual plants have led to those plants taking appropriate actions regarding flooding scenarios. Based on the screening, the NRC staff has recommended that flooding from upstream dam failure be further evaluated as part of implementing recommendations from the agency’s Japan Near-Term Task Force.’

The plants will use present day guidance and analysis methods that have been used in new reactor applications to analyse hazards including stream and river flooding, hurricane storm surges, tsunami, and dam failures. A schedule has also been announced for all US nuclear power plants to complete the hazard re-analysis by March 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.