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NRC Issues Draft Guidance for Seismic Risk Re-Evaluations

The following news article originally appeared in NEI’s Nuclear Energy Overview.

NRC staff has issued for public comment draft interim staff guidance for U.S. nuclear energy facility operators reassessing the ability of their plants to withstand earthquakes.

The guidance describes the NRC’s “acceptable method” for licensees to conduct enhanced “seismic margin assessments,” or SMAs, that would meet the requirements of a March 12 request for information the agency issued as part of its response to the Fukushima accident.

While the approach it outlines is not mandatory, the NRC said it would evaluate alternate approaches when submitted by licensees.

The NRC is requiring all operating U.S. nuclear plants to re-analyze potential earthquake risk using a two-phase process. In the hazard reassessment phase, facility operators will update characterizations of the types of seismic activity to be expected at each site, using seismic source models, seismic ground motion models and geologic data. The analyses will update potential ground motions across a range, or spectrum, of vibration frequencies, and the plants will compare the new spectra to their existing design basis.

Facilities in the central and eastern United States will use a new seismic source model that was developed by the Electric Power Research Institute, DOE and the NRC and should complete their hazard re-evaluations by September 2013. The four facilities west of the Rocky Mountains are in a different seismic region and will have to develop their own seismic source models. They are expected to complete this phase by March 2015.

The NRC endorsed NEI’s guidance for developing a ground motion response spectrum in July.

In the second phase, for facilities whose new ground motion spectrum exceeds that in the plants’ original design bases, one option to further analyze how the plant’s structures and important components would respond to the re-evaluated seismic hazard is to perform a seismic margin assessment, or SMA.

The new guidance provides the NRC’s procedures for performing an SMA. It sets several conditions on the analysis, including covering at least 72 hours after an earthquake or until the plant safely shuts down, whichever takes longer. The guidance also assumes conditions in which the plant loses off-site power and accounts for possible “soil liquefaction” around a plant from an earthquake, the effects of high-frequency earthquake vibrations on electrical systems and mechanical components, and other factors such as human error.

The draft guidance is available in the NRC’s ADAMS document control system under accession number ML12222A327. The NRC will accept comments until Oct. 10. NEI is collecting generic comments to submit to the agency on behalf of industry.

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