Resolution of Generic Safety Issues: Item C-14: Storm Surge Model for Coastal Sites (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–35 )
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DESCRIPTION
Licensees are required to estimate the design-basis flood levels for each nuclear power plant site consistent with the requirements in General Design Criterion 2, "Design Bases for Protection against Natural Phenomena," of Appendix A, "General Design Criteria for Nuclear Power Plants," to 10 CFR Part 50, "Domestic Licensing of Production and Utilization Facilities," and 10 CFR 100.20(c). For coastal and estuarine sites, the design-basis flood could be caused by a storm surge that results from the wind and pressure fields of an intense storm. The primary tool used for estimating storm surge at the time when this generic safety issue was identified was the "bathystrophic" model developed by the U.S. Army Corps of Engineers (USACE), Coastal Engineering Research Center. This model, called SURGE, is based on the bathystrophic approximation, relating sea surface slope to wind stress, bottom stress, and pressure gradient, with a correction for Coriolis force on along-shore currents. The SURGE model served its intended purposes well at that time but is now considered obsolete.
The advent of more powerful computing power allows relative quick solutions to numerical the multidimensional hydrodynamic equations which can account for most conceivable physical wave effects that were not included in the SURGE model. In addition, the modern multidimensional hydrodynamic models can account for irregular shorelines and true long-wave hydrodynamics that are not accounted for by the bathystrophic model.
This task under NUREG-0471, "Generic Task Problem Descriptions (Categories B, C, and D), issued June 1978,3 called for the development of a replacement for the bathystrophic model so that the staff's evaluation of storm surge will reflect state-of-the-art techniques. Because the storm surge modeling is applied at the early site permit or combined license stages, only future license applications for nuclear power plants located at coastal or estuarine sites will be impacted by the issue.
CONCLUSION
This generic safety issue involved the development or application of a multidimensional model that uses state-of-the-art mathematical techniques in estimating hurricane storm surge. The U.S. Nuclear Regulatory Commission staff anticipated that a new multidimensional hydrodynamic model would eliminate the need for initial estimates (required by the bathystrophic model) and would reduce the total required analysis time. Thus, this item is related to increasing knowledge that would increase confidence in the safety assessment; therefore, it is considered to be a licensing issue.
The staff completed a review of this issue in 1988 and concluded that the bathystrophic model (SURGE) existing at the time was adequate for calculating design-basis water levels at proposed nuclear plant sites. The use of SURGE was specified in NUREG-0800, "Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition" (the SRP),11 Section 2.4.5, Revision 2. However, as stated in the SRP, the use of other verified models was not precluded. Thus, the staff concluded that this licensing issue did not require any changes and recommended dropping the issue from further consideration.
Because of subsequent advances in technology, the staff reexamined the issue in 2010. The staff determined that the primary tools used for estimating storm surge could be the ADCIRC (Advanced Circulation) storm surge model as developed by USACE, the Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model developed by the National Oceanic and Atmospheric Administration (NOAA), or other compatible hydrodynamic models that could simulate both storm surge wind setup and wave runup accurately. These models are based on the hydrodynamic processes of offshore and overland surge wave propagation, relating sea surface slope to wind stress, bottom stress, and pressure gradient, with a correction for Coriolis force on along-shore currents. Powerful computers are capable of producing accurate solutions to multidimensional hydrodynamic equations, which account for many meteorological and hydrodynamic effects of storm surges. True long-wave dynamics are also simulated by multidimensional dynamic mathematical models.
The staff believes that existing hydrodynamic models (e.g., ADCIRC, SLOSH) are adequate for calculating design-basis flood levels at proposed nuclear plant sites. The ADCIRC model has been widely used by the Federal Emergency Management Agency in performing storm surge frequency analyses and by USACE for the design of storm damage reduction projects, while the SLOSH model has been extensively used by the NOAA in predicting storm surge levels for hurricane emergency preparations. SRP Section 2.4.5 and Regulatory Guide 1.206, "Combined License Applications for Nuclear Power Plants (LWR Edition)," 1981 provide specific guidance in reviewing the applicant's storm surge flood estimates required for new reactor licensing. Therefore, the staff has determined that this conclusion is consistent with the state of the current regulatory framework and technology and does not change the status of this issue.
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