United States Nuclear Regulatory Commission - Protecting People and the Environment

Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America (NUREG/CR-7046)

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Publication Information

Manuscript Completed: December 2010
Date Published: November 2011

Prepared by:
Rajiv Prasad
Lyle F. Hibler
Andre M. Coleman
Duane L. Ward

Pacific Northwest National Laboratory
P.O. Box 999
Richland, WA 99352

John D. Randall, NRC Project Manager
Thomas J. Nicholson, NRC Technical Monitor

NRC Job Code N6575

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The purpose of this document is to describe approaches and methods for estimation of the design-basis flood at nuclear power plant sites. Chapter 1 defines the design-basis flood and lists the U.S. Nuclear Regulatory Commission's (NRC) regulations that require estimation of the design-basis flood. For comparison, the design-basis flood estimation methods used by other Federal agencies are also described. A brief discussion of the recommendations of the International Atomic Energy Agency for estimation of the design-basis floods in its member States is also included.

Chapter 2 introduces the concept of hierarchical hazard assessment (HHA) and its application to estimation of the design-basis flood at nuclear power plant sites. The HHA consists of a series of progressively refined methods that increasingly use site-specific data to demonstrate whether the plant structures, systems, and components important to safety are adequately protected from the adverse effects of severe floods. The HHA method is illustrated by an example.

Chapter 3 introduces the concept of alternative conceptual models that are used to characterize the severe flooding scenarios at and near the site. The individual flood-causing hydrologic and hydrodynamic mechanisms are also described along with the potentially adverse effects they may cause at the site. A description of the HHA method as applied to several of these flooding mechanisms is provided. A brief discussion of combined events is also included.

Chapter 4 briefly describes two analytical approaches, the deterministic and the probabilistic, used in standard engineering practice for estimation of design-basis floods. The current NRC approach for estimation of design-basis floods uses the deterministic approach.

Chapter 5 describes the NRC's quality assurance criteria for simulation models and provides a description of criteria used in selection of simulation models. A discussion of uncertainty in input data and model parameters is provided. A brief discussion of validation of model-derived estimates is also included. This chapter also provides a discussion of probabilistic approaches to estimation of flood hazard assessment and outlines the components of a formal Probabilistic Flood Hazard Assessment (PFHA) approach.

Chapter 5 also includes a brief summary of the findings of the fourth assessment report on climate change prepared by the Intergovernmental Panel on Climate Change. A brief note is made regarding incorporation of the effects of climate change in estimation of the design-basis floods at nuclear power plant sites. Some future directions for further refinement of design-basis flood estimation methods are also provided.

Chapter 6 provides a few specific recommendations for further research. Two of these are worth noting. Incorporation of more recent site-specific datasets to demonstrate the validity of estimated design-basis flood and available margins would provide additional assurance regarding safety. Development of a comprehensive PFHA methodology that leverages existing techniques in areas of hydrologic and hydraulic simulation, accounting for uncertainty in model inputs and parameters, and probabilistic flood frequency analysis can provide an extremely useful tool for risk-informed designbasis flood estimation at nuclear power plant sites.

The report also contains nine appendices. One of these appendices describes currently available hydrometeorological datasets and geographical information system techniques that are useful in data preprocessing and synthesis of model inputs. Seven of the other appendices describe the flood estimation techniques for various flood-causing mechanisms. The last appendix describes some limitations of the unit hydrograph approach frequently used in estimation of design-basis floods and recommends a method for adjusting unit hydrographs to make them more appropriate for estimation of extreme floods.

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