Application of a Hydrological Uncertainty Methodology to Nuclear Reactor Site Evaluations (NUREG/CR-7211)

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Publication Information

Manuscript Completed: February 2013
Date Published: August 2016

Prepared by:
P.D. Meyer and C.D. Johnson
Pacific Northwest National Laboratory
Richland, WA 99352

R.E. Cady, NRC Program Manager

NRC Job Code Number V6068

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001

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A variety of surface water and groundwater models are often used in the review of applications for new nuclear commercial power reactors. These models are used to evaluate current site hydrology and to provide estimates of future site conditions after facility construction has been completed. Predicted site conditions such as maximum flood and groundwater elevations may be compared to reactor design criteria. The current review guidance emphasizes a conservative approach to hydrologic modeling. The limitations of this approach become apparent when predicted site conditions exceed a design criterion, or when the margin between the two is small. A simple groundwater transport modeling example illustrates how uncertainty concepts can be used to inform regulatory decisions under these conditions. An uncertainty analysis methodology that can be used to estimate the joint impact of parameter, conceptual model, and scenario uncertainties is described, along with available software tools that can be used in a practical implementation of this methodology. The uncertainty methodology is illustrated using an example of reviewing groundwater site characteristics as part of a safety evaluation for a new power reactor. The example uses publicly available information from the Bellefonte Nuclear Station Units 3 and 4 Combined License application. A detailed description of model development is included to illustrate typical modeling decisions that arise in actual reviews. Modeling and uncertainty analyses were carried out using the Groundwater Modeling System (GMS) software, PEST, UCODE, and custom spreadsheets. The example illustrates that following standard procedures for uncertainty analyses can reveal limitations of the hydrologic analyses, e.g., unexamined parameter correlations and outstanding data needs, and results in more consistent reviews. Explicit consideration of alternative models is likely more valuable than quantitative model averaging. However, model averages using linear uncertainty methods can be easily completed and should be considered. Simple approaches to uncertainty evaluation can have great value, contributing to estimates of the degree of conservatism in the analysis demonstrating an understanding of the system, and providing evidence of due diligence.

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