Computational Fluid Dynamics Best Practice Guidelines for Dry Cask Applications: Final Report (NUREG-2152)

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Publication Information

Manuscript Completed: September 2012
Date Published: March 2013

Prepared by:
Ghani Zigh and Jorge Solis

Office of Nuclear Regulatory Research
Office of Nuclear Material Safety and Safeguards
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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Dry storage cask designs for spent nuclear fuel are submitted to the U.S. Nuclear Regulatory Commission (NRC) for certification under Title 10 of the Code of Federal Regulations (10 CFR) Part 72, "Licensing Requirements for the Independent Storage of Spent Nuclear Fuel, High-Level Radioactive Waste, and Reactor-Related Greater Than Class C Waste." The NRC staff technical review of these designs is performed in accordance with 10 CFR Part 72 and the "Standard Review Plan (SRP) for Spent Fuel Dry Storage Systems at a General License Facility" (NUREG-1536, 2010). To ensure that the cask and fuel material temperatures of the dry cask storage system will remain within the allowable limits or criteria for normal, off-normal, and accident conditions, a thermal review is performed as part of the application’s technical review. Recent applications increasingly have used thermal-hydraulic analyses and computational fluid dynamics (CFD) codes (e.g., FLUENT) to demonstrate the adequacy of the thermal design. Therefore, in cooperation with the Division of Spent Fuel Storage and Transportation of the Office of Nuclear Material Safety and Safeguards, the Office of Nuclear Regulatory Research developed this guide to provide practical advice for reviewing CFD methods used in vendor applications and for achieving high-quality CFD simulations of a dry cask. To assist in the analysis, the report includes procedures, analysis methods, and acceptable assumptions.

Deficiencies and inaccuracies of CFD simulations can be related to a wide variety of errors and uncertainties. An error is a recognizable deficiency that is not caused by a lack of knowledge; an uncertainty is a potential deficiency that is caused by lack of knowledge. An error is something that can be removed with proper care, effort, and resources; an uncertainty cannot be removed because it is rooted in a lack of knowledge. This report addresses two categories of uncertainties and provides specific guidelines to minimize them. The first category is modeling uncertainties. The difference between the real flow and the exact solution of the model equations creates these uncertainties. This report provides validation of the modeling approaches used to represent the heat transfer and fluid flow in a dry cask to reduce modeling uncertainties. In particular, the discussion of modeling uncertainties focuses on turbulence modeling, which could greatly influence the predicted results if not applied correctly. Commercial CFD codes have many turbulence models that are not generalized; therefore, they cannot be applied to all types of flows. Depending on the flow characteristics, some models are more applicable than others. Several approaches to model air flow turbulence have been investigated and compared to experimental data, and the results are included in this report. The report provides CFD best practice guidelines (BPGs) to minimize turbulence modeling uncertainties for dry cask analysis.

The second category of uncertainty relates to application uncertainties. These uncertainties are introduced because the application is complex and the precise data needed for simulation are not always available. In this report, the discussion of application uncertainties focuses on the inlet and outlet boundary conditions of ventilated dry storage casks. Usually, boundary conditions and assumptions used by applicants to perform CFD analyses are verified and compared with other equivalent approaches to reduce application uncertainties. As cooling air is naturally induced in ventilated dry casks (VDCs), specifying pressure at the boundaries is the preferred choice at the inlet and outlet ducts. The pressure gradient in the air flow channel affects the magnitude of the potential buoyancy forces because of the heat source (i.e., spent fuel decay heat). As such, the pressure boundary conditions are crucial to the uncertainties that could be introduced in the simulation. The effect of the pressure boundary condition was investigated and compared to experimental data to minimize application uncertainties. The report provides specific guidelines to avoid application uncertainties that could arise in the specification of the pressure boundary conditions at the inlet and outlet ducts of a VDC.

Recently, there has been a growing awareness that computational methods can prove difficult to apply reliably (i.e., with a known level of accuracy). This is, in part, because CFD is a knowledge based activity and, despite the availability of the computational software, the knowledge base embodied in the expert user is not available. This has led to a number of initiatives that have sought to structure existing knowledge in the form of best practice advice. Four notable examples are the BPGs developed by the European Research Community on Flow, Turbulence, and Combustion (ERCOFTAC); the European Thematic Network for Quality and Trust in the Industrial Application of CFD (QNET-CFD); the Organisation for Economic Co-operation and Development /Nuclear Energy Agency/Committee on the Safety of Nuclear Installations CFD working groups; and the American Society of Mechanical Engineers (ASME), "Standard for Verification and Validation in Computational Fluid Dynamics and Heat Transfer" (ASME, 2009). The QNET-CFD knowledge base is currently under the control of ERCOFTAC. The guidelines presented here build on the work of these four initiatives, particularly the ERCOFTAC BPGs that have been used as a template for these guidelines (with some modifications and adaptation).

This document considers the use of CFD programs solving the Reynolds-Averaged Navier-Stokes equations on both structured and unstructured meshes, as well asthe use of large eddy simulation and detached eddy simulation. The report attempts to cover the full range of issues associated with a high-quality CFD analysis. It begins with a proper definition of the problem to be solved, thus permitting selection of an appropriate simulation tool. For the probable range of tools, the report provides generic guidance on selecting physical models and on numerical issues, including the creation of an appropriate spatial grid. To complete the analysis, the report also provides guidance on how to verify the input model, validate results, and document the process.

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