Validation of a Computational Fluid Dynamics Method Using Vertical Dry Cask Simulator Data (NUREG-2238)
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Manuscript Completed: March 2020
Date Published: June 2020
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
Applicants submit spent nuclear fuel dry storage cask designs 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 performs its technical review of these designs in accordance with 10 CFR Part 72 and NUREG–1536, "Standard Review Plan for Spent Fuel Dry Storage Systems at a General License Facility–Final Report," Revision 1, issued July 2010. To ensure that the cask and fuel material temperatures of the dry cask storage system remain within the allowable limits or criteria for normal, off-normal, and accident conditions, the NRC staff performs a thermal review as part of the technical review.
Recent applications increasingly have conducted thermal-hydraulic analyses using computational fluid dynamics (CFD) codes (e.g., ANSYS Fluent) to demonstrate the adequacy of the thermal design. The applicants also want to license casks with decay heat close to 50 kilowatts, resulting in a peak cladding temperature (PCT), close (i.e. small margins) to the temperature limit of 400 degrees Celsius suggested in Interim Staff Guidance 11, "Cladding Considerations for the Transportation and Storage of Spent Fuel," issued November 2003. These PCT predictions presented by the applicants usually are not supported by an uncertainty quantification calculation to assure the thermal reviewer that the calculated temperature margin is adequate. As such, the NRC Office of Nuclear Material Safety and Safeguards asked the NRC Office of Nuclear Regulatory Research to perform validation studies of the ANSYS Fluent CFD code to assist it in making regulatory decisions to provide reasonable assurance of adequate protection for storage casks and transportation packages. The validation studies were based on experimental data documented in NUREG/CR–7250, "Thermal-Hydraulic Experiments Using a Dry Cask Simulator," issued October 2018 .
NUREG/CR–7250 documents a series of tests conducted using a single, prototypic-geometry boiling-water reactor fuel assembly inside a pressure vessel and enclosure to mimic the thermal-hydraulic responses of both aboveground and underground dry storage casks. This simplified test assembly was shown to be similar to prototypic systems through dimensional analysis. The data were collected over a broad range of parameters, including simulated decay power and internal helium pressure.
Previous submissions by applicants and vendors to the NRC have generally employed CFD using finite volume to demonstrate regulatory compliance for the thermal performance of dry cask storage systems. Additionally, when demonstrating compliance, it is valuable to quantify the uncertainty in the simulation result as a function of the computational mesh and simulation inputs. This CFD validation included uncertainty quantification, using American Society of Mechanical Engineers (ASME) Verification and Validation (V&V) 20–2009, "Standard for Verification and Validation in Computational Fluid Dynamics and Heat Transfer" . Additionally, the validation used CFD best practice guidelines  to create the CFD model.
This report discusses validation and uncertainty quantification of a CFD model using the experimental data from NUREG/CR–7250. Air mass flow rate, and PCT were used as the primary variables of interest (i.e., target variables) in this validation. Uncertainty quantification follows the procedures outlined in ASME V&V 20–2009. Sources of uncertainty examined in the analysis include simulation input uncertainty, numerical errors (i.e., iterative, discretization, and round-off), and experimental errors.
The CFD results and experimental data for PCT and air mass flow rate agreed very favorably for the collected cases within the calculated validation uncertainty, which includes the combination of simulation and experimental uncertainty. The simulation uncertainty consists of model input uncertainty and numerical errors. The results show that an ANSYS Fluent thermal model using NUREG–2152  CFD best practice guidelines can demonstrate the safety of the storage of spent nuclear fuel. This report also looks at the quality of the data collected in the DCS experiment documented NUREG/CR–7250  using the calculated validation uncertainty. The low values of the validation uncertainty indicate that the experiment undertaken in this program is considered a CFD-grade experiment. The DCS experiment was designed to minimize the validation uncertainty—a key factor and the basis for thermal model validation. Consequently, a well-validated thermal model will enable thermal reviewers to have confidence in the predictions, even with decreased margins. This document shows that a best estimate analysis is a method that includes a model with design basis, implemented with uncertainty quantification.
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