CFD Validation of Vertical Dry Cask Storage System (NUREG/CR-7260)

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Publication Information

Manuscript Completed: April 2019
Date Published: May 2019

Prepared by:
K. Hall¹, G. Zigh², and J. Solis²

¹Alden Research Laboratory, Inc.
Holden, MA 01541

²U.S. Nuclear Regulatory Commission

Don Algama, NRC Project Manager

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

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Abstract

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," Revision 1, issued July 2010 [6]. 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 have increasingly used thermal-hydraulic analyses using computational fluid dynamics (CFD) codes (e.g., ANSYS FLUENT) to demonstrate the adequacy of the thermal design. The applicants are also looking to license casks with decay heat close to 50 kW, resulting in peak cladding temperature (PCT) close to the ISG-11 suggested temperature of 400 C. 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. To remedy this, the NRC Office of Nuclear Material Safety and Safeguards asked the NRC Office of Nuclear Regulatory Research to be part of a validation study of the FLUENT CFD code to assist it in making regulatory decisions to ensure adequate protection for storage and transportation casks. The validation studies were based on data collected in a demonstration project at the North Anna Power Plant sponsored by Department of Energy (DOE). A TN-32B cask loaded with about 30.5 kW of high burn-up fuel was used for this validation. Extensive temperature measurements throughout the cask were performed, including temperature measurements throughout the height of several fuel assemblies and temperature measurements on the outer surface of the cask [2].

USNRC recognizes that CFD using finite volume is one of the best and most valuable method for the applicants to show compliance with the regulations concerning the dry cask storage systems (DCSS) thermal response. 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. As a participant in this CFD validation, USNRC led the effort to include uncertainty quantification and follow the CFD best practice guidelines in this validation exercise [NUREG 2152].

This report discusses validation and uncertainty quantification of a CFD model using experimental data. Uncertainty quantification follows the procedures outlined in [1]. Sources of uncertainty that were examined in the analysis include iterative uncertainty, spatial discretization, and uncertainty due to approximately twenty input parameters. Input parameters investigated include environmental conditions, material properties, decay heat, and the spacing of the many small gaps in the installation. The uncertainty in gap size was found to be the principal source of uncertainty in this particular installation. The effect of the multiple fluid gap dependence and correlation on the uncertainty quantification was evaluated and discussed. As a result, the uncertainty quantification due to the fluid gaps was determined to be dependent due to the heat path dependence, and as such, a correlated error estimation was used.

Results of a "base case" using the conservative estimates outlined in the safety analysis report (SAR) are presented, as well as a "best estimate case" that uses more realistic values. These results are compared to experimentally measured values, which fall within the uncertainty band of the analysis. The model is then used to predict the peak cladding temperature (PCT) that would be expected when the cask is placed outside in an independent spent fuel storage installation (ISFSI). The method presented herein, showed that all the measured temperature values in all the seven lances inside the fuel region as well as the outside three columns of thermocouples measurements were predicted within the calculated uncertainty bands.

Experimental data to validate a CFD simulation of a dry cask storage system for both design and safety studies, suffers from a lack of local measurements, an insufficient number of measured flow variables, a lack of well-defined initial and boundary conditions, and a lack of information on experimental uncertainty. V&V 20-2009 and the work by the working group on CFD application to nuclear safety of the OECD-NEA-CSNI-WGAMA [10] established some requirements for CFD-grade experiments able to properly validate single phase CFD models. The work in this report highlights the quality of experiment used for the validation as a concern, especially due to the many lacking inputs that are of valuable interest to the modeler. The discussion on this topic establishes the criteria to judge whether an experiment can be considered of a CFD grade. CFD grade experiments should be able to validate CFD and the main concern is to minimize the validation uncertainty on some selected figure of merit (FoM). Usually, the Peak Cladding Temperature (PCT) is used as the figure of merit for dry cask applications. Other local temperature throughout or flow variable can also be a figure of merit.

Even though this validation was worth the time and the effort, the experiment cannot be classified as a CFD grade experiment due to lack of geometry specifications which resulted in large validation uncertainty. The lack of geometrical specification includes all the gaps that were part of the design specifications but not measured for the experiment performed in North Anna Power Plant.

A validation uncertainty of 63 K (113°F) was calculated for the current experiment to validate the model's prediction of PCT. Therefore, this experiment cannot be classified as a CFD grade experiment because of the relatively large calculated validation uncertainty. Cask vendors have been submitting applications within margins close or less than 10-20 K (18-36°F) from ISG-11 specified temperature limit of 400°C (752°F). In order to be useful as a demonstration of the accuracy of the CFD modeling process or to improve the model's capabilities, the validation uncertainty should be minimized to the desired margin of 10-20 K (18-36°F) or less. The primary simulation uncertainties consisted mainly of lack of knowledge of the size of fluid gaps that existed in the dry cask. The gap sizes are an important simulation input, and greatly influence the value of PCT predicted by the model.