Thermal Behavior of the TN-32B Spent Nuclear Fuel Cask under Storage and Vacuum Drying Conditions
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Authors
Higley, Megan
Issue Date
2024
Type
Dissertation
Language
en_US
Keywords
Computational Fluid Dynamics , Heat Transfer , Spent Nuclear Fuel
Alternative Title
Abstract
In this work, computational models are developed to predict the temperatures in a spent fuel cask under storage and vacuum drying conditions. In particular, the TN-32 spent nuclear fuel cask was modeled because it is one of the few demonstration casks with publicly available experimental data to validate computational models for storage and vacuum drying conditions. Computational fluid dynamics models of this cask were developed to investigate common modeling techniques, assumptions, and uncertainties to identify and quantify their effects on temperature predictions. Two-dimensional and three-dimensional models of the TN-32 spent nuclear fuel cask are constructed and simulated in the ANSYS package for comparison with experimental data. The gaps between the cask components were found to have a strong influence on assembly temperatures. The size of these gaps is unknown due to the potential shifting of fuel and cask components, manufacturing tolerancing, and thermal expansion. During vacuum drying, the effect of rarefaction at the helium-solid interfaces caused significant temperature increases in the fuel region. This effect is strongly dependent on pressure, gas species, and gap sizes, which are all unknown during the vacuum drying process. This dissertation was divided into three main sections, each one is a stand-alone journal paper. In the first paper, two-dimensional detailed models of the TN-32B were used to investigate different peripheral basket/rail gap geometries and their effect on temperatures under storage and vacuum drying conditions. The basket/rail gap is explicitly modeled to have three different widths, 0 mm, 2.54 mm, and 4.78 mm. The basket and fuel assemblies are also explicitly shifted within the model to investigate two leaning geometries where the basket contacts one or two sides. Fuel region temperatures are shown to be very sensitive to gap width and pressure with vacuum drying induced rarefaction potentially causing as much as a 51°C increase in fuel region temperatures. In the second paper, full homogenous three-dimensional models were developed and used to conduct an uncertainty quantification (UQ) analysis of the collective and individual effects of nine gaps, between solid components in the TN-32B, on temperature prediction. A global UQ method that uses the Latin Hypercube Sampling technique is used to quantify uncertainty in temperature prediction due to uncertainty in gap widths at all locations of internal temperature measurement in the TN-32B from the High Burnup Demonstration cask. Results indicate that the effects of different gaps are location-dependent with the total effect ranging from ±12°C to ±21°C and that uncertainty in the basket/rail gap accounts for approximately 50% of all uncertainty in temperature predictions due to uncertainty in gap widths. In the third and final paper, a one-eighth three-dimensional model of the TN-32B is constructed with a detailed representation of the fuel assemblies. The same computational mesh was used to generate a homogenous model, with the fuel regions represented as porous heatgenerating blocks. An assembly-of-interest fuel region heat loading that allows for reasonable comparisons with experimental data is proposed and used in the simulations. Effective thermal and flow properties for homogenous model were derived from sub-models. The temperature and flow velocity predictions were compared on a point-to-point basis between the detailed and homogenous models throughout the fuel region. The temperatures at 63 locations obtained from the detailed model are compared to data from the High Burnup Demonstration project and the results showed a difference ranging from -7°C to +10°C. The homogenous model accurately predicts temperatures relative to the detailed model, with the most notable differences observed at lower elevations within the fuel regions. 95% of fuel region temperatures are within 2°C between the models.