Date of Award

Fall 2020

Document Type

Open Access Thesis

Department

Mechanical Engineering

First Advisor

Travis W. Knight

Abstract

The United States Department of Energy (DOE) currently manages approxi- mately 2500 metric tons of heavy metal (MTHM) of spent nuclear fuel (SNF), pri- marily from naval sources and domestic research reactors, and has agreements in place to accept 10,000 MTHM of aluminum-clad SNF from foreign reactors by 2029. However, due to the closure and the Yucca Mountain site and difficulties in the vitrification process, the majority of this fuel is expected to be placed in intermediate dry storage.

Before being placed in interim storage, all assemblies must be subjected to a drying process, either by forced gas recirculation (FGR) or by vacuum. Failure to do so can result in a variety of undesirable outcomes are possible. In spite of the well-established acceptance criteria for these processes, there is at present no reliable method of judging their efficacy.

The purpose of this work is to develop a model on the basis of computational uid dynamics (CFD) by which a reasonable and accurate estimate of any residual water content may be estimated. The resulting framework is built on the basis of the classical Navier-Stokes equations for the conservation of mass, momentum, and energy, with the addition of drying physics driven by a long-known mass ux expression known as the Hertz-Knudsen relation. To complete the drying model, an accomodation constant was derived from past characterization work performed at USC. These physics were combined with the finite volume software STAR-CCM+ and a geometry based on the aluminum-clad fuel assemblies used at Idaho National Laboratory's (INL) Advanced Test Reactor (ATR) to complete the model.

A limited number of test cases are presented illustrating the sensitivity of FGR to temperature and ow rate. An increase in inlet temperature will, unsurprisingly, lead to a higher ultimate temperature for the system and a higher peak temperature lag in the assemblies, but will not cause the system to arrive at them earlier. On the other hand, increasing either the inlet temperature or the recirculation rate (or both) will reduce the time required to fully dry the assemblies, although in relative terms increasing the inlet temperature is up to 55% more effective. In any case, results from these studies show complete drying within 6 to 8 hours at moderate to high ow rates regardless of inlet temperature.

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