Date of Award

Spring 2018

Project Type

Thesis

Program or Major

Civil Engineering

Degree Name

Master of Science

First Advisor

Ricardo A Medina

Second Advisor

Robert Henry

Third Advisor

Erin Bell

Abstract

The disposal of nuclear waste is a safety matter that requires in-depth knowledge of the behavior of spent nuclear fuel assemblies. To reliably transport spent nuclear fuels, it is necessary to understand their dynamic response, especially when material degradation is present. As contribution to this effort, joint experimental and numerical analyses with Abaqus have been conducted in this study to investigate the effect of boundary conditions on the modal frequencies of intact (un-irradiated) surrogate nuclear fuel rods. To do so, a single cell spacer, a simplified version of the actual fuel rod support (spacer grid), was modeled, designed and fabricated. Cases of surrogate copper cladding alone, which represents Zircaloy-4 cladding, and surrogate copper cladding enclosing unbonded steel pellets, which represent the uranium dioxide fuel pellets, were examined using different span configurations to simulate the effect of damaged supports due to irradiation. Also, to evaluate separately the effects of the two main components of a spacer grid support, dimple and spring, on the modal behavior of the fuel rod, the single cell spacer was used as open box in two configurations. A set of results for the three first modal frequencies for different cases and configurations has been provided as a benchmark for further studies.

This study demonstrated that an intact single-cell spacer behaves analogously to a fixed support. An evaluation of the vibration response of the fuel rod with partial intermediate single-cell spacer showed that the dimples have a greater effect than springs on modal frequencies of the fuel rod. This is mainly due to the geometric configuration of the single cell spacer. It is shown that the loss of intermediate supports (e.g., due to irradiation or temperature) has the potential to expose the fuel rod to a risk of failure by resonance, as the shift in modal frequencies (in the order of 15 to 71%) is expected to expose the rod to higher levels of stress cycles. It was also observed that the pellets increase the modal damping ratios of the rods by an amount equivalent to approximately 2% of critical for the fundamental mode of vibration.

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