Multi-Scale Characterization of Residual Stresses in Spent AGR Fuel Cladding

Abstract

Residual stresses inside the cladding of spent nuclear fuel pins are suspected to have a significant influence on their structural integrity. Currently, any potential investigation of these suspicions is hindered by the lack of a viable measurement method, resulting from the fuel pins’ high radioactivity and complex geometry. In this thesis, potential measurement techniques were identified, and their feasibility for adaptation and application on spent nuclear fuel pins was investigated. Moreover, since residual stresses are known to act on multiple
length scales, for instance the macro-scale of components, but also the micro-scale of single crystalline grains, both these length scales were considered. Investigating the macroscale was relevant from an industrial point of view, as at this length scale, mechanical stresses are known to have a decisive influence on structural integrity. Failure modes on the micro-scale have been
hypothesized as well, and have been the subject of a wide range of research activities. Therefore, there was also a significant academic interest in investigating micro-scale measurement techniques. For the macro-scale, the Incremental Centre Hole Drilling (ICHD) technique was
selected. Firstly, its adaptation for application on spent nuclear fuel pins involved finite element modelling to characterize the cladding’s deformation response when subjected to ICHD experiments. Secondly, lab-based trials were performed on non-radioactive samples to validate the technique. For these trials, it was necessary to replace the classic contact measurement
using strain gauges with an optical method. Afterwards, a prototype for performing ICHD on radioactive samples was designed, built, and tested in a relevant environment with the aim to further demonstrate the feasibility of the technique outside the lab as well. Finally, targetting the micro-scale, a Focused Ion Beam (FIB) was used, in combination with a Scanning Electron
Microscope (SEM), to map grain-level stresses as they exist within the microstructure of the cladding’s material.

Date of Award	7 May 2024
Original language	English
Awarding Institution	University of Bristol
Sponsors	Nuclear Decommissioning Authority & National Nuclear Laboratory
Supervisor	Harry Coules (Supervisor), Christopher E Truman (Supervisor) & Ronald Clark (Supervisor)