There is an economical drive to use uranium oxide fuel with higher enrichment and burn-up in the current UK nuclear reactor fleet. Higher burn-up will result in physical changes within stainless steel fuel rods, potentially impacting their performance at the end of the fuel cycle, specifically during post-discharge handling and storage prior to final disposal in a Geological disposal facility (GDF). Higher burn-up will result in larger quantities of helium generated due to the alpha-decay of the minor actinides in the fuel, and greater in-line temperatures during storage. Within the stainless steel system, these temperatures may be sufficient for helium atom diffusion and agglomeration at neutron-induced defect clusters, dislocations and grain boundaries, possibly resulting in features such as cracking, surface exfoliation, hardening and disintegration, especially during post-discharge handling for final storage. Similarly, these agglomerations may act as hydrogen getters, which will become problematic in later life. This affect may also be applicable to zircaloy cladding to some extent.
The overarching aim of this project is to enhance our understanding of the fundamental mechanisms which influence cladding behaviour in storage, focusing on the UK system. If successful, the summation of the project will allow proposal of a model for bubble formation, bubble agglomeration and distribution within irradiated stainless steels.
Specifically, this project will: 1. Determine post-discharge microstructures within ex-service AGR material, using transmission and scanning electron microscopies (TEM and SEM), and ex-service zircaloy if available. 2. Simulate post-discharge microstructures using cold work, ion-implantation and thermal annealing to produce non-active analogue samples, 3. Determine the impact of post-discharge microstructure on mechanical and corrosion properties, to assess the impact of high burn-up fuels on long-term storage and handling.
This project includes a 3 month secondment at Sellafield, NNL, working with neutron irradiated ex-service materials. This is a rare opportunity as data collected on recently irradiated material is minimal, and there has been a significant gap in data production over the last 20 years.