The sintering behaviour of Cr₂O₃-doped UO₂

Abstract

This thesis presents an investigation into the sintering behaviour of UO₂ and Cr₂O₃-doped UO_2. Throughout this work, experimental methods have been implemented to contribute towards the establishment of the under- pinning mechanism for enlarged grain growth in Cr₂O₃-doped UO₂ pellets. Factors that affect grain growth in Cr₂O₃-doped UO₂ are well documented, for example sintering atmosphere, sintering temperature and Cr₂O₃ concentration, however the mechanism by which large grain nucleation begins and proceeds is still unclear.

Sintering behaviours of UO₂ and Cr₂O₃-doped UO₂ have been studied through a variety of experiments. A freeze-frame study was conducted to assess the grain growth and densification in both doped and undoped samples at varying sintering temperatures ranging from 550 ^◦C to 1650 ^◦C; all samples were analysed by scanning electron microscopy (SEM) upon cooling to assess the extent of grain growth. Special attention was given to sintering temperatures where there was evidence of pore closure and densification that could be attributed to a grain-initiating phase change occurring.

For the first time in this work, optical dilatometry was implemented to visually observe UO₂ and Cr₂O₃-doped UO₂ pellets as they sintered, giving an observable insight into the expansion and shrinkage behaviour undergone by the samples as they were subject to high temperature. Differences between the undoped and doped samples were discussed, and connections drawn to the freeze frame study, highlighting that the grain growth process proceeds once densification/shrinkage is near to completion.

Liquid-phase sintering was investigated as a cause of enlarged grains in Cr₂O₃-doped UO₂ by determining the behaviour of the CrUO₄ secondary phase at high temperatures in a reducing environment. Molecular dynamics simulations were utilised to predict the melting temperature of CrUO₄, hence deducing the possibility of the secondary phase being involving in liquid- phase sintering, and therefore contributing to the enhanced grain growth. This same investigation was then conducted on a laboratory scale using DSC to assess the phase behaviour of CrUO₄, Cr₂O₃-doped UO₂ and UO₂ up to 1500 ^◦C. These experiments showed a phase change occurring at temperatures which had been highlighted in the freeze frame study and during optical dilatometry experiments relating to grain nucleation and growth.

This work also presents preliminary results for the impedance spectroscopy of UO₂ and Cr₂O₃-doped UO₂ pellets, which has been implemented as a method for further assessing grain growth within Cr₂O₃-doped UO₂. This contributes to the development of an overall picture of the underpinning mechanism for large grain growth in Cr₂O₃-doped UO₂ pellets.

Date of Award	2 Jun 2026
Original language	English
Awarding Institution	Bangor University
Sponsors	National Nuclear Laboratory (NNL) & Department of Energy Security and Net Zero (DESNZ)
Supervisor	Simon Middleburgh (Supervisor)