Microstructural and Mechanical Property of TRISO-Coated Fuel Particles under Extreme Conditions

Abstract

The development of generation-IV High-Temperature Gas-Cooled reactors (HTGRs) is essential for meeting global Net-Zero energy goals, with TRISO (TRi-structural ISOtropic) coated fuel particles forming the foundation of their safety and performance. The capability of these modern TRISO particles to maintain their structural integrity and offer improved thermal fission efficiency is crucial for these advanced thermal fission reactors. It is therefore considered critical to understand the microstructural and mechanical property evolution of TRISO particles under the extremes of high temperature and neutron irradiation.
Specifically, mechanical properties in terms of elastic modulus, hardness and interfacial bonding strength have been studied by ex-situ nanoindentation and microcantilever bending. Considering that the values from open literatures were obtained using different sample preparation methods, which hampers direct cross-comparison and interpretation, a ‘standardised’ sample and surface finishing preparation procedure has been developed to ensure reliable and repeatable micromechanical properties measured by nanoindentation. As a direct consequence, spatially resolved mapping of elastic modulus and hardness across coating layers has been implemented, suggesting the existence of a radial gradient of elastic modulus and hardness changes across all coating layers. The established experimental procedure for performing nanoindentation has been further implemented for high-temperature nanoindentation measurements at 800°C, resulting in a notable decrement of hardness in the SiC layer. Other mechanical responses of IPyC and OPyC layers were also investigated. Additionally, the ex-situ microcantilever method has been performed to directly quantify the IPyC-SiC and OPyC-SiC interfacial bonding strength at room temperature, supplemented by high-resolution microstructural characterisation evidence from Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM).
Furthermore, microstructural evolution of pyrocarbon layers in two types of PYCASSO particles (namely “Buffer-1” and “PYC-1” ), which underwent neutron irradiation in High Flux Reactor (HFR) in Petten (NRG, The Netherlands) at 1000°C has been conducted, revealing mechanistic insights to the irradiation induced dimensional change mechanisms, including buffer and PyC densification, PyC layer creep, and a reversal of residual stress from tensile to compressive in the PyC layer. These insights contribute essential data for refining TRISO fuel performance models and enhancing the safety and reliability of next-generation nuclear reactors.

Date of Award	17 Mar 2026
Original language	English
Awarding Institution	University of Bristol
Supervisor	Lilly Liu (Supervisor), Martin H H Kuball (Supervisor) & Neil A Fox (Supervisor)