X-ray tomography study on the crushing strength and irradiation behaviour of dedicated tristructural isotropic nuclear fuel particles at 1000 °C

Two types of dedicated Tristructural isotropic (TRISO) nuclear fuel particles, PyC-1 (Kernel/Buffer/PyC) and PyC-2 (Kernel/Buffer/SiC/PyC) from PYCASSO (Pyrocarbon irradiation for creep and swelling/shrinkage of objects) neutron irradiation experiments, were studied. For unirradiated particles, crushing experiments using a unique hot cell, combined with in situ X-ray computed micro-tomography (XCT) imaging, were conducted at room temperature (RT) and at 1000 °C. Although the SiC layer on the particles is presumed to provide 'mechanical stability' to the TRISO particles, results showed a remarkable reduction (~45%) in the crushing strength of the PyC-2 particles at 1000 °C compared to RT. The fracture patterns of the two types of particles, both at the contact zone and on subsequent propagation, differ significantly at RT and 1000 °C. Further, irradiated particles (irradiation temperature: 1000 ± 20 °C; irradiation doses: 1.08–1.23 dpa and 1.49–1.51 dpa) were imaged by XCT; 250 PyC-1 particles and 223 PyC-2 particles were studied in total and the change in radius/layer thickness in each type was examined. It was found that the buffer densification was lower in PyC-1 particles compared to PyC-2 particles, and the PyC layer shrank in the PyC-I particles, whereas it expanded in PyC-2. Results are discussed in terms of how the residual stresses can impact the high-temperature and post-irradiation behavior of these particles.