Role of Built-in Residual Stress and Interfacial Fracture Toughness on the Irradiation Behaviour of Fuel Particles at 1000°C

The fuel particles fabricated by CEA consist of surrogate kernels (Al₂O₃) with different types of SiC/PyC combinations were irradiated during the PYCASSO programme performed in the High Flux Reactor (HFR) Petten (coordinated by NRG). These particles were irradiated at a well-defined temperature (980-1020°C) at two different doses. In this work, the change in radius/thickness of the layers in two types of particles - Kernel/buffer/OPyC (Type I) and Kernel/buffer/SiC/OPyC (Type II) - are studied using non-destructive lab-based X-ray computed micro-tomography with a pixel size 1.99 x 1.99 μm at NRG (The Netherlands). For Type I particles, the swelling of the kernel is negligible while a densification of the buffer layer was observed. As a consequence the OPyC radius was reduced accordingly with the buffer layer shrinkage. For Type II particles, a similar reduction in the buffer layer occurred and a delamination gap was formed as the SiC remained almost unchanged in radius. The OPyC layer displayed little change in inner radius but its thickness was reduced. In addition to the examination of the irradiated particles, it is important to understand the deformation of the unirradiated specimens at similar temperatures as the irradiation. Therefore, uniaxial compression tests on both types of particles were conducted at 1000±5°C with in situ 3-D imaging using synchrotron X-ray. Post-test experiments were designed to characterize the residual stresses in individual layers and the fracture toughness of the interfaces; these methods can be applied to irradiated particles to explore the evolution of the residual stresses change with fluence. (author)