The impact of the matrix and buffer properties on residual stresses in TRISO particles during manufacture and early life

A three-dimensional finite-element model was developed using Abaqus to simulate the fabrication and initial power ramp-up of a tri-structural isotropic (TRISO) particle, featuring a UO2 kernel, encapsulated in either a SiC or graphite matrix. After sintering, the residual compressive hoop stress in the SiC coating layer reached -540 MPa when encapsulated in a graphite matrix, 94 MPa more compressive than in a SiC matrix. However, following the initial power ramp-up, the predicted compressive hoop stresses in the SiC layer of a particle embedded in a SiC matrix (-388 MPa) were significantly greater than in a graphite matrix (-222 MPa), emphasizing the matrix material's critical role in the stress state of the SiC layer. Model validation attempts were made with the experimental measurements of the residual stresses of a zirconia-kernel surrogate particle. We found that in a fully-bonded surrogate TRISO particle, the stress state of the SiC layer is highly sensitive to the buffer porosity with compressive SiC layer hoop stresses ranging from up to -1.06 GPa at a porosity of 0.14 to -0.77 GPa at a porosity of 0.60. Partial kernel/buffer delamination simulations revealed a significantly varied geometric stress distribution, with tensile stresses reaching up to +54 MPa and compressive stresses up to -546 MPa at different axes of the sectioned plane in the model. This finding suggests that contrary to the common assumption of complete delamination at the kernel/buffer interface during fabrication, partial delamination is a more plausible explanation for the high compressive stresses observed in the SiC layer experimentally.