Residual stresses in as-manufactured TRISO Coated Particle Fuel (CPF)

Abstract

TRistructural ISOtropic coated particle fuels (TRISO CPF) have been developed as a possible fuel solution for high temperature nuclear reactors, which offer the possibility of nuclear cogeneration-powered industrial facilities and hydrogen production. A finite element model was developed to simulate the fabrication process of TRISO particles. Understanding the initial stress and bonding state of the layers is crucial in predicting performance and failure in service, however this factor has not received an in depth consideration in the available literature. The simulations of a fully bonded model of TRISO (layers perfectly attached to each other) revealed the presence of high values of tensile hoop stresses in the inner fuel kernel (up to 250 MPa, sufficient to cause fracture in UO2) and even higher compressive stresses (up to 600 MPa) in the silicon carbide layer. Simulations conducted without bonding between kernel and buffer found the residual stress state to be consistently more relaxed with respect to the fully bonded model. This was most evident in the radial stress, which drops to less than 10 MPa (tensile or compressive) throughout the particle. In the hoop direction, compression of 150 MPa remained in the SiC layer. Such results are consistent with the empirical evidence of the occurrence of kernel-buffer debonding during the fabrication of TRISO particles. Finally, a brief investigation of the effect of ovality on the model with kernel-buffer debonding showed an overall increase in the magnitude of the hoop stress in the SiC and PyC layers in a flat spot caused by reduced buffer material.