Silicon carbide integrity in TRISO fuel particles

Abstract

Chemical vapor deposited (CVD) silicon carbide (SiC) is used as a pressure vessel material for tristructural isotropic (TRISO) coated fuel particles for the high temperature pebble-bed reactor. Experience has shown that sometimes these particles fail in operation. The fracture toughness (KIC) was measured at room temperature to determine the maximum internal pressure that the CVD SiC layer in TRISO-coated fuel particles can withstand without fracturing. The KIC was measured using the indentation fracture toughness method on the curved-surface and cross-section of the test piece. The most suitable test load for measuring the KIC and Vickers hardness (HV) values of the CVD SiC layer was 100 g. The Evans-Davis model for calculating the KIC of ceramic materials was found to be the most appropriate for the CVD SiC layer. The average curved-surface KIC value was 3.47 MPa.√m, for the 51 mm test sample, which was tougher than the average cross-section KIC value of 3.17 MPa.√m. The wall thickness values of the samples tested were 28, 36, 39 and 51 mm and their corresponding measured mean curved-surface KIC values were 3.13, 3.07, 3.15 and 3.47 MPa.√m, respectively, which were on par with values reported in literature. The corresponding minimum curved-surface KIC values were 1.43, 1.67, 1.65 and 2.52 MPa.√m, respectively. These values were obtained from the average curved-surface KIC values minus five sigma (KIC = μ-5σ). The internal pressure that the SiC shell can handle without failing will depend on the initial crack length and the value of KIC. The pressure that a particle can withstand assuming the leak-before-break (i.e. a particle with a crack that extends right through the shell) was also used. This represents a more conservative value for the maximum pressure allowable without fracture when the fracture toughness is known. In fact, there is a considerable wide distribution in the measured fracture toughness. Using the average fracture toughness (μ) minus five times the standard deviation (σ) represents a conservative approach. The calculated maximum allowable pressure represents an even more conservative approach when both the LBB condition and the μ-5σ requirement are assumed. This is probably representative for the operation of the PBMR.