FUEL PARTICLE FAILURE PROBABILITIES AND MATERIAL STRENGTHS DETERMINED FROM CRUSH TEST DATA

摘要

The prototypical nuclear fuel of the New Production Modular High-temperature Gas-Cooled Reactor (NP-MHTGR) consists of spherical TRISO-coated particles suspended in graphite cylinders. The coating layers surrounding the fuel kernels consist of pyrolytic carbon layers and a silicon carbide (SIG) layer. These coating layers act as a pressure vessel that retains fission product gases. The structural integrity of this pressure vessel relies on the strength of the SiC layer. If the SiC fractures because of the internal pressure loading, then the particle fails. Results obtained from a series of crush tests on unirradiated fuel particles were used to estimate the strength of the SiC layer for internal pressure loading. The SIC strength for a particle was defined in terms of the maximum stress level in the SiC layer at which the particle failed. The transformations between the test results, which involve compressive crushing loads, and the internal pressure loadings experienced in NP-MHTGR reactor conditions were made utilizing the Weibull statistical theory. The test results were also used to estimate failure probabilities for fuel particles that are lacking an outer pyrolytic carbon layer (OPyC). The transformation from the crush test failure data to equivalent SIC strengths under an internal pressure load allowed for a direct comparison in strengths between fully coated particles and particles that lack the OPyC layer. In the latter case, the OPyC has typically been removed through a ''burn-back'' process. Results showed that strengths for fully coated particles are somewhat higher than for burned-back particles. Whether this is attributable to an actual loss of strength because of the burn-back process or is an artifact of the testing process is a subject for further study. Other effects on particle strength measured by making similar comparisons were particle compacting and particle flaws (large ''gold spots''). These generally did not have a significant effect on SiC strengths. Another finding from calculations performed was that application of a ''spread ring'' load rather than a concentrated ''point'' load in the crush tests was more representative of the internal pressure loading experienced in NP-MHTGR reactor conditions. Results of the failure probability calculations showed that the failure probability for a batch of burned-back fuel particles was governed by a weak ''tail'' group of particles, which constitute a small percentage of the total particle batch. References: 9