Consideration of the effects on fuel particle behavior from shrinkage cracks in the inner pyrocarbon layer

The fundamental design for a gas-cooled pebble bed reactor relies on an und erstanding of the behavior of coated particle fuel. The coating layers surr ounding the fuel kernels in these spherical particles consist of pyrolytic carbon layers and a silicon carbide (SiC) layer. These coating layers act a s a pressure vessel that retains fission product gases. A small percentage of fuel particles may fail during irradiation in the mode of a traditional pressure vessel failure. Fuel performance models used to predict particle b ehavior have traditionally been one-dimensional models that focus on this f ailure mechanism. Results of irradiation experiments, however, show that ma ny more fuel particles fail than would be predicted by this mechanism alone . Post-irradiation examinations indicate that multi-dimensional effects, su ch as the presence of shrinkage cracks in the inner pyrolytic carbon layer (IPyC), contribute to these unexplained failures. Results of a study perfor med to evaluate the significance of cracking in the IPyC layer on behavior of a fuel particle are presented herein, which indicate that shrinkage crac ks could contribute significantly to fuel particle failures. (C) 2001 Elsev ier Science B.V. All rights reserved.