Mathematical Model for Predicting Coated-Particle Behavior

A mathematical model for investigating the irradiation behavior of pyrolytic-carbon-coated fuel microspheres has been formulated. The model can be used to study the influence on coated-particle life of a number of design parameters, such as fuel particle density, number of coating layers, and coating thickness, density, and strength. For typical two-layer coated particles, the model predicts two modes of coating failure: 1) failure initiated at the inner coating surface from the combined effects of fuel swelling, fission-gas pressure, and outer coating shrinkage; and 2) failure initiated at the outer coating surface resulting from anisotropic thermal expansion and fast-neutron damage to the pyrolytic carbon structure. To assure long-term irradiation stability, a coated-particle design must incorporate: 1) free volume to accommodate fission gas and fuel swelling; and 2) an outer coating material that exhibits small dimensional changes under fast-neutron irradiation.