A MATHEMATICAL MODEL FOR CALCULATING STRESSES IN A PYROCARBON- AND SILICON CARBIDE-COATED FUEL PARTICLE

The analysis employed by previous authors in calculating the stresses developed during service in a pyrolytic carbon coating on a spherical fuel particle has been extended to consider composite carbide-carbon coatings. In the mathematical model, stresses are considered to result from (1) a pressure buildup inside the coatings due to gaseous fission products; (2) anisotropic dimensional changes induced in the carbon by fast-neutron irradiation; (3) differential thermal expansion, and are considered to be relieved by creep induced in the carbon by fast-neutron irradiation. The silicon carbide coating has been assumed to be rigid since present data seem to indicate that creep and fast-neutron-induced dimensional changes in silicon carbide are negligible under operating conditions. The stresses calculated using the model have been compared with observed fractional failure of coatings during accelerated testing of fuel particles with a number of different dimensions, carbon properties, burnups, and fast-neutron doses. In general, the fraction of failed coatings was high when the calculated stresses were high and was low when the calculated stresses were low. © 1969.