Calculation of irradiation-induced stresses in restrained pyrolytic carbons

Models for the calculation of stresses generated in pyrolytic carbon and silicon carbide coatings on the fuel particles used in high-temperature gas-cooled reactors generally use the irradiation-induced dimensional changes of unrestrained carbons. However, restraint during irradiation has been shown to alter the behaviour of the carbon. Two effects produced by restraint are a reduction in the irradiation-induced densification and increase in the preferred orientation of the carbon, the latter leading to an increase in the dimensional change rate. The reduction in densification can be incorporated in the models by the use of appropriate values for Poisson's ratio in creep, μsub(c). By comparison of calculated densities with experimentally measured densities, it seems that μsub(c) = 0.4 for 0<=γ<=2.0 x 1021 n/cm2 and μsub(c) = 0.5 for γ>2.0 x 1021 n/cm2 (E>0.18 MeV) are the correct values. Experimental data indicate that the increase in dimensional change rates can be incorporated in the models by using crystallite averaging expressions after accounting for dimensional changes due to changes in density. Introduction of the restraint effects in the calculation of the stresses in an outer carbon coating on a fuel particle containing silicon carbide layer causes a considerable alteration in the calculated stress-fluence history. With inclusion of the restraint effects, the stress-fluence history is in much better agreement with observed particle behaviour. (author)