Resonance treatment for double heterogeneity

The tristructural isotropic (TRISO) particles have been widely applied in many reactor designs in the form of pebble and fully ceramic microencapsulated fuel. Those fuels yield double heterogeneity in the aspect of geometry compared with typical pin cells in current Pressurized Water Reactor (PWR) assembly. The combination of the usual heterogeneity from the fuel element lattice along with the heterogeneous mixture of microspheres constituting the elements yields a doubly heterogeneous environment for particle fuel. Due to the double heterogeneity in the fuels, traditional self-shielding calculation methods, such as the equivalence theory, subgroup method, and hyperfine-group method cannot be directly applied to treat the self-shielding effect. This chapter introduces the Markovian distribution and the method of chord length sampling. Then multigroup effective cross-sections are corrected by Dancoff factors to consider the double heterogeneity effect. Equivalence theory method for treating double heterogeneity and Sanchez-Pomraning method are also introduced.