Interface-currents integral transport model for treating doubly-heterogeneous, multisystem geometries

An analytical model for calculating neutron spectra in the doubly-heterogeneous fuel-moderator geometries of the pebble bed reactor concept is presented. The model is capable of simultaneously treating more than one type of fuel grain in the fuel matrix and more than one type of pebble in the reactor core. The model was developed to assess the need for treating various levels of material heterogeneity in processing neutron multigroup cross sections in the resolved resonance energy range. The slowing-down calculation is performed over a pointwise energy mesh tailored to the cross section structure for the nuclides present in the problem. Isotropic, elastic scattering theory is applied in an explicit calculation of down-scattered sources due to neutron interaction with all materials in all zones. At each energy point, neutron transport between zones is calculated with the interface-currents integral transport technique. Here, this technique is extended to include the simultaneous treatment of coupled, one-dimensional, multiregion systems. The coupling between the two levels of heterogeneity (grain systems and pebble systems) is accomplished by a sequence of source normalization and cross section averaging treatments. The equations applied in the slowing-down and spatial transport models are presented. Results from the analyses of single pebble and double pebble systems indicate the importance of resonance shielding as a function of fuel kernel diameter, fuel loading in each pebble, and the presence of more than one type of pebble in the system