Comparative study of random and uniform models for the coated fuel particles distribution in pebble bed reactors

The use of TRISO fuel is a main feature of the high temperature reactors. In these reactors, the heterogeneity in the internal composition of the TRISO particle and the random arrangement of coated fuel particles inside graphite matrix creates a significant challenge in the modelling tasks. To simulate spherical elements using MCNPX code repetitive structures are usually created, generating a uniform distribution of the coated fuel particles. The use of these repetitive structures introduces two major limitations: the no randomness of the TRISO particles inside the pebbles and the intersection of the pebble surface with the particles. These limitations could affect significantly the multiplicative properties of the core. In order to study the influence of these approximations in the multiplicative properties of pebble-bed reactor cores, four different configurations for the TRISO particles distribution inside pebble were analysed. On pebble scale, the results showed significant differences between the mod...