Effect of TRISO-particles distributions in pebble fuel

Abstract Background: Packing fraction of tristructural-isotropic (TRISO) particles in pebble fuel is lower than 10% in general, which leads to great randomness in distribution, resulting in various challenges in neutronics modeling, such as double heterogeneity effect, preserving exact number of particles, and their random distributions in pebble. Purpose: This study aims to verify the calculations, the infinite pebble fuel with refectory boundary conditions based on the thorium-based molten salt reactor with solid fuel (TMSR-SF1) pebble design using Monte-Carlo code MCNP6 ver1.0 and ENDF/B VⅡ.0. Methods: Four types of models of TRISO particles have been performed based on MATLAB, the equivalently filled regular model, the disturbed regular model, the randomly filled regular model, and the fully random model. Results: The neutronic effect of various modeling approaches was quantitatively evaluated. Conclusion: The results show that the four kinds of modeling methods form different randomness. The clustering of the coated particles enhances the Dancof effect which raises the neutron absorption probability in the fuel. As a result, the infinite multiplication factor is increased while the fuel temperature coefficient is decreased. (authors)