Modelling of HTRs with Monte Carlo: from a homogeneous to an exact heterogeneous core with microparticles

Recently a worldwide interest in HTR technology has been experienced. In this context the gas turbine modular helium-cooled reactor (GT-MHR) is a potential candidate for the maximum 239Pu destruction in a once-through cycle. A particular feature of GT-MHR is that its coated fuel (TRISO particles) is supposed to provide an impermeable barrier to the release of fission products and, at the same time, to resist very deep burn-up rates (more than 90% for 239Pu). In this paper a Monte Carlo approach is employed to characterise the neutron fluxes and the fuel evolution inside the tiny 200-μm diameter fuel kernels with an exact and finite geometry description. Our major goal is to obtain a quantitative comparison of different geometry sets, namely homogeneous versus single-heterogeneous and double-heterogeneous, in terms of keff eigenvalues, the length of the fuel cycle, neutron characteristics and the evolution of fuel composition in particular. In all cases the same Monteburns (MCNP+ORIGEN) code system is used. We show that the performance of GT-MHR is considerably influenced by the way its geometry is modelled within the Monte Carlo approach. The spatial and energy shielding of the neutron flux even in such small particles cannot be neglected for important isotopes which have high resonance cross sections as 240Pu, 241Pu and 167Er. Namely, the formation of 241Pu and burn-up of 167Er are responsible for the different length of the fuel cycle, being the shortest for a double-heterogeneous geometry. On the other hand, the evolution of 239Pu at a constant reactor power and comparable neutron fluence is very similar for all three geometry configurations.