Effect of pebble packing on neutron spectrum and the isotopic composition of HTGR fuel

Fission products play an important role in the safety and fuel integrity of high-temperature gas-cooled reactor (HTGR) and they depend on temperature, burnup, neutron energy distribution, and fast fluence. Energy distribution of neutrons in a fuel region determines the isotopic distribution of the fission products to be produced. The local concentrations of these isotopes are considered to be functions of temperature and burnup as well as the amount transported from the kernel to the coating layers where they interact and may degrade layers. Thus, the integrity of the fuel particle may be lost and fission products can be released into the reactor coolant inventory. In this study, it is the main purpose to perform neutron energy spectrum shift in spherical HTGR fuels and to investigate its effect on fission products. Moreover, it is also intended to analyze the effect of unit cell geometries on criticality of the system. The calculations for group fluxes based on ENDF4 library with 27 neutron energy groups are accomplished by the MCNP5 neutron transport code. Burnup and criticality analyses are performed by using the MONTEBURNS2 code (MCNP5 coupled with ORIGENS). To simplify the neutron transport problem, instead of full core modeling, two fundamental unit cell arrangements, body-centered cubic (BCC) and hexagonal close-packed (HCP) lattices, are considered to be as reference geometry models. Unit cells are defined with proper boundary conditions and random packing for TRISO particles provided by the stochastic geometry card specified in MCNP5 for HTGR pebbles is used.