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.