The chemical states of fission products in nuclear fuel pins are usually evaluated by thermodynamic analysis. The thermal equilibrium is assumed in the analysis, so that atoms included in the system are to obey Maxwell-Boltzmann energy distribution. However, nuclear fuel pins are subjected to a strong radiation field and, hence, molecules in the system have non-equilibrium energy distribution, which may affect the chemical state of fission products. In this paper, collision processes initiated by fission fragments in a solid fuel were calculated by the TRIM Monte Carlo code. In the gas phase in a fuel-cladding gap, the same processes were also simulated. Based on the calculation results, it has been evaluated how the energy distribution of gas atoms in fuel-cladding gap is different from Maxwell-Boltzmann distribution corresponding to the temperature of the system. It has been concluded that the radiation effect of fission fragments on chemical reactions are significant when the threshold energy of the chemical reaction is high and the reaction occurs at low temperature. The radiation effect on the decomposition reaction of CsI vapor has been demonstrated at 650 K, a fission density of 4 × 1013 fissions cm-3 s-1 and a Xe pressure of 10 atm in the fuel-cladding gap. The CsI decomposition reaction rate has been found to increase the iodine partial pressure from 3.7 × 10-23 atom in the absence of radiation to 3.1 × 10-10 atoms in the radiation field.