RADIATION EFFECT ON THE CHEMICAL-STATE OF FISSION-PRODUCT IODINE

The chemical states of fission products in nuclear fuel pins are usual ly evaluated by thermodynamic analysis. The thermal equilibrium is ass umed 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 sy stem have non-equilibrium energy distribution, which may affect the ch emical state of fission products. In this paper, collision processes i nitiated by fission fragments in a solid fuel were calculated by the T RIM Monte Carlo code. In the gas phase in a fuel-cladding gap, the sam e processes were also simulated. Based on the calculation results, it has been evaluated how the energy distribution of gas atoms in fuel-cl adding gap is different from Maxwell-Boltzmann distribution correspond ing to the temperature of the system. It has been concluded that the r adiation effect of fission fragments on chemical reactions are signifi cant when the threshold energy of the chemical reaction is high and th e reaction occurs at low temperature. The radiation effect on the deco mposition reaction of CsI vapor has been demonstrated at 650 K, a fiss ion density of 4 X 10(13) fissions cm(-3) s(-1) and a Xe pressure of 1 0 atm in the fuel-cladding gap. The CsI decomposition reaction rate ha s been found to increase the iodine partial pressure from 3.7 X 10(-23 ) atom in the absence of radiation to 3.1 X 10(-10) atoms in the radia tion field. (C) 1997 Elsevier Science B.V.