Evaluation of fission-product gases in program GAPCON series and FREG-3 to estimate the gap heat transfer coefficient

In safety evaluation of a fuel rod, estimation of the stored energy in the fuel rod is indispensable. For this estimation, the temperature distribution in the fuel rod is calculated. Most important in determination of the temperature distribution is the gap heat transfer coefficient (gap conductance) between pellet surface and cladding inner surface. Under fuel rod operating condition, the mixed gas in the gap is composed of He, Xe and Kr. He is initial seald gas. Xe and Kr are fission-product gases, of which the quantities depend on the fuel burn-up. In program GAPCON series (GAPCON and GAPCON-THERMAL-1 and -2) and FREG-3, these quantities are given as a function of the irradiation time, power rating and neutron flux in estimation of the thermal conductivity of the mixed gas. The methods of calculating the quantities of Xe and Kr in the programs have been examined. Input of the neutron flux which influences F.P. gas production rates is better than the determination from the fuel-rod power rating. (auth.)