Diffusion and Evaporation of Fission Products in Coated Fuel Particles

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Fractional releases of 133Xe, 140Ba and 89Sr from slightly-irradiated pyrolytic-carbon-coated and SiC-coated particles were measured over a temperature range of 1, 200°1, 750°C. The results are analyzed mathematically in order to obtain the diffusion and evaporation coefficients relevant to PyC and SiC. The resulting expressions for the coefficient of diffusion in PyC are 2.9X10-7 exp(-61X103/RT) for 133Xe and 4.7X10-2 exp(-51X103/RT) for 140Ba. For the coefficient of evaporation of 140Ba from PyC, the expression is 3.5X10-3 exp(-42X103/RT). As for SiC, the diffusion and evaporation coefficients of these nuclides are given for a temperature of 1, 750°C. A high diffusivity path for the diffusion of 140Ba is postulated to explain the difference in diffusion behavior between 133Xe and 140Ba in PyC.