Fission Gas Release from Fuel Particles and Fuel Rods

A review and analysis of available data on the release of fission gases from high-temperature gas-cooled reactor fuel particles indicates that the release of short-lived nuclides (half-life ≲5.3 days) occurs from the fissile material primarily by a mechanism involving recoil followed by gas-phase and bulk diffusion in the surrounding material at temperatures up to at least 1300°C (1573 K). The gas-phase contribution to the release of short-lived nuclides is dominant at temperatures below 600°C (873 K), and bulk diffusion is dominant at temperatures above 900°C (1173 K). Fission gas release to birth ratio (R/B) varies with half-life to a power of ∼0.2 at 300°C (573 K) and ∼0.5 at 1100°C (1373 K). For the short-lived isotopes, R/B is independent of burnup over the range from 0 to 63% FIMA in UC₂ kernels; for dense ThO₂ kernels, the data are insufficient to determine the burnup dependence. For the long-lived and stable isotopes, release from dense ThO₂ kernels is strongly dependent on burnup. Iodine and tellurium isotopes may be treated as if they were xenon isotopes.