Release characteristics of volatile metallic fission products from the coated fuel particle and the reactor core for a very high temperature gas-cooled reactor during its power operation has been studied using numerical analysis. A computer code FORNAX, based on Fick’s diffusion law and the evaporation mass transfer relation, has been developed, which considers, in particular, distributions and time histories of power density, fuel temperature, and failed and degraded fuel particle fractions in the core. Applicability of the code to evaluate the core design has been shown and the following have been indicated on the release of cesium from the reactor:

  1. The release from the intact fuel particles by diffusion through their intact coatings shows larger contribution in the total core release at higher temperature.
  2. The diffusion release from the intact particle is governed not only by the diffusion in the silicon carbide layer but also by that in the fuel kernel.