Advanced full-core modeling of fission product release in pebble-bed high-temperature gas-cooled reactors

Accurate modeling of fission product release is essential for the safety of pebble-bed high-temperature gas-cooled reactors. Traditional models oversimplify by neglecting non-uniform temperature fields, variable nuclide production rates, and the randomness of pebble flow. This study introduces an enhanced diffusion calculation method that incorporates variable production rates from the burnup equation and non-uniform temperature profiles from steady-state heat conduction models. A rapid full-core release calculation refined to individual pebble dynamics is developed to better capture equilibrium core behavior. Comparing the refined model to traditional methods, results show stepwise accumulation of concentrations, reduced release rates from particles and pebbles, and increased concentration gradients within the graphite matrix. Release rate distributions for short-lived nuclides are influenced by both core temperature and neutron flux distributions, while those for long-lived nuclides are primarily temperature-dependent. The variable-temperature model indicates that, compared to uniform particle temperature models, a greater diffusion distance is required for nuclide release.