Gaseous and metallic fission product transport characteristics of a modular pebble bed HTGR during loss of core cooling accidents

Models have been developed to predict the transport of metallic and gaseous fission products (FPs) through the multilayered fuel particle coatings and the graphite matrix of the core under accident conditions. Using these models, FP transport and releases were calculated for a loss of core convective cooling accident in a 250-MW(t) 3.8-W/cc pebble bed HTGR. This accident was selected for this study since the fuel particles are exposed to above normal temperatures, thereby affecting the retention of fission products within the coated fuel particles. Fission-product transport through the particle kernel and coatings, the graphite pebbles/reflectors, the reactor vessel, and the confinement were assessed. These studies extended the models and release results reported earlier. The amount of fission products released is small and the release occurs slowly over a period of days as the pebble heats up. Over 90% of the amount of gaseous FPs that are released is due to the small amount of initially failed particles and heavy metal contamination. The metallic cesium release during this transient is due to release from failed fuel particles and diffusion through intact particles. However, the net cesium release from the reactor core is negligible, as the release from the hotter regions plates out in the cooler parts of the core. The results of this study show that the most effective barrier to fission products is the coated fuel particle. The reactor vessel and the confinement provide additional attenuation for the small amount released from the core. The small release to the environment occurs over a period of days and is so low that the safety criterion of 5 rem thyroid dose (to avoid offsite sheltering) is satisfied with a margin of more than an order of magnitude