FISSION-PRODUCT BEHAVIOR AND GRAPHITE CORROSION UNDER ACCIDENT CONDITIONS IN THE HTR

This paper primarily gives an overview of methods and data in source term estimations for the HTR with pebble bed core. For medium size HTRs the risk dominating accidents are tied to core heat-up events, where a significant portion of the fission product inventory may be released from the coated fuel particles. Here the research mainly is focused on temperature-induced coated particle failure and the interaction of metallic fission products with the core graphite. For small HTRs, with their limitation of maximum temperatures below coated particle failure limits, core heat-up accidents virtually play no role with respect to source terms. Here the risk is dominated by accidents like water ingress or rapid depressurization which may lead to a partial release of fission products accumulated on primary circuit surfaces like the steam reformer. Deposition of fission products and remobilization under the conditions mentioned above are predominant research areas. It can be expected that the ongoing and planned improvements of models and data base, in particular for the medium size HTR, will result in a further reduction of the already low source terms. A principal possibility for core degradation and hence destruction of fission product barriers is graphite corrosion caused by massive air ingress. The research effort in this field as well as for graphite corrosion during water ingress accidents is described in Part B of this paper. From the viewpoint of risk for this type of accident no significant contribution to that of present reactor concepts was found. © 1990.