ABSTRACT
Analysis of fission product release from the molten ceramic pool in the Three Mile Island Unit 2 accident indicates that fission product chemical form and concentration dominate the release of medium and low volatile fission products and that bubble dynamics dominates the release of volatile fission products. Two factors that strongly influence the location and chemical form of fission products are the chemistry within the core and the morphology of the fuel-bearing debris. Information has been obtained recently on phenomena affecting the release of fission products from fuel and the reactor vessel during the in-vessel melt progression phase of a severe accident. The influence of a number of these phenomena will be reviewed, including fuel chemistry, fuel liquefaction, molten pools, and debris beds. Boric acid in the reactor coolant and in the emergency coolant is used as a reactivity control material in Pressurized Water Reactors, and boron carbide encased in stainless steel control blades is used in boiling water reactors.
