Analysis of Fission Product Release Behavior from the Three Mile Island Unit 2 Core

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Abstract
An analysis of fission product release during the Three Mile Island Unit 2 (TMI-2) accident has been performed to provide an understanding of fission product behavior that is consistent with both the best-estimate accident scenario and fission product results from the sample acquisition and examination efforts. “Firstprinciples” fission product release models are used to describe release from intact, disrupted, and molten fuel during the various phases of the TMI-2 accident. Extensive gaseous and volatile fission product release is calculated to have occurred, with local regions of the core experiencing up to 100% release. Diffusion is calculated to have dominated release during the initial core heatup, while bubble coalescence and rise dominated release from the large consolidated region of molten material. The calculations are generally consistent with fission product retention data from upper and lower plenum debris bed samples. An exception to this is the small retention of cesium in the lower plenum samples, suggesting that cesium may have been in a low-volatile chemical form. The small release fractions measured for the less volatile fission product oxides (SrO, Eu2O3, and Ce2O3) are calculated to have resulted from the low partial pressures of these species in the melt coupled with the low surface-to-volume ratio of the consolidated melt region. Metallic species (ruthenium and antimony) are thought to be associated with metallic core structural material debris.