The TMI-2 accident evaluation program

This paper discusses an evaluation of the accident at the Three Mile Island Unit 2 (TMI-2) reactor. It remains as the United States' worst commercial nuclear reactor accident. Although the consequences of the accident were restricted primarily to the plant itself, the potential consequences of the accident, should it have progressed further, are large enough to warrant close scrutiny of all aspects of the event. The TMI-2 accident resulted in extensive oxidation and melting of the reactor core and in significant release of fission products from the fuel. At least 45% (62 metric tons) of the core melted and about 20 metric tons of molten core material relocated into the lower plenum of the reactor vessel. However, in spite of severe core damage, water in the reactor vessel prevented failure of the reactor vessel's lower head. The TMI-2 accident was similar to a station blackout or smallbreak LOCA accident sequence like those postulated in reactor safety studies. The accident progressed into the latter stages of core melt progression, that is, generally much further into the progression of a severe accident than core melt experiments have explored. As such, the accident greatly expands and enhances the current understanding of core melt progression and fission product behavior during severe accidents and provides a unique database for assessment of severe accident computer models