The behavior of melts in severe accident sequences affects the nature (composition and fission product inventory) of the debris released from the vessel upon lower head failure in unmitigated accidents and the coolability of debris at various stages in managed accidents. Core melting progressed further in the Three Mile Island Unit 2 (TMI-2) accident than in any of the severe core damage experiments that have been conducted since the accident, and, therefore, TMI-2 represents a valuable source of information that extends into later phases of core melt progression, including melt relocation into the lower plenum. Examination and evaluation of melts within the TMI-2 reactor vessel indicate that melts can form uncoolable geometries in the core but they can also break through the surrounding crust, massively relocate into the lower plenum, and fragment upon interaction with water resident in the lower plenum to form a rubble bed of coolable geometry. The chemistry of melts, particularly the oxygen potential, affects fission product chemical form and, therefore, retention in the melt. The chemistry also determines interactions of the melts with reactor pressure vessel components.