Analysis of the TMI-2 accident using ATHLET-CD

On March 28, 1979, the only loss-of-coolant accident so far in a light water reactor exceeding the design basis occurred in unit 2 of the U.S. PWR of Three Mile Island (TMI-2) near Harrisburg. The accident entailed massive core degradation accompanied by the formation of a bed of debris and a molten pool. Approx. 30 t of the core inventory were moved to the bottom of the reactor pressure vessel; the vessel sustained this thermal load. Also because of the worldwide use of light water reactors, this accident constitutes an outstanding event with respect to technical safety which can be used to describe the phenomenology of the early as well as the late phases of core degradation, for modeling and, above all, for validation of codes analyzing very severe accidents and, in this way, can serve to enhance the safety of plants in operation. After a brief introduction, the accident scenario is outlined. On this basis, the ATHLET-CD code is introduced, and the approach used in modeling the plant and the accident is described. Finally, the results of the simulation carried out with ATHLET-CD are summarized and evaluated. It is seen that the code is able, in principle, to describe the accident with good accuracy. However, further development with respect to modeling of the late phase is required.