Analysis of the TMI-2 accident with the code ATHLET-CD

Thermal-hydraulic phenomena play a decisive role for the course of severe accidents in light water reactors. Therefore, the simulation of such accidents with computer codes requires comprehensive and detailed modelling of these processes. The system code ATHLET-CD is being developed for a realistic simulation of accidents with core degradation and for the evaluation of accident management measures. It applies the detailed and validated models of the thermal-hydraulic code ATHLET in an efficient coupling with dedicated models for core degradation and fission product behaviour. The capabilities of the current code version are demonstrated by a calculation of the Three Mile Islands unit 2 (TMI-2) accident. The first three phases of the accident, up to the melt relocation into the lower plenum, were successfully simulated by the code in a reasonable computing time. The calculated system pressure and pressurizer level after pump trip, during pump restart and until core slump are in acceptable agreement with the measured data. The calculated hydrogen production up to pump restart is in accordance with the value deduced from plant examination. However the hydrogen generation during the quench phase following the pump restart as well as the amount of liquid melt at the end of Phase 3 are underestimated in comparison with deduced values. Further model extensions mainly regarding the quenching of degraded core material, fracture and relocation of solid fuel rods as well as the simulation of the debris bed behaviour are necessary to further improve the simulation. (authors)