A thorough analysis of the thermal-hydraulic behavior of the TMI-2 reactor during the first three hours of the accident on March 28, 1979 was carried out by use of RELAP4/MOD6/U4/J2 computer program. The analysis includes the loss of feedwater transient, the small break loss-of-coolant accident (LOCA) through the pressurizer pilot-operated relief valve (PORV) and the distribution of hydrogen generated by the zirconium-water reaction. It took account of the effects of the pressurizer heaters and of the presence of the hydrogen. The calculated result shows good agreement with the accident record, and the effects of some key parameters have become better understood. This analysis shows that the tops of active fuel rods would have begun to be exposed at 6490 s into the accident and the core mixture level remained at a height of 3.4 to 4 ft from the bottom of the core for about 40 min.
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