MARCH calculations performed for the TMI-2 (Three Mile Island Unit) analysis exercise program

As part of the validation effort for the MARCH portion of the Source Term Code Package, the U.S. Nuclear Regulatory Commission has requested Battelle Columbus to participate in the Three Mile Island Unit (TMI-2) Analysis Exercise Program. Previous TMI-2 calculations (1980) had been performed using the original MARCH 1.1 version of the code. MARCH was written to calculate thermal-hydraulic phenomena for reactor accidents leading to core meltdown. The accident sequences normally considered by MARCH users are relatively uncomplicated. For example, multiple changes in emergency core cooling injection, steam generator auxiliary feedwater flow rates, and cycling of primary system steam relief valves are not generally considered in severe-accident analysis. All of these things were done during the TMI-2 accident. In addition, at TMI-2 the core meltdown sequence was interrupted by the restoration of core cooling after achieving significant melting. In order to perform meaningful TMI-2 calculations, it has been found necessary to add a number of modeling enhancements to MARCH. These modeling changes are discussed along with the results of the TMI-2 MARCH calculations in this paper.