TMI-2 Accident Analysis with SCDAP/RELAP5 Code

Analysis of the TMI-2 accident has been performed using the SCDAP/RELAP5/MOD3.2 system code, which is being developed by INEEL for the U.S. NRC. The nodalization of the TMI-2 plant included a detailed modeling of the primary coolant system with the reactor pressure vessel, the two coolant loops with once-through steam generators and main coolant pumps, the pressurizer with surge-line and pilot operated relief valve. The nodalization of the secondary system was limited to the secondary side of steam generators. The boundary conditions for the calculations were fixed according to the OECD TMI-2 Analysis Exercise. The first three phases of the accident including small break LOCA, core heat-up and melting, and core reflood were adequately modeled with SCDAP/RELAP5. Hydrogen generation and relevant primary system pressure increase were well predicted during both core heat-up and reflood. The code predicted the formation of a large molten pool in the central region of the core and complete quenching and fragmentation of embrittled fuel rod in the upper part of the core, in reasonable agreement with the TMI-2 accident scenario. No molten core material slumping to the lower head was computed by the code in the late phase of the accident.