top of page

TMI2 Historical Information 

Three Mile Island Unit 2 

TMI-2 is a non-operational, pressurized water reactor that was rated at a core thermal power level of 2,772 megawatts with a corresponding turbine generator gross output of 959 megawatt-electric. TMI-2 utilized a two-loop pressurized water reactor nuclear steam supply system designed by Babcock and Wilcox Corporation.


GPU Nuclear was issued an operating license for TMI-2 on February 8, 1978, with commercial operation declared on December 30, 1978. On March 28, 1979, TMI-2 experienced an accident initiated by interruption of secondary feedwater flow. The lack of secondary feedwater resulted in the reduction of primary-to-secondary heat exchange that caused an increase in the reactor coolant temperature, creating a surge into the pressurizer and an increase in system pressure. The pressure-operated relief valve opened to relieve the pressure but failed to close when the pressure decreased. The reactor coolant pumps were turned off and core heat-up began as the reactor coolant system water inventory continued to decrease, resulting in a reactor vessel water level below the top of the core. This led to a core heat-up that resulted in damage to the fuel.


Most of the fuel material travelled down through the southeastern assemblies and into the core bypass region. A portion of the fuel material moved around the bypass region and migrated into the lower internals and lower head region. Overall reactor vessel integrity was maintained throughout the accident.


As a result of the accident, small quantities of damaged core material (DCM) and fission products were transported through the reactor coolant system and the reactor building. In addition, a small quantity of core debris was transported to the auxiliary and fuel handling buildings. Further spread of the debris occurred as part of the post-accident water processing cleanup activities.


The quantity of damaged fuel remaining in TMI-2 is a small fraction of the initial fuel load. In fact, approximately 99 percent (%) of the damaged fuel was successfully removed in the post-accident, defueling effort. Additionally, large quantities of radioactive fission products released into various systems and structures were removed as part of the waste processing activities during the TMI-2 Cleanup Program which spanned over a decade. The NRC staff’s post-accident, safe storage criteria for cleanup were met and accepted by the NRC, permitting TMI-2 entrance into PDMS in 1993.


The 99% of damaged fuel and core debris removed from the plant was packaged and shipped to the Idaho National Engineering and Environmental Laboratory (INEEL) under the responsibility of the U. S. Department of Energy (DOE). The reactor coolant system was decontaminated to the extent practicable to reduce radiation levels to as low as is reasonably achievable (ALARA). As part of the decontamination effort, accident-generated water was removed to the extent practicable from the reactor coolant system and the fuel transfer canal and the fuel transfer tubes were isolated. Radioactive wastes from the major cleanup activities have since been shipped offsite.

Following decontamination activities, only the reactor building and a few areas in the auxiliary and fuel handling buildings continued to have general area radiation levels higher than those of an undamaged reactor facility nearing the end of its operating life.

Three Mile Island 4K

Three Mile Island 4K

bottom of page