Three Mile Island

core melt

45% of the core - 62 tonnes - had melted and 19 tonnes of this had ended up in the lower plenum

What happened:

After shutting down the fission reaction, the TMI-2 reactor's fuel core became uncovered and more than one third of the fuel melted. Inadequate instrumentation and training programs at the time hampered operators' ability to respond to the accident. The accident was accompanied by communications problems that led to conflicting information available to the public, contributing to the public's fears A small amount of radiation was released from the plant. The releases were not serious and were not health hazards. This was confirmed by thousands of environmental and other samples and measurements taken during the accident. The containment building worked as designed. Despite melting of about one-third of the fuel core, the reactor vessel itself maintained its integrity and contained the damaged fuel.

Longer-term impacts

Applying the accident's lessons produced important, continuing improvement in the performance of all nuclear power plants. The accident fostered better understanding of fuel melting, including improbability of a "China Syndrome" meltdown breaching the reactor vessel and the containment structure. Public confidence in nuclear energy, particularly in USA, declined sharply following the Three Mile Island accident. It was a major cause of the decline in nuclear construction through the 1980s and 1990s.

root cause

Deficient control room instrumentation and inadequate emergency response

failure

It involved a relatively minor malfunction in the secondary cooling circuit which caused the temperature in the primary coolant to rise. This in turn caused the reactor to shut down automatically. relief valve failed to close, but instrumentation did not reveal the fact, and so much of the primary coolant drained away that the residual decay heat in the reactor core was not removed. The core suffered severe damage as a result.

Date

March 28th 1979

TMI-2 clean-up

The clean-up of the damaged nuclear reactor system at TMI-2 took nearly 12 years and cost approximately $973 million 100 tonnes of damaged uranium fuel had to be removed from the reactor vessel

Training improvements

Training reforms are among the most significant outcomes of the TMI-2 accident. Training became centered on protecting a plant's cooling capacity, whatever the triggering problem might be led to the establishment of the Atlanta-based Institute of Nuclear Power Operations (INPO) and its National Academy for Nuclear Training

Unit 1

from its restart in 1985, TMI-1 has operated at very high levels of safety and reliability In 1997, TMI-1 completed the longest operating run of any light water reactor in the history of nuclear power worldwide The unit was finally shut down in September 2019.

health

no health problems from release, radioactive release was very small

malfunction

secondary cooling circuit