Three Mile Island

1979: A major accident that led to the destruction of a reactor

The Three Mile Island plant (TMI) is located on the Susquehanna River in Pennsylvania USA. Unit No. 2 included a pressurized water reactor (PWR) of design similar to those built in France, but with a different disposition of the steam generator and the core vessel.

The reactor operated normally during the first 3 months following its start-up when, on March 28, 1979 at 4:00 occurred an operating incident on the “secondary” circuit. This incident caused the loss of the main water supply.

The accident began with the loss of sealing of the enclosure of the primary circuit of water – the second barrier of protection – a discharge valve of the pressurizer having remained permanently open. As a result of wrong diagnostics and inappropriate actions, the cooling of the reactor core was no longer assured, leading to a partial fusion of the nuclear fuel, that is to say the loss of the first barrier of protection.

The reactor building containment has played its role of ultimate barrier perfectly with the exception of a small radioactive leak (limited in scope and duration) due to non-isolation of transfer water from the sump of the reactor building to that of the nuclear auxiliary building, a conception error.

These low environmental releases were caused by the system pumping water from the sump located below the primary circuit and that remained active. Pumps have, for several hours, transported in the “auxiliary building” water increasingly loaded with radioactive materials. Contaminated hot water was sprayed in this building, releasing xenon and a part of iodine contained in the environment.

The accident was classified at level 5 on the INES scale. When six years later, it has been possible to enter the compound, a camera inserted into the vessel showed that some fuel had melted but did not escape through the vessel. The reactor was lost, but there were no victims.

In the former Soviet Union, the authorities were silent for a time about the severity of the accident at Chernobyl. Nothing like that in an open society such as America. The accident was highly publicized. Thousands of motorists fled along the roads of Pennsylvania, before returning to their homes once the alert was lifted. The accident, the exodus, the dramatization of newspapers and television caused a deep trauma.

Three Mile Island led the U.S. to abandon the construction of new power plants, following a decision taken by President Jimmy Carter. The U.S. nuclear industry, hitherto dominant, lost its leadership in favor of Europe and Japan. Carter doctrine lasted nearly 30 years. But, on January 27, 2009, in his State of the Union address, President Barack Obama has called for “a new generation of nuclear power plants safe and clean.”

The accident had finally positive impacts in terms of “reactor safety.” Many lessons were learned, leading to multiple measures to improve safety. A safety culture has been developed. The absence of accidents recorded since 1979 in the park of Western reactors is a legacy of Three Mile Island.