Three Mile Island Release

Three Mile Island is a nuclear power plant located near Middleton, Pennsylvania.

A couple of days before the accident in Unit 2 (TMI-2), tests of the emergency feedwater system were carried out. During these tests, a valve was meant to be closed and opened, but remained closed after the test, making the system inoperative.

At roughly 4:00 a.m. on March 28th, 1979 the first signs of failure became visible to the operators. Maintenance in the non-nuclear section of the plant caused water to leak into the instrument air system, tripping the main feedwater pumps. With the steam generators unable to remove the heat from the system, the turbines shut down.

The effects of this failure spread to the nuclear section of the plant, causing the reactor to shut down.

To relieve the pressure, a relief valve was opened. This valve was meant to close once the pressure was lowered sufficiently, but remained open while indications did not show this to the operators. For eight minutes more, the pressure continued to drop before the incorrect valve position from the maintenance was noticed and rectified.

Water flowed back into the steam generators but the pressure had decreased and voids without water started to form. However, these voids were outside the pressurizer, which is where the pressure levels indicated that the system was full of water.

With insufficient cooling, the fuel overheated and the zirconium cladding reacted with the water to produce hydrogen that was released into the containment building.

Two days later hydrogen was still present in the primary coolant system, a gaseous bubble that would reduce the water cooling the reactor if the pressure were allowed to drop. Gradual degassing eventually rectified the problem.

A meltdown was avoided, but high levels of radioactivity were measured in the plant and some radioactive materials were released into the environment.

The causes of the accident were argued over after a number of investigations, and included combinations of personnel error, and deficiencies in the design and components.


Of the 19 pages in the enquiry report, only 2 addressed technical issues and 17 address management, training and institutional issues.


The report from the USA Nuclear Regulatory Commission.