Three Mile Island

Three Mile Island ¹⁾

Three Mile Island is a twin Pressurised Water Reactor plant. The accident occurred in Unit 2 on 28 March 1979, when the plant experienced a failure in the secondary, non-nuclear section. The main feedwater pumps stopped running resulting in an increase in pressure in the primary system and the opening of a pressure-relief valve. The valve should have closed when the pressure decreased, but this did not happen and signals available to the operators failed to show that the valve was still open. As a result, cooling water poured out of the stuck-open valve and caused the core to overheat. As a result of misinterpretation of instrument readings, the operators took a series of actions that made conditions worse by reducing the flow of coolant through the core and the reactor suffered a severe core meltdown. However, no breach of containment occurred. Nevertheless, there was a release of radioactivity to the environment from the plant auxiliary building. This arose from an action taken to relieve pressure on the primary system and avoid curtailing the flow of coolant to the core.

Detailed studies of the radiological consequences of the accident indicate that the average effective dose to about two million people in the area was about 0.01 mSv.