See glossary pages 33 to 36 Three Mile Island, the first nuclear accident that attracted worldwide attention The accident involving partial meltdown of the core of reactor 2 of the Three Mile Island (TMI) power plant demonstrated that combinations of human and technical failures could lead to a severe accident. Rated level 5 on the INES scale, the accident was a major turning point for the nuclear industry and gave rise to an overall review of the risks and approach to reactor safety. A year after it was commissioned, reactor 2 of the TMI NPP situated on an island on the River Susquehanna, suffered a technical failure. The TMI NPP, situated in Pennsylvania in the east of the United States, was commissioned in 1974. In 1979, it was equipped with separate 900 megawatt electric (MWe) PWRs. Wednesday 28 March 1979, 04:00 (4 am) The accident began with a simple operating incident, failure of the main feedwater pumps supplying the steam generator cooling system. The planned safety mechanisms – emergency shutdown (reactor trip) by inserting control rods into the fuel core and activation of the auxiliary feedwater pumps supplying water to the reactor – functioned perfectly. Succession of failures and negligence But then a second failure occurred: despite the activation of the auxiliary feedwater pumps, the water did not reach the Steam Generators (SGs) because, due to an operator omission, the valves situated between the SGs and the pumps were closed instead of being open. These valves were reopened manually eight minutes later. Core meltdown Reactor core meltdown occurs when the nuclear fuel rods which contain uranium or plutonium and highly radioactive fission products start to overheat and then melt. It occurs in particular when a reactors stops being properly cooled. It is considered to be a severe nuclear accident because fissile materials can contaminate the environment with the release of numerous highly radioactive radioisotopes outside the reactor containment. UNIT 2 OF THE THREE MILE ISLAND NPP Reactor containment Relief valve Pressuriser Turbine building Radioactivity coming from the auxiliary building Primary cooling system Condenser Secondary cooling system Auxiliary building Turbine 12 • Les cahiers Histoire de l’ASN • November 2023
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