Key dates at a glance
Three Mile Island (TMI), near Middletown, Pennsylvania, is best known for the March 1979 partial core meltdown in TMI-2. The plant ultimately closed in stages: TMI-1 was shut down in 2019 following a decision announced in 2017, and TMI-2 never returned to service after the accident. This article clarifies the shutdown timeline with verified dates and context.
| Date | Event | Why it matters |
|---|---|---|
| March 28, 1979 | TMI-2 partial loss of coolant accident (LOCA) | Most significant U.S. commercial nuclear incident; no public health impact confirmed but led to long-term shutdown discussions. |
| May 1979 | TMI-2 declared permanent loss of core | Unit 2 effectively ended operations; defueling began in 1982. |
| 1985 | TMI-2 core removal complete | Active fuel removed; unit remained in SAFSTOR (monitored storage). |
| 2017 | Exelon Generation announces TMI-1 closure decision | Company cited market conditions, low prices, surplus generation, and energy efficiency reducing demand. |
| September 20, 2019 | TMI-1 finally shuts down | 36 years after the accident; unit formally retired and entered SAFSTOR decommissioning phase. |
Status clarification: Three Mile Island did not shut down immediately after the accident
Following the March 28, 1979 incident, TMI-1 continued operating for years. Nuclear regulatory and medical studies determined the small radioactive releases inside containment posed no public health threat. TMI-2, however, never restarted; it was permanently retired and defueled. TMI-1 operated until Exelon’s 2017 market-driven decision and closure in 2019. The plant’s current status is retired; TMI-2 remains in SAFSTOR with decommissioning planned.
Understanding the March 1979 accident at Three Mile Island
The sequence on March 28, 1979 began with a cooling malfunction and stuck-open relief valve in TMI-2. Automation responded as designed to maintain cooling, but operator misinterpretation and instrumentation issues led to partial fuel damage inside the reactor core. No breach occurred, and radiation releases were small. The accident changed nuclear plant design, operator training, and emergency planning across the U.S., and it kept TMI-2 from ever generating power again.
Unit-by-unit timeline and current status
TMI-1 and TMI-2 shared a site but had separate histories. TMI-1 operated from 1974 until the long-term decision to close in 2019; TMI-2 effectively ended on March 28, 1979. The table below summarizes their milestones in concise, comparable form.
| Attribute | Verified Detail | Source Type |
|---|---|---|
| TMI-1 commercial operation start | December 1974 | NRC / Exelon records |
| TMI-2 accident date | March 28, 1979 | NRC investigation reports |
| TMI-2 declared a permanent core loss | May 1979 | NRC / utility filings |
| TMI-1 closure decision announced | 2017 | Exelon Generation public statement |
| TMI-1 final shutdown date | September 20, 2019 | NRC license surrender and confirmation |
| TMI-2 defueling complete | 1985 | NRC historical records |
| Current disposition | Units in SAFSTOR decommissioning | NRC site status |
What actually happened in the 1979 accident
The loss of coolant event allowed part of the core to overheat and form a damaged mass, or “corium.” Very small amounts of radioactive gases and iodine escaped, primarily through engineered releases designed to limit pressure. Independent health reviews concluded these releases did not cause observable health effects. The lesson was procedural and training reform, not an indictment of nuclear power as an energy source. For TMI-2, there was no restart plan after 1979; the unit transitioned into SAFSTOR until permanent decommissioning activities. TMI-1 kept operating under stricter regulation and culture changes imposed by the NRC.
Why TMI-1 closed in 2019, not sooner
Exelon Generation’s 2017 decision to close TMI-1 reflected energy market realities rather than safety issues. Factors included low wholesale prices, abundant natural gas generation, growth in renewables, and declining demand driven by efficiency and customer-side resources. Operating a nuclear unit under these conditions was not economically sustainable. Additional regulatory and tax incentives did not materially alter the calculus. Unit 1 remained safe and compliant but not viable in a competitive resource portfolio, leading to the September 2019 shutdown and entry into SAFSTOR decommissioning.
Post-shutdown status and decommissioning path
Both TMI units are now retired. TMI-1 is in SAFSTOR, a monitored storage state with systems maintained for 60 years while radiation levels decline; full decontamination and dismantlement follow. TMI-2 has been in SAFSTOR since the 1980s, with ongoing environmental monitoring and institutional control. Waste management continues on secure on-site dry cask storage, coordinated with federal policy and the Yucca Mountain framework. No immediate radiological hazards remain, and the sites are managed under rigorous NRC oversight.
Key takeaways
- TMI-2 effectively ended on March 28, 1979 and never returned to service.
- TMI-1 shut down on September 20, 2019, 36 years after the accident, due to market conditions.
- Health studies found no public harm from the small radioactive releases.
- Both units are now in SAFSTOR decommissioning, with long-term waste management plans in place.
Frequently asked questions
| Question | Answer |
|---|---|
| Did the 1979 accident cause immediate deaths? | No; health studies found no immediate or observable health effects. |
| Is Three Mile Island still radioactive? | Yes, but contained; units are in SAFSTOR and monitored, with low-level waste managed on-site. |
| Will the site be cleared entirely? | Decommissioning is planned after SAFSTOR; final land use decisions remain with regulators and stakeholders. |
| What changed for nuclear plants after 1979? | Stricter regulation, improved operator training, enhanced safety systems, and more robust emergency planning. |