Three Mile Island does still contain radioactive material, but radiation levels in the surrounding area are indistinguishable from natural background. The 1979 accident released far less radiation than most people assume, and decades of cleanup have removed 99% of the damaged fuel from the reactor. What remains is a decommissioning project, not an ongoing hazard to nearby communities.
Radiation Levels Around the Site Today
The area surrounding Three Mile Island receives about 100 to 125 millirem of natural background radiation per year, which is actually below the national average of roughly 300 millirem. Even at the peak of the 1979 accident, the maximum dose a person standing at the site boundary would have received was less than 100 millirem above that background level. For comparison, a single chest X-ray delivers about 10 millirem, and a cross-country flight exposes you to around 3 to 5 millirem.
Pennsylvania’s Department of Environmental Protection runs an ongoing monitoring program that samples air, water, and milk near the plant. The 2022 results found no reactor-related isotopes in surface water or milk samples collected from surrounding farms. Airborne radioactivity readings near Three Mile Island matched readings at monitoring stations near other Pennsylvania nuclear plants and at a control station in Harrisburg. In practical terms, living near Three Mile Island exposes you to no more radiation than living anywhere else in central Pennsylvania.
What Happened to the Damaged Reactor
The accident occurred in Unit 2, one of two reactors at the site. About half of the reactor core melted during the event, but the containment building held. A massive cleanup effort ran from 1979 into the 1990s and removed approximately 99% of the fuel and radioactive debris from the reactor vessel. That material was shipped to the Department of Energy’s facility in Idaho, where it sits in dry storage canisters at a dedicated spent fuel storage installation.
Roughly 1% of the original fuel and debris remains inside the Unit 2 reactor vessel. This residual material is radioactive, but it’s sealed within the containment structure and continuously monitored. The storage facility in Idaho undergoes regular NRC inspections covering radiation safety, air sampling, surface contamination surveys, and direct radiation measurements using dosimeters placed along the perimeter fence. Monthly air sampling and annual contamination surveys have consistently shown readings well within safe thresholds.
The Decommissioning Timeline
Unit 2 sat in a monitored storage state for decades after the initial cleanup. In March 2023, the NRC approved a transition from that holding pattern to active decommissioning, known as the DECON method, which means dismantling the facility rather than letting it sit for additional decades. A company called TMI-2 Solutions is managing the work.
The project is broken into phases. The first phase of field work is scheduled for completion by March 2029. A second phase covering final decommissioning, site surveys, and restoration runs from 2029 through 2037. After that comes a lengthy administrative process: final status surveys to confirm the site meets regulatory cleanup standards are planned for 2033 to 2037, with full license termination projected for 2053. That distant date reflects the regulatory paperwork and verification involved, not an expectation of ongoing danger.
Unit 1 and the Restart Plan
Unit 1 is the reactor that wasn’t involved in the 1979 accident. It operated safely for decades afterward before its owner, Constellation Energy, shut it down in September 2019 for economic reasons. The reactor and its systems are intact.
In late 2024, Constellation notified the NRC that it wants to bring Unit 1 back online, now rebranded as the Crane Clean Energy Center. The restart is tied to a deal to supply power to Microsoft’s data center operations. The NRC created a dedicated restart panel to evaluate whether the plant can safely return to service after sitting idle for several years. Public meetings began in early 2025, and the review process is ongoing. If approved, Unit 1 would generate carbon-free electricity, which is the primary motivation behind the effort.
Trace Releases That Still Occur
Even in its current shutdown state, Three Mile Island releases tiny amounts of tritium, a weakly radioactive form of hydrogen. The 2024 annual effluent report shows that Unit 1 released about 1.26 curies of tritium into the atmosphere over the entire year, and small quantities entered the Susquehanna River through groundwater. Unit 2’s releases were far smaller, on the order of thousandths of a curie.
These numbers sound alarming only if you’re unfamiliar with the scale. Tritium is the least energetic radioactive isotope commonly encountered. It cannot penetrate skin and poses a health risk only if consumed in large quantities. The amounts released from Three Mile Island are a fraction of what operating nuclear plants routinely and legally discharge, and they register as undetectable in the environmental water and air samples collected around the site. No reactor-related isotopes have been found in local milk, drinking water, or air monitoring stations.
What the Health Data Shows
The original accident exposed the roughly 2 million people living within a 50-mile radius to an average additional dose of about 1 millirem, a negligible amount. Multiple epidemiological studies conducted in the years and decades after the accident found no measurable increase in cancer rates among the surrounding population. Pennsylvania’s environmental radiation report summarizes it plainly: residents have not been exposed to radiation levels from the facility that would affect their health.
So while Three Mile Island is technically still radioactive in the sense that radioactive materials exist on site and trace tritium is released, the practical answer is that it poses no meaningful radiation risk to the surrounding community. The residual contamination is contained, monitored, and in the process of being permanently cleaned up.