Three Mile Island is not dangerous to people living nearby. The radiation dose to the public from the facility in 2023 was 0.013 millirem per year, a tiny sliver of the 311 millirem Americans receive annually from natural background sources like soil, rocks, and cosmic rays. That means living next to Three Mile Island exposes you to roughly 24,000 times less radiation than you already get from nature.
What Happened to the Damaged Reactor
The 1979 accident involved Unit 2, which partially melted down when cooling systems failed. That reactor was permanently shut down and fully defueled in the years following the accident. The reactor fuel and core debris were shipped off-site to a Department of Energy facility. Radioactive water was decontaminated and evaporated, and radioactive waste was sent to appropriate disposal sites. What remains is an empty, drained reactor vessel under long-term monitored storage.
Unit 1, the undamaged reactor, operated independently for decades after the accident and was shut down in 2019 for economic reasons. Constellation Energy announced plans to restart Unit 1 to supply power under a deal with Microsoft, but that process involves fresh regulatory review. Spent nuclear fuel from Unit 1’s years of operation is stored on-site in water-filled pools and dry storage casks, which are sealed metal cylinders enclosed in metal or concrete outer shells. This is standard practice at every nuclear plant site in the country, since no permanent national repository for spent fuel exists yet.
Current Radiation Monitoring
The NRC requires ongoing environmental monitoring around Three Mile Island, and the 2023 annual report confirmed that doses to the public from the facility’s remaining materials were “well below applicable dose limits and only a small fraction of the doses received from natural background radiation.” Crucially, the monitoring also found no permanent buildup of radioactive materials in the surrounding environment and no increase in background radiation levels over time. The site is not leaking, and the area around it reads essentially the same as anywhere else in central Pennsylvania.
Did the Accident Cause Health Problems?
This is the question that has lingered for over four decades. The Pennsylvania Department of Health tracked 32,135 residents who lived near the plant at the time of the accident, following them from 1979 through 1998. Overall cancer death rates in that group were essentially the same as the surrounding local population. Radiation exposure from the accident was not a significant predictor of lung cancer, overall cancer, or heart disease after adjusting for other risk factors.
A few findings stood out without being conclusive. Men with the highest estimated radiation exposure showed elevated rates of lymphatic and blood cancers compared to men with the lowest exposure, in a pattern that suggested a possible dose-response relationship, though the statistical test for a true trend was not significant. Women with higher exposure showed a modest upward pattern in breast cancer rates, but again without reaching statistical certainty. The study’s authors summarized it plainly: there is no consistent evidence that the accident significantly impacted overall mortality, but certain elevations persist, and a connection cannot be definitively ruled out.
The radiation released during the 1979 accident was far smaller than what escaped from disasters like Chernobyl or Fukushima. Most estimates put the average dose to people living within 10 miles at about 8 millirem, roughly equivalent to a chest X-ray.
Is the Susquehanna River Contaminated?
Three Mile Island sits on an island in the Susquehanna River, so water contamination has been a persistent concern. A 25-year monitoring program that began in 1979 tracked radionuclides in the river using periphyton (algae that cling to rocks and are especially good at absorbing trace radioactive particles) along with fish, mussels, crayfish, insects, deer, mushrooms, sediment, and garden vegetables like cabbage and tomatoes. The program was sensitive enough to detect fallout from the Chernobyl disaster thousands of miles away in 1986. Its conclusion: releases from the area’s nuclear operations have had no known environmental or human health impact on the river system.
Emergency Planning Around the Site
Nuclear plants in the U.S. maintain a 10-mile Emergency Planning Zone, or EPZ, with siren systems, evacuation routes, and potassium iodide distribution plans. These protocols remain in place around active nuclear facilities. For Three Mile Island specifically, the level of emergency preparedness depends on the operating status of Unit 1. If Constellation Energy succeeds in restarting the reactor, full emergency planning infrastructure would be required and reviewed as part of the licensing process.
For residents near any operating nuclear plant, the practical guidance is straightforward: if you hear a siren, do not evacuate immediately. Tune to an Emergency Alert System radio station for instructions. Evacuations, if ever ordered, are designed to give people plenty of time to act safely, alert neighbors, gather family members, and head to designated reception centers.
The Bottom Line on Safety Today
The damaged Unit 2 reactor is empty, defueled, and poses no radiological threat. Spent fuel from Unit 1 sits in the same type of storage used safely at dozens of sites nationwide. Environmental monitoring shows no contamination in the air, water, or surrounding land. The 1979 accident was the worst in American nuclear history, but what it left behind is a closely watched, well-contained site that contributes virtually zero radiation to the community around it.