The Three Mile Island nuclear power plant, located along the Susquehanna River in Pennsylvania (USA), was the scene of one of the most dramatic accidents in US nuclear history, classified at level 5 out of 7 on the International Nuclear and Radiological Event Scale (INES). To be clear, the Chernobyl nuclear disaster was classified as level 7.
The accident at Three Mile Island occurred at 04:36 on March 28, 1979, when the TMI-2 reactor was operating at 97% of its rated power. Even in the absence of victims or injuries, the event had significant consequences on a socio-perceptive level, substantially compromising public confidence in the safety of nuclear plants.
Characteristics of the Three Mile Island power plant in Pennsylvania
The Three Mile Island power plant, located along the Susquehanna River, was characterized by the presence of two pressurized water nuclear reactors (PWR):
- Unit 1, with an installed capacity of approximately 800 MW, has been in commercial operation since 1974
- Unit 2 (TMI-2), of approximately 900 MW, had been operational since 30 December 1978, just 90 days before the accident
The plant featured a technologically advanced engineering configuration for the 1970s, designed to ensure high standards of operational safety and long-term energy reliability. This structure resulted from the implementation of specific technical solutions and safeguard systems aimed at preventing and mitigating fault conditions. The PWR type reactor was equipped with the Emergency Core Cooling System, designed to automatically activate in the event of a loss of coolant accident. The latter carries out the essential function of removing the heat generated by the nuclear fission reaction, contributing to maintaining the core temperature within operating limits.
The dynamics of the most serious nuclear accident in the United States
At 04:36 on 28 March 1979, the simultaneous shutdown of the auxiliary circuit pumps resulted in the interruption of the correct supply of feed water to the steam generator, compromising the cooling of the TMI-2 reactor core, which at that time was operating at 97% of its nominal power. Although there was a sudden increase in the temperature of the core, the emergency protection and safety system intervened promptly by activating the “scram” of the reactor, i.e. the immediate arrest of the nuclear fission reaction by inserting the control rods into the core (core).
However, a combination of technical-system failures, the inadequacy of emergency protocols and human error factors transformed an initially manageable situation into a potentially catastrophic scenario. The complexity of the field instrumentation, together with control sensors that provided ambiguous and sometimes misleading indications on the state of the reactor, led the plant operators – with limited experience in managing emergency conditions – to misinterpret the nature of the event, leading to the adoption of operational maneuvers that worsened its evolution. The outcome was a partial meltdown of the reactor core; however, the integrity of the containment building was preserved, preventing significant release of radioactive material into the external environment.
The days following the accident were marked by a chaotic communication flow and little transparency. The plant managers, federal agencies and state authorities provided inconsistent information on several occasions, while numerous public officials learned of the event through media channels, rather than through official institutional communications. In that climate of uncertainty, the governor of Pennsylvania ordered the evacuation of pregnant women and preschool children within a five-mile radius of the plant area. Furthermore, thousands of residents of the island decided to voluntarily move away from their homes, waiting to receive clearer indications on the evolution of the event.
What were the consequences?
Although a limited amount of radioactive material was released into the atmosphere during the accident, no casualties or injuries were reported. Subsequent radiological monitoring activities and environmental sampling analyzes highlighted that the population’s exposure levels to ionizing radiation were negligible, allowing the onset of acute health effects on individuals to be excluded.
Nonetheless, the event led to a significant crisis of confidence in nuclear energy as a source of energy supply (for civil purposes). The Nuclear Regulatory Commission (NRC) suspended approval of new construction licenses for nuclear plants for several years, while numerous ongoing projects were substantially delayed or permanently canceled. The accident triggered a process of review of the regulatory framework and operational practices of the US nuclear sector. Emergency response procedures were updated, training and qualification programs for power plant personnel were strengthened and more stringent regulatory standards regarding the safety and management of nuclear plants were introduced.
The Three Mile Island nuclear power plant today and future prospects
The TMI-2 reactor involved in the accident has been definitively out of service since 1979. Unit 1, not affected by the event, continued commercial operation until September 2019, when it was deactivated following economic assessments and critical issues related to plant maintenance. Unit 1, however, could be reactivated starting from 2028 as part of an agreement stipulated between Microsoft and Constellation Energy, aimed at guaranteeing a twenty-year supply of sustainable electricity, intended to power the data centers and digital infrastructures owned by the technological giant founded by Bill Gates. A study commissioned by the Pennsylvania Building & Construction Trades Council estimated that the Crane Clean Energy Center (CCEC) project, planned to redevelop the plant, could generate approximately 3,400 jobs, significantly helping to stimulate the island’s economy.
The development of the program is associated with a series of technical-regulatory critical issues, with the disposal of radioactive waste representing one of the most controversial aspects. The irradiated (exhausted) fuel rods would initially be cooled in the site’s swimming pools and subsequently transferred to dry cask storage, located near the power plant, awaiting final transfer to the centralized national depot, once available. This temporary on-site storage solution is an element of concern for the resident communities, who fear a possible prolonged stay of radioactive material on their territory, transforming the island into a “permanent nuclear landfill”.
At the same time, the authorization process is particularly complex and long-lasting, as the NRC has never previously approved the reactivation of an already decommissioned nuclear plant. In this context, Constellation Energy, one of the main operators in the US energy sector, will be required to present technical evidence and detailed engineering documentation to demonstrate that Unit 1 can be brought back into operation in compliance with nuclear safety and public health protection requirements. Some environmental activists argue that the authorization outcome could be favorable in the medium term, considering the relevance of the economic and strategic interests connected to the project, despite the fact that critical positions remain regarding the reactivation of the plant.
