Remembering the Three Mile Island Accident and Its Impact on the Nuclear Power Industry

David Passehl

The 1979 Three Mile Island nuclear accident in PA is the most commercial nuclear power plant accident in U.S. history.

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Recently, Netflix released a new docuseries on the 1979 nuclear accident at Three Mile Island, Unit 2 (TMI-2) nuclear power plant. Given the interest the show has garnered, it seems like an appropriate time to share my perspective of the accident and its outcome, as someone who worked in both the military (Navy nuclear propulsion) and civilian (commercial plant) side of nuclear power. The safety and reliability of today's nuclear power is vastly improved since the days of TMI-2.

What Happened at TMI-2?

On March 28, 1979, the TMI-2 accident began with a loss of feedwater to the steam generators. Without adequate feedwater flow to remove the heat generated by the reactor, primary system pressure increased until the reactor automatically shut down. Seconds later, a pilot-operated relief valve (PORV) on the pressurizer opened to relieve the high pressure. However, because the valve did not reclose as intended, primary coolant in the form of steam continued to escape out the open valve and into the containment building. It took about 140 minutes for a plant operator to close another" block" valve to stop the outflow.

Despite numerous alarms and warning lights, operators did not realize that a loss-of-coolant accident was in progress. Complicating matters were inadequate and misleading instrumentation readouts which erroneously led operators to believe that the PORV was closed and that the pressurized water level height was enough to properly cover the reactor core with cooling water.

The full extent of the accident was not understood until later, including the realization that partial melting of the reactor fuel had occurred. Communications released to the public about the impact of the event in the first days were muddled and ambiguous. It soon became apparent that TMI station and government officials were ill-prepared to deal with nuclear emergencies.

Aftermath of the Accident

Although the TMI-2 accident caused no deaths or injuries at the time, it was the most serious incident in U.S. commercial nuclear power history and became the benchmark for evaluating nuclear risk. Public interest in the nuclear industry increased dramatically in the aftermath, and the accident's effects on nuclear safety and regulations are still felt today.

The NRC and the U.S Department of Energy (DOE) have intensively studied and documented the TMI-2 accident. The accident led the U.S. Nuclear Regulatory Commission (NRC) to clamp down its oversight of the nuclear power industry by mandating numerous safety improvements. For its part, the nuclear industry took aggressive action to improve plant safety as well. The industry made improvements in emergency response planning, reactor operator training, human factors engineering, radiation protection, and many areas of plant operations and design.

In 1985, the industry formed the Institute of Nuclear Power Operations (INPO), whose mission is to promote the highest levels of safety and reliability of nuclear plants. To improve training, INPO formed the National Academy for Nuclear Training, which reviews and accredits nuclear utilities training programs for all key plant positions.

Current Status of TMI-2

Today, TMI-2 is permanently shut down. Its fuel has been removed, the coolant system is fully drained, and the radioactive water has been decontaminated and evaporated. The accident's radioactive waste was shipped off-site to an appropriate disposal area, and the reactor fuel and core debris were shipped to the DOE's Idaho National Laboratory.

In 2020, ownership of TMI-2 was transferred to EnergySolutions for completing reactor decommissioning. In-plant and off-site monitoring of TMI-2 continues until the plant it is fully decommissioned. More than a dozen studies conducted since 1981 have found no discernible direct health effects on the population in the vicinity of the plant. Regular reports are made to the NRC, the Commonwealth of Pennsylvania and the public. TMI-1 permanently shut down on September 20, 2019.

Our Commitment to Safe Nuclear Energy

Nuclear energy needs to be a part of the global climate change solution. Nuclear power is a zero-emission clean energy source, has a small land footprint and produces minimal waste, which can be reprocessed and recycled. Jensen Hughes is committed to providing the highest quality engineering, research, consulting and management services to the nuclear industry. Our services help support the safe, economical, continuous and secure delivery of electric power in concert with supply from other carbon-free sources, such as solar and wind.


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