What Did We Learn From Three Mile Island?

By Rod Adams

Thirty-five years ago this week, a nuclear reactor located on an island in the Susquehanna River near Harrisburg, Pennsylvania, suffered a partial core melt.

On some levels, the accident that became known as TMI (Three Mile Island) was a wake-up call and an expensive learning opportunity for both the nuclear industry and the society it was attempting to serve. Some people woke up, some considered the event a nightmare that they would do anything to avoid repeating, and some hard lessons were properly identified and absorbed. Unfortunately, some people learned the wrong lessons and some of the available lessons were never properly interpreted or assimilated.

The melted fuel remained inside the TMI unit 2 pressure vessel, nearly all the volatile and water-soluble fission products remained inside the reactor containment, and there were no public health impacts. The plant was a total loss after just three months of commercial operation, the plant buildings required a clean-up effort that took 14 years, the plant owner went bankrupt, and the utility customers paid dearly for the accident.

The other unit on the same site, TMI-1, continues to operate well today under a different owner.

Although the orders for new nuclear power plants had already stopped several years before the accident, and there were already people writing off the nuclear industry’s chances for a recovery, the TMI accident’s emotional and financial impacts added another obstacle to new plant project development.

In the United States, it took more than 30 years to finally begin building new nuclear power plants. These plants incorporate some of the most important lessons in their design and operational concepts from the beginning of the project development process. During the new plant construction hiatus, the U.S. electricity industry remained as dependent as ever on burning coal and burning natural gas.

Aside: A description of the sequence of events at TMI is beyond the scope of this post. There is a good backgrounder—with a system sketch—about the event on the Nuclear Regulatory Commission’s web site. Another site with useful information is Inside TMI Three Mile Island Accident: Moment by Moment. End Aside.


The TMI event was the result of a series of human decisions, many of which were made long before the event or in places far from the control room. Of those decisions, there were some that were good, some that were bad, some that were reactions based on little or no information, and many made without taking advantage of readily available information.

One of the best decisions, made long before the event happened, was the industry’s adoption of a defense-in-depth approach to design. From the very beginning of nuclear reactor design, responsible people recognized that bad things could happen, that it was impossible to predict exactly which bad things could happen, and that the public should be protected from excess exposure to radioactive materials through the use of multiple barriers and appropriate reactor siting.

The TMI accident autopsy shows that the basic design of large pressurized water reactors inside sturdy containment buildings was fundamentally sound and adequately safe. As intended by the designers, the defense-in-depth approach and generous engineering margins allowed numerous things to go wrong while still keeping the vast majority of radioactive materials contained away from humans. Here is a quote from the Kemeny Commission report:

We are convinced that if the only problems were equipment problems, this Presidential Commission would never have been created. The equipment was sufficiently good that, except for human failures, the major accident at Three Mile Island would have been a minor incident.

Though it is not well-known, the NRC completed a study called the State of the Art Reactor Consequences Analysis (SOARCA aka NUREG-1935) that indicated that there would be few, if any, public casualties as the result of a credible accident at a U.S. nuclear power plant, even if there were a failure in the containment system.

One of the most regrettable aspects of TMI was that the heavy investment that the United States had made into the infrastructure for manufacturing components and constructing large nuclear power plants—factories, equipment, and people— was mostly lost, even though the large components and basic design did what they were supposed to do.

There were, however, numerous lessons learned about specific design choices, control systems, human machine interfaces, training programs, and information sharing programs.

Emergency core cooling

The Union of Concerned Scientists and Ralph Nader’s Critical Mass Energy Project had been warning about a hypothetical nuclear reactor accident for several years, though it turns out that they were wrong about why the emergency core cooling system did not work as designed.

The core damage at TMI was not caused by a failure of the cooling system to provide adequate water in the case of a worst case condition of a double-ended sheer of a large pipe; it was caused by a slow loss of cooling water that went unnoticed for 2 hours and 20 minutes. The leak, in this case, was a stuck-open relief valve that had initially opened during a loss of feedwater accident.

While the slow leak was in progress, the operators purposely reduced the flow of water from the high pressure injection pumps, preventing them from performing their design task of keeping the primary system full of water when its pressure is low.

It’s worthwhile to understand that the operators did not reduce injection flow by mistake or out of malice. They did what they had been trained to do. Their instructors had carefully taught them to worry about the effects of completely filling the pressurizer with water because that would eliminate its cushioning steam bubble. Their instructors and the regulators that tested them apparently did not emphasize the importance of understanding the relationship between saturation temperature and saturation pressure.

The admonition to avoid “going solid” (filling the pressurizer with water instead of maintaining its normal steam bubble) was a clearly communicated and memorable lesson in both classroom and simulator training sessions. When TMI control room operators saw pressurizer level nearing or exceeding the top of its indicating range, they took action to slow the inflow of water. At the time, they had still not recognized that cooling water was leaving the system via the stuck open relief valve.

The physical system had responded as it had been designed, but the designers had neglected to ensure that their training department fully understood the system response to various conditions that might be expected to occur. It’s possible that the designers did not know that a pressurizer steam space leak could cause pressure to fall and the pressurizer level to rise at the time that they designed the system. There was not yet much operating experience; the large plants being built in the 1960s and 1970s could not be fully tested at scale, and computer models have always had their limitations, especially at a time when processing power was many orders of magnitude lower than it is today.

There was also a generally accepted assumption that safety analysis could be simplified by focusing on the worst case accident.  If the system could be proven to respond safely to the worst case conditions, the assumption was that less challenging conditions would also be handled safely. The focus on worst case scenarios, emphasized by very public emergency core cooling system hearings, took some attention away from analyzing other possible scenarios.

Lessons learned

  • Following the TMI accident, there was a belated push to complete the loss of flow and loss of coolant testing program that the Atomic Energy Commission had initiated in the early 1960s. For a variety of political, financial, and managerial reasons, that program had received low priority and was chronically underfunded and behind schedule.
  • Today’s plant designs undergo far more rigorous testing programs and have better, more completely validated computer models.
  • Far more attention has been focused on the possible impact of events like “small break” loss of cooling accidents.
  • All new operators at pressurized water reactors learn to understand the importance of the relationship between saturation pressure and saturation temperature.

At the time of the accident, there was no defined system of sharing experiences gained during reactor plant operation with all the right people. TMI might have been a minor event if information about a similar event at Davis-Besse, a similar but not identical plant, that happened in September 1977 had made it to the control room staff at TMI-2.

Certain sections of the NRC knew about the Davis-Besse event, engineers at the reactor supplier knew about it, and even the Advisory Committee on Reactor Safeguards was aware of the event, but there was no established process for sharing the information to other operating units.

Lesson learned: After the accident, the industry invested a great deal of effort into a sustained program to share operating experience.

The plant designers also did not do their operators any favors in the design and layout of the control room. Key indicators were haphazardly arranged, there were thousands of different parameters that could cause an alarm if out of their normal range, and there was no prioritization of alarming conditions.

Lesson learned: After the accident, an extensive effort was made to improve the control rooms for existing plants and to devise regulations that increased the attention paid to human factors, man-machine interfaces, and other facets of control room design. All plants now have their own simulators that are designed to mimic the particular plant and are provided with the same operating procedures used in the actual plant. Operators are on a shift routine that puts them in the simulator for a week at a time every four to six weeks.

The initiating failures that started the whole sequence took place in the steam plant, a portion of the power plant that was not subject to as much regulatory or design scrutiny as the portions that were more closely associated with the nuclear reactor and its direct cooling systems.

Lesson still being learned: An increased level of attention is now paid to structures, systems, and components that are not directly related to a reactor, but there is still a confusing, expensive, and potentially vulnerable system that attempts to classify systems and give them an appropriate level of attention.

For at least 10 years prior to March 28, 1979, there had been an increasingly active movement focused on opposing the use of nuclear energy, while at the same time the industry was expanding near many major media markets and was one of the fastest growing employment opportunities, especially for people interested in technical fields. The technology was often in the spotlight, with the opposition claiming grave safety concerns and the industry—rather arrogantly, quite frankly—pointing to what had been a relatively unblemished record.

The industry did not do enough in the way of public outreach or routine advertising to explain the value of their product. They rarely compared the characteristics of nuclear energy against other possible electricity sources—mainly because there are no purely nuclear companies. In addition, the electric utility industry has a long tradition of preferring to be quiet and left alone.

The accident at TMI developed slowly over several days, but it became a major news story by mid-morning on the first day. Not only was it a “man bites dog” unusual event, but it was an event that the nuclear industry, the general public, the government, and the news media had been conditioned to take very seriously. Although nuclear experts from around the United States sprang into action to assist where they could at the plant itself, there was no established group of communications experts who could help reporters understand what was happening.

No reporter on a deadline is motivated or willing to wait for information to be gathered, evaluated, and verified. In the absence of real experts willing to talk, they turned to activists with impressive sounding credentials who were quite willing to speculate and spin tall tales designed to generate public interest and concern.

Lesson not yet learned: Although most decision makers in the nuclear industry understand the importance of planned maintenance systems to keep their equipment in top condition and the importance of a systematic approach to training to keep their employees performing at the top of their game, they have not yet implemented an effective, adequately resourced, planned communications program that helps to ensure that the public and the media understand the importance of a strong nuclear energy sector.

Planned communications efforts have a lot in common with planned maintenance systems. They might appear to be expensive with little immediate return on investment, but repairing a broken public image is almost as challenging and expensive as repairing a major plant component that failed due to a decision to reuse a gasket or postpone an oil change. As the guy in the commercial says, “You can pay me now or pay me later.”

That is probably the most tragic part of the TMI event. Despite being the subject of several expensively researched and documented studies, countless articles, thousands of documented training events, and more than a handful of books, the event could have—and should have—made the established nuclear industry stronger and the electric power generation system around the world cleaner and safer.

So far, however, TMI Unit 2’s destruction remains a sacrifice made partially in vain to the harsh master of human experience.

Note: I have purposely decided to avoid attempting to discuss the performance of the NRC or to judge their implementation of the lessons that were available to be learned. That effort would require a post at least twice as long as this one.

Additional Reading

General Public Utilities (March 28, 1980) Three Mile Island: One Year Later

Gray, Mike, and Rosen, Ira The Warning: Accident at Three Mile Island a Nuclear Omen for the Age of Terror W. W. Norton, 1982

Ford, Daniel Three Mile Island: Thirty Minutes to Meltdown Penguin Books, 1981

Hampton, Wilborn Meltdown: A Race Against Disaster at Three Mile Island A Reporter’s Story Candlewick Press, 2001

Report of the President’s Commission On The Accident At Three Mile Island. The Need for Change: The Legacy of TMI, October 1979

Three Mile Island A Report to the Commissioners and to the Public, January 1980

three mile island 300x237




Rod Adams is a nuclear advocate with extensive small nuclear plant operating experience. Adams is a former engineer officer, USS Von Steuben. He is the host and producer of The Atomic Show Podcast. Adams has been an ANS member since 2005. He writes about nuclear technology at his own blog, Atomic Insights.

14 thoughts on “What Did We Learn From Three Mile Island?

  1. radams


    Thank you for your comment. I think you will be interested in a guest post that I will soon publish on Atomic Insights from Michael Derivan. He was the senior reactor operator at Davis-Besse in November 1977 when that plant experienced a loss of feed accident and a resulting increase in primary pressure with a lifting of the PORV that stuck open.

    His event ended differently, but up until minute 20, its progress was virtually identical to TMI, including the operator actions. Mike has been thinking and digging through every piece of information he could find about the event ever since.

    His conclusions might cause some disturbance in the force.

  2. Rob Brixey

    Lesson Learned : Hands Off!

    To me, one of the most profound lessons of TMI was that if Operators picked up their lunchboxes and went home when the trip occurred, no fuel damage would have resulted.

    At the time in 1979, that was a tough concept for me. I was in the Navy operating a very “Hands On” type of reactor plant. I had to “unlearn” that tendency when becoming NRC Licensed as a Reactor Operator.

    High Pressure Safety Injection actuated at TMI. Low RCS Pressure associated with the open PORV is an initiation signal. Cold water was being supplied to the core automatically. Only operators could have stopped it, and they did.

    A combination of causal factors resulted in Pressurizer Level indication rising. Initially, the steam flow out the open PORV and the injection flow.
    After operators (not realizing they lost RCS subcooling) throttled SI flow, and secured vibrating (cavitating) RCPs – the steam bubble formation in the Reactor further pushed Pressurizer Level up.

    Had they taken an hour break or so to discuss the event (exaggeration), the overflowing Pressurizer Relief Tank (PRT) may have provided a sufficient cue to deduce that the PORV was still open. That leakage to the containment would have been comparatively free of fission products, as the cladding would have remained sufficiently cool. A mess for sure, but an easily recoverable one.

    During operator training, I still emphasize that automatic actuation signals are there for a reason. Know the basis, correct the cause, and make sure you don’t need the function to assure cooling or containment prior to resetting or overriding such signals. In the post TMI environment – it takes two licensed operators to ascertain an invalid actuation – prior to defeating the function.

  3. Alex DeVolpi

    Rod, reflecting on your TMI assessment, I don’t think you pursued one of the logical conclusions about available lessons not implemented. As you indicated, the meltdown “was caused by a slow loss of cooling water that went unnoticed for 2 hours and 20 minutes.” Ergo, wouldn’t it make sense to provide dependable instrumentation t0 monitor water level in a reactor?

    That was a conclusion I reached after TMI, while working at Argonne on reactor safety, leading to a patent filed for autonomous ex-vessel gamma-ray instrumentation that would independently monitor actual coolant level and density. Years later, French scientists independently validated the correlation of in-vessel water level with emerging high-energy gamma-rays level.

    Regrettably, builders and regulators have ignored that conspicuous capability, relegating autonomous water-level monitoring to the very bottom of all assessments (e.g. Fukushima Tier 3 and TMI NUREG-0933 IIF-1,2,3).

    The three loss-of-coolant accidents at Fukushima should have been another wake-up call, considering that resources might conceivably and arguably have been diverted to pre-empt one or two of the meltdowns — had the operators known that they were losing so much water over a couple of days.

    Determining that a container is emptying is really not such a difficult technical task. Placing so much blame on reactor operators was and is inappropriate, especially when these highfalutin commissions have failed to prioritize the obvious.

  4. Virgil Cox

    Thank you. The articles was great. Lots of details about various things that needed to be improved. From an over all perspective however, I see the biggest lesson learned being that business as usual was not going to work in the nuclear electric industry. Political and business decisions following that incident essentially instantiated a culture of safety in commercial nuclear power that it “borrowed” from the military (mostly Admiral Rickover). That was the game changer! It came out of the world of submarines. It is still the aspect of nuclear power in this country that differentiates us from many other industries locally and from many international nuclear power systems.

  5. Dave Hancock

    Why didn’t General Electric apply forewarned knowledge to Fukushima? Mechanical Mechanics, do not serve society, nor the nuclear industry. Selfish interest only serve the maintenance operations of the designing corporation.

  6. mjd

    This is one of the best high level synopsis of the TMI accident event I have ever read. It absolutely touches on all the significant issues, the pluses and the minuses of nuclear power operation. And it cuts right to the bone on the issue. That is nuclear power demands respect, at absolutely all levels involved, be it operations, regulatory, and including accurate media attention. Unfortunately it has to compete with other options on an un-level playing field due to special interests and political short sightedness. Which in any world results in risk taking to survive. Compared to all alternatives for the long-range survival and prosperity of planet earth it represents the solution, not the problem. It is really simple, do you want door number 1 or door number 2? Don’t believe the hype, from either side, learn what is behind both doors. Then you choose. An old saying, be part of the solution or part of the problem. That choice is clearly yours.

  7. Rod Adams

    @K. K. S. Pillay

    Nuclear professionals need to take responsibility for helping people, including the press, to understand enough about our technology so that they become comfortable and understand the full spectrum of risk from zero, to concern, to need to take action.

    We cannot expect people to “educate themselves” and we cannot expect the media to automatically tell our story. There is a proven method available for helping people to become comfortable with complex, useful, but sometimes scary technology like commercial airplanes or personal computers. (Yes, I am old enough to remember working with people who were scared of PCs).

    This is not an effort that should be arrogantly dismissed as something that should have been the responsibility of schools, the education system, or someone else. It is our job. It will not happen without an investment that should be considered to be as important as planned maintenance, or regulatory relations.

    We need the public’s approval and we need to do the work necessary to obtain it.

  8. Mitch

    >>> K. K. S. Pillay Part of the problem was an ignorant free press taking liberties. This still remains a problem. <<<

    The press is NOT ignorant or "misinformed"! They have an anti-nuclear bias & agenda based on nuclear gripes and guilt-trips all the way back to kids getting "uniquely and evilly" crisped at Hiroshima. NPR just as much stated that it was a civic duty for journalists and reporters to malign and dispose of nuclear power (a "child of war") by any means and misinformation as possible.

    You have to fight FUD in the public arena with Ads — LOTS of good educational ads, because the media is NEVER going to be fair about nuclear.

    Not in our grandkid's lifetimes!

  9. K. K. S. Pillay

    I read with interest Rod Adam’s excellent summary of TMI-2 accident and five of the comments so far. Since I was there at that time and actively participated in many of the recovery operations both by the State, Federal Govt. and especially by the largest private industry (Hershey) there. I was delighted to read this report as I had read most of the reports that was ever written on the subject for a decade after the accident.

    Yes, there was no public health impact in the sense of large radiation release etc. But, the trauma and psychological strains people suffered were immeasurable. Part of the problem was an ignorant free press taking liberties. This still remains a problem.

    I had the privilege of planning to feed 4000 milking cows and monitoring their milk for a very long period. Because all the fodder stored on site were considered “contaminated” and industry competitors were already designing publicity commercials with a trefoil sign on a chocolate bar.

    To me one thing that stood out was the poorly trained group of people at that particular shift was one of the primary reasons that accident happened and the way it happened. I had known some of them. I assume all the managers of the industry properly address this important problem and should continue to be cognizant of this one avoidable problem.

  10. Rod Adams

    @Thomas Murray

    If perfection is your requirement, we might as well give up. No human activity is perfect or perfectly safe. We cannot assure anyone that there will not be another event, even if all of our new plants are much better than existing ones.

    There are more than 400 nuclear power plants operating in the world today. There are a variety of operating companies, regulatory bodies, and geographical locations. I expect there will be more events, and probably some that will release radioactive material.

    Though I’d love for perfection to be possible, that is just not reality.

    In my opinion, however, the historical record of nuclear energy, even when you include Windscale, TMI, Chernobyl and Fukushima is acceptably safe, especially when compared against the alternative power sources.

    Nuclear with occasional accidents is far better than coal on a day to day basis, and coal is not immune to accidents and large area spills.

    Have you paid any attention to the Duke Energy coal ash spill into the Dan River? Do you have any idea how long that material will be harming the environment?

    This is why I say we need better communications. We need to help people understand that the benefits brought by using emission free nuclear energy are worth some level of risk and an occasional accident. We can always seek to get better, but we will never be perfect.

  11. Thomas Murray

    As a trained nuclear engineer who worked at both the Humboldt Bay Power Plant (a BWR) and the Diablo Canyon Power Plant (PWRs), I appreciate the brief outline of the TMI event. I’m concerned about the communications concern of this article. How will an improved communications plan prevent the next event? Since Fukushima, the industry has again had to face the issue: how do we affirm to the public that nuclear power is safe. Therefore, a communications plan by the industry seems irrelevant until we can assure that public that another TMI or Fukushima event will not occur. Clearly, TMI was nothing compared to the outcome of Fukushima. Nonetheless, we (the nuclear industry) have much work to convince ourselves and the public that another TMI or Fukushima will not occur.

  12. Roger Blomquist

    I think there is still much more the industry must do to foster public acceptance. In particular, advertising. For an example, see the Norfolk Southern Railway (and other) commercials on the Sunday morning CNN news shows. And all of us nuclear professionals need to cultivate relationships with journalists to give them actual expert sources instead of the “public policy” ones. This is a professional duty, not an option.

  13. David Walters

    Rod, this was the most concise and interesting rendition of the events leading up to, and the effects of, the accident at TMI. I was living in Pittsburgh when this event took place. I still remember, when going to TMI a month later for the first big anti-nuke demonstration, the stream of moving vans heading west along the Penns. Turnpike.

    I suggest anti nukes read the “lessons learned” which is part of my response to “what about TMI?” when it is raised.

    I thought the point about control room layout was insightful. In the fossil plants I’ve worked in both alarm, and critical freed back indications (pressures, turbine speed, temperatures of all sorts) are super important during times of crisis, and knowing where to look, to use a kind of ‘triage’ for prioritizing alarms and then operator response simply cannot be underestimated in it’s importance. Most anti-nukes thinks “nothing has changed” since TMI. It is so sad they are stuck in the past.

    David Walters

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