Chapter 34: Three Mile Island

The very first dreadful nuclear accident at the Three Mile Island Unit 2 (TMI-2) Nuclear Power Plant, Pennsylvania, USA, was occurred at 4 in the morning on March 28, 1979.  It lasted only five days as the crisis was officially declared to be over on April 2, 1979 but it provoked massive fear, stress, and confusion.  Rumors were dispersed piercingly about an uncontrolled release of radiation from the plant and on March 30th, the governor of Pennsylvania ordered the evacuation of children and pregnant women living within 5 miles of the plant.  In spite of the fact that there were no deaths or injuries to plant workers or members of the nearby community, it was considered to be the most serious accident in US commercial nuclear power plant operating history and it amplified the long global squabble over the safety of nuclear energy.Slide1Here is a summarized version of the activities based on the World Nuclear Association that took place on March 28, 1979:

1.     4:00 am on that day:

  • The reactor was operating at 97 percent and it involved a relatively minor malfunction in the secondary cooling circuit (TMI-2) which caused the temperature in the primary coolant to rise.  This in turn caused the reactor to shut down automatically which only took about one second;
  • Within seconds of the shutdown, the pilot-operated relief valve (PORV) on the reactor cooling system opened, as it was supposed to. About 10 seconds later it should have closed. But it remained open, leaking vital reactor coolant water to the reactor coolant drain tank;
  • The operators believed the relief valve had shut because instruments showed them that a “close” signal was sent to the valve. However, they did not have an instrument indicating the valve’s actual position;
  • Responding to the loss of cooling water, high-pressure injection pumps automatically pushed replacement water into the reactor system. As water and steam escaped through the relief valve, cooling water surged into the pressurizer, raising the water level in it;
  • Based on their training, operators responded by reducing the flow of replacement water;
  • It caused steam to form in the reactor primary cooling system, pumping a mixture of steam and water which caused the reactor cooling pumps to vibrate;
  • Because the severe vibrations could have damaged the pumps and made them unusable, operators shut down the pumps; and
  • This ended forced cooling of the reactor core. However, as reactor coolant water boiled away, the reactor’s fuel core was uncovered and became even hotter. The fuel rods were damaged and released radioactive material into the cooling water.

2.      At 6:22 am on that day:

  • Operators closed a block valve between the relief valve and the pressurizer;
  • This action stopped the loss of coolant water through the relief valve. However, superheated steam and gases blocked the flow of water through the core cooling system
  • Throughout the morning, operators attempted to force more water into the reactor system to condense steam bubbles that they believed were blocking the flow of cooling water; and
  • During the afternoon, operators attempted to decrease the pressure in the reactor system to allow a low pressure cooling system to be used and emergency water supplies to be put into the system.

3.     At 7:50 pm on that day:

  • Operators restored forced cooling of the reactor core when they were able to restart one reactor coolant pump. They had condensed steam so that the pump could run without severe vibrations.

Radioactive gases from the reactor cooling system built up in the makeup tank in the auxiliary building. During March 29 and 30, operators used a system of pipes and compressors to move the gas to waste gas decay tanks. The compressors leaked, and some radioactive gas was released to the environment. These went through high-efficiency particulate air (HEPA) filters and charcoal filters which removed most of the radionuclides, except for the noble gases, the estimated total of which was about 370 PBq (the Kemeny Commission said “a maximum of 480 PBq noble gases” and NRC also quotes 1.6 PBq of krypton released in July). With short half-life and being biologically inert, these did not pose a health hazard.Slide2When the reactor’s core was uncovered, on the morning of 28 March, a high-temperature chemical reaction between water and the zircaloy metal tubes holding the nuclear fuel pellets had created hydrogen gas. In the afternoon of 28 March, a sudden rise in reactor building pressure shown by the control room instruments indicated a hydrogen burn had occurred. Hydrogen gas also gathered at the top of the reactor vessel.

From 30 March through 1 April operators removed this hydrogen gas “bubble” by periodically opening the vent valve on the reactor cooling system pressurizer. For a time, regulatory officials from Nuclear Regulatory Commission (NRC) believed the hydrogen bubble could explode, though such an explosion was never possible since there was not enough oxygen in the system.Slide3On 27 April, after a month of the crisis, operators established natural convection circulation of coolant. The reactor core was being cooled by the natural movement of water rather than by mechanical pumping. The plant was in “cold shutdown” (i.e. with the water at less than 100°C) at atmospheric pressure.

The head of the reactor pressure vessel was removed in July 1984 allowing access to the remains of the core. Subsequent investigation revealed that at least 45 percent of the core – 62 tonnes – had melted and 19 tonnes of this had ended up in the lower plenum, mostly in the lower head of the reactor pressure vessel, but without seriously damaging the vessel. Most of the melted core material (corium) had remained in the core region. In 1988 a multinational OECD Vessel Investigation Project (VIP) took samples to evaluate the situation in detail and confirmed that there was much less damage than anticipated.

As far as the health effects are concerned, the Three Mile Island accident caused concerns about the possibility of radiation-induced health effects, principally cancer, in the area surrounding the plant. Because of those concerns, the Pennsylvania Department of Health for 18 years maintained a registry of more than 30,000 people who lived within five miles of Three Mile Island at the time of the accident. The state’s registry was discontinued in mid-1997, without any evidence of unusual health trends in the area.  Furthermore, more than a dozen major, independent health studies of the accident showed no evidence of any abnormal number of cancers around TMI years after the accident. The only detectable effect was psychological stress during and shortly after the accident.

Furthermore, the studies found that the radiation releases during the accident were minimal, well below any levels that have been associated with health effects from radiation exposure. The average radiation dose to people living within 10 miles of the plant was 0.08 millisieverts, with no more than 1 millisievert to any single individual. The level of 0.08 mSv is about equal to a chest X-ray, and 1 mSv is about a third of the average background level of radiation received by U.S. residents in a year.

In June 1996, 17 years after the TMI-2 accident, Harrisburg U.S. District Court Judge Sylvia Rambo dismissed a class action lawsuit alleging that the accident caused health effects. The plaintiffs have appealed Judge Rambo’s ruling. The appeal is before the U.S. Third Circuit Court of Appeals. However, in making her decision, Judge Rambo cited:

  • Findings that exposure patterns projected by computer models of the releases compared so well with data from the TMI dosimeters (TLDs) available during the accident that the dosimeters probably were adequate to measure the releases;
  • That the maximum offsite dose was, possibly, 100 millirem (1 mSv), and that projected fatal cancers were less than one; and
  • The plaintiffs’ failure to prove their assertion that one or more unreported hydrogen “blowouts” in the reactor system caused one or more unreported radiation “spikes”, producing a narrow yet highly concentrated plume of radioactive gases.

Slide4Judge Rambo concluded: “The parties to the instant action have had nearly two decades to muster evidence in support of their respective cases.  The paucity of proof alleged in support of Plaintiffs’ case is manifest. The court has searched the record for any and all evidence which construed in a light most favourable to Plaintiffs creates a genuine issue of material fact warranting submission of their claims to a jury. This effort has been in vain.”

More than a dozen major, independent studies have assessed the radiation releases and possible effects on the people and the environment around TMI since the 1979 accident at TMI-2. The most recent was a 13-year study on 32,000 people. None has found any adverse health effects such as cancers which might be linked to the accident.

When TMI-1 restarted in October 1985, General Public Utilities pledged that the plant would be operated safely and efficiently and would become a leader in the nuclear power industry. Those pledges have been kept.

  • The plant’s capability factor for 1987, including almost three months of a five-month outage of refueling and maintenance was 74.1 percent, compared to an industry average of 62 percent. Capability factor refers to the amount of electricity generated compared to the plant’s maximum capacity;
  • In 1988 a 1.3 percent (11 MWe) uprate was licensed;
  • For 1989, TMI-1’s capability factor was 100.03 percent and the best of 357 nuclear power plants worldwide, according to Nucleonics Week;
  • In 1990-91, TMI-1 operated 479 consecutive days, the longest operating run at that point in the history of US commercial nuclear power. It was named by the NRC as one of the four safest plants in the country during this period;
  • By the end of 1994, TMI-1 was one of the first two plants in the history of US commercial nuclear power to achieve a three-year average capability factor of over 90 percent (TMI-1 had 94.3 percent);
  • In October 1998, TMI workers completed two full years without a lost workday injury;
  • Since its restart, TMI-1 has earned consistently high ratings in the NRC’s program, Systematic Assessment of Licensee Performance (SALP);
  • In 2009, the TMI-1 operating licence was renewed, extending it life by 20 years to 2034; and
  • Immediately following this, both steam generators were replaced as TMI’s “largest capital project to date”

The accident in Three Mile Island was indeed a wakeup call to the custodians of nuclear energy around the world and as a result, it brought about widespread improvements with the focus on emergency response planning, radiation protection, reactor operator training, human factor engineering and many other areas of nuclear power plant operations.  Furthermore, the nuclear regulatory organizations around the world decided to tighten and heighten its regulatory oversight which resulted in enhanced nuclear safety for nuclear power plants.Slide5Here are some of the major changes which have occurred since the accident:

  • Upgrading and strengthening of plant design and equipment requirements. This includes fire protection, piping systems, auxiliary feedwater systems, containment building isolation, reliability of individual components (Pressure relief valves and electrical circuit breakers), and the ability of plants to shut down automatically;
  • Identifying human performance as a critical part of plant safety, revamping operator training and staffing requirements, followed by improved instrumentation and controls for operating the plant, and establishment of fitness-for-duty programs for plant workers to guard against alcohol or drug abuse;
  • Improved instruction to avoid the confusing signals that plagued operations during the accident;
  • Enhancement of emergency preparedness to include immediate NRC notification requirements for plant events and an NRC operations center that is staffed 24 hours a day. Drills and response plans are now tested by licensees several times a year, and state and local agencies participate in drills with the Federal Emergency Management Agency and NRC;
  • Establishment of a program to integrate NRC observations, findings, and conclusions about licensee performance and management effectiveness into a periodic, public report;
  • Regular analysis of plant performance by senior NRC managers who identify those plants needing additional regulatory attention;
  • Expansion of NRC’s resident inspector program – first authorized in 1977 – whereby at least two inspectors live nearby and work exclusively at each plant in the U.S. to provide daily surveillance of licensee adherence to NRC regulations;
  • Expansion of performance‑oriented as well as safety‑oriented inspections, and the use of risk assessment to identify vulnerabilities of any plant to severe accidents;
  • Strengthening and reorganization of enforcement as a separate office within the NRC;
  • The establishment of the Institute of Nuclear Power Operations (INPO), the industry’s own “policing” group, and formation of what is now the Nuclear Energy Institute to provide a unified industry approach to generic nuclear regulatory issues, and interaction with NRC and other government agencies;
  • The installing of additional equipment by licensees to mitigate accident conditions, and monitor radiation levels and plant status;
  • Employment of major initiatives by licensees in early identification of important safety‑related problems, and in collecting and assessing relevant data so lessons of experience can be shared and quickly acted upon; and
  • Expansion of NRC’s international activities to share enhanced knowledge of nuclear safety with other countries in a number of important technical areas.


The whole world learned a great deal from the accident and those lessons produced important, continued improvement in the performance of all nuclear power plants around the world.  Unfortunately, public confidence in nuclear energy, particularly in the USA, declined sharply following the Three Mile Island accident. It was a major cause of the decline in nuclear construction through the 1980s and 1990s.


  1. The World Nuclear Association – Three Mile Island Accident;
  2. The United States Nuclear Regulatory Commission – Background of Three Mile Island Accident; and
  3. Pittsburgh – Three Mile Island – 25 Years Later.
  • This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on September 4, 2012; and
  • This chapter was updated on 28 June 2020.

Chapter 35