The following explanation of the Three Mile Island accident and the changes to regulations that resulted from it has been compiled by NucNet for communicators and executives in the civil nuclear energy industry. It addresses questions that might arise in the run-up to the 30th anniversary of the accident, on 28 March 2009.
23 Mar (NucNet): The 1979 accident at unit 2 of the Three Mile Island (TMI) nuclear power plant in Harrisburg (Pennsylvania) had the greatest impact on nuclear regulation of any single event in history. It led to “permanent and sweeping changes” in how plants are regulated in the US, the Nuclear Regulatory Commission (NRC) has said.
In an information paper* released to coincide with the 30th anniversary of the accident on 28 March 2009, the commission said that since the accident, there have been substantial regulatory changes and other improvements that have contributed to making nuclear plants safer and better.
Some of the major changes that have strengthened the NRC’s regulation include immediate NRC notification by plants of serious events, and expanded emergency preparedness including an NRC operations centre staffed around the clock.
Similar lessons were taken on board by nuclear safety authorities outside the US. TMI speeded up plans to launch the International Atomic Energy Agency’s Operational Safety Review Team (OSART) system, which was officially launched in 1982. France said the accident led to improved instrumentation, better control room ergonomics, better training, the introduction of “beyond-design-base-accident” instructions for the control room operator crew, and regular emergency drills.
Three Mile Island led to the establishment of the Atlanta-based Institute of Nuclear Power Operations (INPO), the industry's own ‘policing’ group, and the formation of what is now the Nuclear Energy Institute to provide a unified industry approach to generic nuclear regulatory issues, and interaction with the NRC and other government agencies.
The NRC said the TMI accident also led to increased identification, analysis and publication of plant performance information, and recognising human performance as “a critical component of plant safety”.
Key indicators of plant safety performance in the US have improved dramatically. Those indicators show:
• The average number of significant reactor events over the past 20 years has dropped to nearly zero.
• Today there are far fewer, much less frequent and lower risk events that could lead to a reactor-core damage.
• The average number of times safety systems have had to be activated is about one-tenth of what it was 22 years ago.
• Radiation exposure levels to plant workers have steadily decreased to about one-sixth of the 1985 exposure levels and are well below national limits.
• The average number of unplanned reactor shutdowns has decreased by nearly ten-fold. In 2007 there were about 52 shutdowns compared to about 530 shutdowns in 1985.
Other major changes that came about as a result of the accident include:
• Upgrading and strengthening of plant design and equipment requirements. This includes fire protection, piping systems, auxiliary feedwater systems, containment building isolation, reliability of components and the ability of plants to shut down automatically.
• Enhancement of emergency preparedness to include immediate NRC notification requirements for plant events.
• Establishment of a programme 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.
The accident at TMI-2 was the most serious in US commercial nuclear power plant operating history, even though it led to no deaths or injuries to plant workers or members of the nearby community.
However, it brought about sweeping changes involving emergency response planning, reactor operator training, human factors engineering, radiation protection, and many other areas of nuclear power plant operations not only in the US but worldwide.
A sequence of events – equipment malfunctions, design-related problems and worker errors – led to a partial meltdown of unit 2’s reactor core, but only very small off-site releases of radioactivity.
Background: How The Accident Happened
The accident began about 04:00 local time with a failure in the secondary, non-nuclear section of the plant. The main feedwater pumps stopped running, caused either by a mechanical or electrical failure, which prevented the steam generators from removing heat.
First the turbine, then the reactor automatically shut down. Immediately, the pressure in the primary system of the pressurised water reactor began to increase. In order to prevent that pressure from becoming excessive, the pilot-operated relief valve automatically opened. The valve should have closed when the pressure decreased by a certain amount, but it did not. Signals available to the control-room operators failed to show that the valve was still open and as a result, cooling water poured out of the stuck-open valve.
As coolant flowed from the core through the pressuriser, the instruments available to reactor operators provided confusing information. As alarms rang and warning lights flashed, the operators did not realise that the plant was experiencing a loss-of-coolant accident. They took a series of actions that made conditions worse by simply reducing the flow of coolant through the core since they thought that there was too much water in the system when there was not enough.
Because adequate cooling was not available, the nuclear fuel overheated to the point at which the zirconium cladding – the metal tubes that hold the nuclear fuel pellets – ruptured and the fuel pellets began to melt. It was later found that about 50 percent of the core melted during the early stages of the accident.
Although the unit suffered a severe core meltdown, the most dangerous kind of nuclear power accident, it did not produce the “worst-case consequences” that reactor experts had long feared. In a worst-case accident, the melting of nuclear fuel would lead to a breach of the walls of the reactor pressure vessel and of the containment building resulting in the release of massive quantities of radiation to the environment. This did not happen at Three Mile Island.
Detailed studies of the radiological consequences of the accident have been carried out by the NRC, the Environmental Protection Agency, the Department of Health and Human Services, the Department of Energy, and the State of Pennsylvania. Several independent studies have also been conducted.
Estimates are that the average dose to about 2 million people in the area was only about 1 millirem. To put this into context, exposure from a chest x-ray is about 6 millirem. Compared to the natural radioactive background dose of about 100-125 millirem per year for the area, the collective dose to the community from the accident was very small. The maximum dose to a person at the site boundary would have been less than 100 millirem, the NRC has said.
In the months following the accident, questions were raised about possible adverse effects from radiation on human, animal, and plant life in the TMI area. However, none could be directly linked to the accident. Thousands of environmental samples of air, water, milk, vegetation, soil, and foodstuffs were collected by various groups monitoring the area.
Very low levels of radionuclides could be attributed to releases from the accident. However, the NRC said comprehensive investigations and assessments by several well-respected organisations have concluded that in spite of serious damage to the reactor, most of the radiation was contained and that the actual release had negligible effects on the physical health of individuals or the environment.
Today, Three Mile Island-2 is permanently shut down and defueled, with the reactor coolant system drained, the radioactive water decontaminated and evaporated, radioactive waste shipped off-site to a disposal site, reactor fuel and core debris shipped off-site to a Department of Energy facility, and the remainder of the site being monitored.
In 2001, FirstEnergy acquired Three Mile Island-2 from GPU Nuclear Corporation. FirstEnergy has contracted the monitoring of the unit to Exelon, the current owner and operator of Three Mile Island-1. The companies plan to keep Three Mile Island-2 in long-term, monitored storage until the operating licence for the Three Mile Island-1 plant expires, at which time both plants will be decommissioned.
Links and Sources
* The NRC fact sheet is at: www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html
* What Happened and What Didn’t Happen (American Nuclear Society)
* American Journal of Epidemiology
* Tell It Like It Is (Communications Lessons From Three Mile Island)
* The IAEA’s OSART mission system
>>Related reports in the NucNet database (available to subscribers)
Three Mile Island - Twenty Years On (Insider No. 1, 22 February 1999)
New US Study Confirms Lack of Health Effects From TMI Accident (News No. 347, 4 November 2002)
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