Reactor accident at the Three Mile Island nuclear power plant

Three Mile Island nuclear power plant, where the accident occurred

When reactor accident at the nuclear power plant Three Mile Iceland in Pennsylvania in the United States one occurred on March 28, 1979 Serious accident ( INES stage 5), which is of the reactor Unit 2 nuclear power plant Three Mile Iceland in a partial meltdown came in the course of about a third of the reactor core has been fragmented or melted.

the accident

Quick shutdown

On March 28, 1979 at 4:00:36 a.m. local time , during maintenance work on the condensate cleaning system, a valve in the feed line from the condenser to the two main feed pumps in the secondary circuit closed due to a malfunction of the pneumatic control. The pumps switched off immediately, as a result of which the reactor was not cooled by the two steam generators .

As a result of the pump failure, the emergency shutdown was activated, which means that the control rods fall between the fuel rods and end the chain reaction . As a result, the nuclear heat output suddenly drops, but not to zero. In the case of the TMI reactor, this so-called decay heat output was approximately 6% of the nominal thermal output of the reactor, i.e. approx. 155 MW, immediately after shutdown. It had to be removed through the emergency cooling. Auxiliary feed pumps started as expected, but could not deliver water to the steam generators because several valves were closed. The emergency feed water system had been tested 42 hours before the accident and the subsequent opening of the valves was neglected.

Open pressure relief valve

Without cooling, the temperature and pressure in the primary circuit of the reactor rose rapidly. Against a dangerous pressure there was on top of the pressurizer a safety valve . During normal operation this should close tightly up to 151 bar so that no radioactivity escapes. At a slightly higher pressure, however, it should open wide in order to fulfill its safety function by blowing off a ton of steam per minute. For this purpose, it is structured in two stages (pilot-operated relief valve, PORV ).

Simplified schematic representation of block 2

It was a full-lift safety blow-off valve (SBV) that opens fully with hysteresis at the upper pressure value, 158 bar, and then remains open until the lower switching point, 155 bar, is reached. 13 seconds would have been enough. In this case the valve stuck and stayed open. In the control room there was no direct display of the valve position, so this malfunction went unnoticed. The radioactively contaminated steam first shot into a large, water-filled blow-off tank and condensed there while heating the tank contents until its rupture disc broke at almost 1 bar overpressure and the coolant escaped into the containment , the reactor's safety container .

That too went unnoticed, so that it didn't help much that after eight minutes the closed emergency feed valves were noticed and opened: The primary circuit was now cooled again, but a steadily growing vapor bubble formed at the upper end of the reactor pressure vessel due to the loss of coolant. Actually, the pressurizer was supposed to prevent steam bubbles in the primary circuit by connecting it to a high, hot point in the primary circuit during normal operation with a cylinder filled with 22 m³ of water and above that with 19 m³ of steam. High means lower hydrostatic pressure and hot means higher vapor pressure, so that the only vapor bubble in the primary circuit should arise there in the pressure holder according to the reactor design. The evaporation of a ton of cooling water per minute, however, cooled the contents of the pressurizer and lowered the vapor pressure there. The vapor bubble collapsed in favor of the bubble in the reactor. In any case, coolant was constantly being lost, but the excessively filled pressurizer - the only sensor for the level indicator of the primary circuit was located there - simulated the opposite. During the training, the reactor technicians were taught to prevent the pressurizer from filling up completely with water under all circumstances, so that its function of absorbing pressure surges softly is guaranteed. That is why one of the reactor operators finally stopped the previously automatic supply of coolant to the primary circuit.

Molten reactor core in the Three Mile Island accident.
1. 2B connection
2. 1A connection
3. cavity
4. loose fragments of the core
5. crust
6. molten material
7. fragments in lower chamber
8. possible uranium-depleted region
9. destroyed penetration
10. perforated shield
11. layer of molten Material on surfaces of the bypass channels
12. Damage to the upper grille

Overheated reactor core

When the heat output fell only slowly, a loss of coolant accident was looming : just one hour after the reactor was shut down, the decay heat output was still around 30 MW. After almost an hour and 20 minutes of slow temperature rise, the pumps in the primary circuit began to cavitate due to the increasing vapor pressure . The pumps were turned off and it was believed that natural convection kept the water flowing. But the large vapor bubble in the reactor pressure vessel blocked the convection. After a total of around 130 minutes, the fuel rods began to dry out and overheat. The shell of the fuel rods oxidized through a zirconium - water reaction, hydrogen was released, and the fuel elements melted. The released hydrogen, together with the coolant, which was already mixed with hydrogen to protect against corrosion, reached the containment via the open pressure relief valve (PORV) via the blow-off tank and formed oxyhydrogen with the oxygen present there .

The outflowing coolant, which is now heavily radioactively contaminated due to the destroyed fuel elements, collected at the deepest point of the containment, the so-called sump. From there it was pumped into a collecting tank in an outbuilding due to a switching error. The tank eventually overflowed, the water was released and a smaller part of these radioactive gases got into the environment through inadequate filters.

Partial meltdown

The shift change in the control room at 6:00 a.m. The newcomers indirectly concluded, also due to the high temperature readings from the reactor, that there was a loss of coolant and that only the PORV could be responsible for this. They used a shut-off valve to stop the loss of cooling water. By this time, 150 m³ of cooling water had already escaped from the primary cooling circuit. 165 minutes had passed since the start of the incident when radioactively contaminated water reached the sensors. At this point in time, the readings in the primary cooling circuit were 300 times higher than expected: the meltdown was in full swing.

For a long time, the operators in the control room were unaware of how little water the primary cooling circuit still contained. About three and a half hours after the incident began, the experts who had rushed to the area became aware of the consequences - new water was pumped into the primary circuit. It was later determined that about half of the inventory had melted and that in this molten mass a supercritical state would have been quite possible.

Visit of the control room of Three Mile Island-2 by the then US President Jimmy Carter on April 1, 1979

Venting into the area

A reserve safety valve was then opened to reduce the pressure in the primary circuit. After nine hours, the oxyhydrogen gas mixture ignited in the containment, the internal pressure of which rose briefly to almost 2 bar overpressure, close to the design pressure. It had been almost 16 hours when the pumps in the primary circuit turned on again and the core temperature began to drop. During the next week, both hydrogen and water vapor were removed from the reactor containment . This was done on the one hand by condensers and on the other hand, which was very controversial, by venting into the atmosphere . According to estimates, radioactive gas (in the form of krypton -85; 10.75 years half-life ) with an activity of around 1.665 · 10 ¹⁵ Bq escaped during the incident .

Measures after the accident

The cause of the accident was justified with the poor equipment of the control room and the inadequate training of the employees. An investigation found that the accident could have been avoided if the staff had noticed that the PORV on the pressurizer was open and closed it - the accident at Three Mile Island would have remained an insignificant event. But in addition to the misconduct of the staff, there was no so-called fill level probe that would have indicated the respective fill level of the reactor vessel to the operators.

Demonstration on September 4, 1979

In a first long-term study carried out over a period of 18 years, according to medical examinations, no consequential health damage was found in around 30,000 residents. As a result, thousands of complaints from those affected were dismissed by the courts. Citizens' initiatives such as “Three Mile Island Alert” and the “Union of Concerned Scientists” questioned the statements made by the industry and the NRC nuclear control agency . According to “TMI Alert” there were numerous residents within a mile who fell ill or died after the accident and whose relatives were compensated by the operating company MetEd. In addition, the residents of the nearby cities of Harrisburg, Royalton and Middletown were found to have enormous psychological stress, which was exacerbated by the evacuation measures - mainly caused by the fact that ionizing radiation is not immediately perceptible.

An independent study has shown that, six years after the accident, the incidence of cancer on the leeward side of the power plant (facing away from the wind) was significantly higher than on the windward side, in some places by more than 150 percent.

Cleanup

Cleaning work of contaminated areas in the reactor auxiliary building in October 1979

The reactor core was largely destroyed by the accident in Unit 2, so that this power plant unit could not be put back into operation. The clean-up work from August 1979 to December 1993, which was limited to dismantling the reactor, cost $ 979 million. In 1984 the top of the reactor was removed, from 1985 to 1990 100 tons of fuel were removed, and in the following two years eight million liters of cooling water were decontaminated from the containment and evaporated. In 1988 the control authority reported that, although further decontamination of the building was possible, the remaining radiation from the contaminated cooling water that seeped into the concrete of the building would not pose a threat to the population. A further dismantling was postponed into the future, with the justification of a lower radiation exposure and a probably higher economic efficiency with a simultaneous dismantling of Unit 1.

In January 2010, the NRC announced that the generator from the destroyed second block would be used for the Shearon Harris Nuclear Power Plant in New Hill, North Carolina. For this purpose, the 670-ton generator is to be dismantled into two parts.

additional

In the shadow of the reactor accident at the TMI, an accident with uranium mining overburden occurred that same year , which radiologically is considered to be the more serious. The rupture of the dam of a uranium mining retention basin on the Rio Puerco in New Mexico (USA) caused around 400,000 tons of radioactive water to flow into the Rio Puerco, which mainly serves as a water reservoir for the Diné , Hopi and Pueblo Indians. A measurement made immediately showed a value for drinking water that was 7000 times higher than the limit value. Informing and educating the population turned out to be extremely difficult due to the lack of electronic means of communication and educational deficits. A large number of cancer deaths are expected.

Cultural adaptations

• The film The China Syndrome was shown in the United States two weeks before the accident . The latter takes a critical look at the economic use of nuclear energy by describing a fictitious incident in a nuclear power plant. Due to the actual incident at the Three Mile Island power plant, this film received a lot of media coverage.
• In 1991, the German music group Kraftwerk published a modified version of their song Radio Activity , which had already been released in 1975 , which now begins with the call to stop radioactivity and a list of well-known locations for nuclear accidents, and names Harrisburg as a metonym for the damaged nuclear power plant in addition to Chernobyl and Sellafield .
• The band Midnight Oil remembered the reactor accident with the song Harrisburg .
• On the album cover to Till The Cows Come Home German Metal - band Farmer Boys is an American to see a farmer with a stuffed two-headed calf, which a few years came after the nuclear accident at Three Mile Iceland on a nearby farm to the world.
• In the movie X-Men Origins: Wolverine, there is a secret mutant prison on the nuclear facility. Since the action takes place at the time of the incident and the power plant is severely damaged during the showdown , the film provides a fictional explanation of the damage.
• In the book Robots and Empire (German title: Das Galaktische Imperium ) Isaac Asimov describes Three Mile Island as "the scene of an 'incident' which took the people of Earth forever against nuclear fission as a source of energy".

See also

• List of accidents in nuclear facilities
• Chernobyl nuclear disaster
• Fukushima nuclear disaster

Web links

Commons : Accident at Three Mile Island Nuclear Power Plant  - Pictures, Videos and Audio Files Collection

• US Nuclear Regulatory Commission - Fact Sheet (PDF)
• Dossier Three Mile Island (French)
• Scott Johnson: Inside TMI - Minute By Minute
• Article on zeit.de
• List of links to current articles 30 years TMI 2009
• Book Cristina Perincioli "Die Frauen von Harrisburg" in full text (PDF, 55S, accessed on January 31, 2011; 242 kB)

literature

• Matthias Hofmann: Learning from disasters. After the accidents in Harrisburg, Seveso and Sandoz , Edition Sigma , Berlin 2008, ISBN 978-3-89404-559-3
• Robert Jungk (Ed.): The Harrisburg Incident - The official report of the Commission appointed by President Carter on the reactor accident on Three Mile Island. Erb Verlag, Düsseldorf 1979, ISBN 3-88458-011-6
• Peter S. Houts / Paul D. Cleary / Teh-Wei Hu: The Three Mile Island Crisis - Psychological, Social and Economic Impacts on the Surrounding Population , The Pennsylvania State University Press, 1988, ISBN 0-271-00633-1
• Charles Perrow: Normal Accidents - Living with High-Risk Technologies , Princeton University Press, 1984, ISBN 0-691-00412-9
• Steve Wing, David Richardson, Donna Armstrong, Douglas Crawford-Brown (School of Public Health, University of North Carolina, Chapel Hill, USA): A reevaluation of cancer incidence near the Three Mile Island nuclear plant: the collision of evidence and assumptions. In: Environmental health perspectives. Volume 105, Number 1, January 1997, pp. 52-57, PMID 9074881 , PMC 1469835 (free full text).
• Cristina Perincioli : The women of Harrisburg, or: "We won't let our fear talk us out" , Rowohlt aktuell, Reinbek 1980, new editions 1986, 1991. Total print run 20,000. Note: This high-circulation book has contributed to awareness-raising in Germany. The functionality of the Harrisburg nuclear power plant is also well-founded, clearly explained with graphics, even for laypeople. The full text of the out of print book is available under [1] .

Individual evidence

1. ↑ Archived copy ( Memento of November 30, 2010 in the Internet Archive ) (PDF; 3.1 MB) Three mile Island; A REPORT TO THE COMMISSIONERS AND TO THE PUBLIC; Mitchell Rogovin, George T. Frampton; 4/5/79
2. ↑ atomic economy from June 1987, article by Dr. J. Wolters (Research Center Jülich)
3. ^ S. Wing, D. Richardson, D. Armstrong, D. Crawford-Brown: A reevaluation of cancer incidence near the Three Mile Island nuclear plant: the collision of evidence and assumptions. In: Environmental health perspectives. Volume 105, Number 1, January 1997, pp. 52-57, PMID 9074881 , PMC 1469835 (free full text).
4. ↑ http://www.nytimes.com/1993/08/15/us/14-year-cleanup-at-three-mile-island-concludes.html
5. ↑ http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html
6. ^ N. Stöcklin: Uranium Economy in North America - The Consequences for the Indigenous People , 2001