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Safety in the Nuclear Industry
IntroductionCanadian Nuclear Safety CommissionHow Safe Are CANDU?The Three C'sNuclear Power AccidentsNuclear WasteHow is the Environment Protected in Uranium MiningFuel ReprocessingNon-proliferationTime Distance ShieldingTransport of Nuclear MaterialsInsurance Oilrig
 
Did You Know?

Reprocessing of spent nuclear fuel produces plutonium-239, a fissile material.

For more information, see:
Fuel Reprocessing

For a lesson about this topic, see:
Keeping the Genie in the Bottle: Fuel Rod Reprocessing and Nuclear Non-Proliferation

accident_title

Chernobyl

Ukraine Map

On April 26, 1986 the worst accident in the history of the nuclear power industry occurred in reactor 4 at the Chernobyl nuclear power plant in Ukraine (then part of the former Soviet Union). Two explosions brought about a rupture in the reactor, causing radionuclides to travel several kilometres into the atmosphere and contaminating the surrounding area. The radionuclides in the atmosphere caused widespread contamination as they spread over much of Europe and around the world.  

The accident occurred as a result of a flawed reactor design and dangerous operating procedures. The automatic shutdown mechanisms were disabled prior to a test to determine how long the turbines would spin and supply power after the loss of main electrical power. As the flow of coolant water decreased, the power increased.  When the operator attempted to shut down the unstable reactor, the power surged causing fuel elements to rupture.  The force of the steam blew the cover lid off the reactor, releasing fission products into the atmosphere and surrounding area.

Chernobyl

The explosion led directly to the deaths of 28 workers as a result of acute radiations sickness (ARS) the year of the accident. Three other workers died of other, non-radiation related causes. Of the 134 emergency workers diagnosed with ARS, 19 others died between 1987 and 2004, but some of these were not a result of the radiation sickness. The general public received relatively low doses of radiation and no cases of ARS occurred.

Chernobyl

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Five million people live in “contaminated” areas of Belarus, Russia and Ukraine. These people would now receive less that a 1 mSv dose of radiation in addition to background radiation. The average annual background radiation dose (around the world) is 2.4 mSv, with a range of 1-10 mSv. These people would now receive a lower annual dose of radiation than those who live in areas with high levels of radon. In the spring of 1986, 116,000 people were evacuated from a highly contaminated zone, and an additional 220,000 were later relocated. In addition to the above three countries, Austria, Bulgaria, Finland, Greece, Italy, Norway, Republic of Moldova, Slovenia, Sweden and Switzerland had areas which could be considered contaminated.


Summary of average accumulated doses to affected populations from Chernobyl fallout
Population catergory
Number
Average dose
(mSv)
Liquidators
(1986-1989)
600,000
~100
Evacuees from highly-contaminated zone (1989)
116,000
33
Residents of "strict-control" zones
(1986-2005)
270,000
>50
Residents of other 'contaminated' areas
(1986-2005)
5,000,000
10-20
Source: Chernobyl’s Legacy: Health, Environmental and Socio-Economic Impacts and Recommendations to the Governments of Belarus, the Russian Federation and Ukraine, p. 14. Chernobyl Forum, 2006

In the years since the accident, there has been an increase in the cases of thyroid cancer, particularly among those who were children or adolescents at the time. As of 2002, of the nearly 5,000 cases reported, there were 15 deaths due to the progression of the cancer. The increase in thyroid cancer was a result of the radioactive iodine-131 which was released. Iodine-131 has a short half-life (eight days), and so would have been ingested shortly after the accident. Cesium-137, with a half-life of 30 years, is the radionuclide of greatest significance after the initial period as it provides, and will provide in the near future, the most significant doses of radiation.

According to the Chernobyl Forum's 20-year report (could hyperlink this), the additional deaths due to cancer are very difficult to determine. The timeframe to develop cancer after exposure can be as low as five years for some forms, and 20 years or more for others. However, based on what has been learned from the higher radiation doses received at Hiroshima and Nagasaki, there are estimates of an additional 4,000 cancer-related deaths among the 600,000 workers known as “liquidators” who worked to clean up the Chernobyl site. Since about 200,000 of these workers will develop cancer over their lifetime from all causes, this increase of less than 1% in cancer risk will not be noticeable. Among the 5,000,000 people residing in contaminated areas, the increase is expected to be much less than 1%. There are those who question these forecasts and have conducted their own independent studies (Torch, for example); however, it has its own critics claiming bias and ignorance in developments of radiobiology.

As part of a 2006 press release, the World Health Organization (WHO) recommended renewed support for survivors of Chernobyl and, “renewed efforts to provide the public and key professionals with accurate information about the health consequences of the disaster, as part of the efforts to revitalize the people and areas affected by Chernobyl. WHO continues the efforts to improve the health care for affected populations through the establishment of telemedicine and educational programmes, and supporting research.”

References:

www.world-nuclear.org/info/chernobyl/inf07.html
www.who.int/mediacentre/news/releases/2006/pr20/en/index.html
www.cna.ca/english/pdf/nuclearfacts/NEI-Chernobyl-Accident-Consequences.pdf
www.iaea.org/Publications/Booklets/Chernobyl/chernobyl.pdf
www-pub.iaea.org/MTCD/publications/PDF/Pub1239_web.pdf
www.greens-efa.org/

Three Mile Island: 1979

accidentThe Three Mile Island (TMI) power station is near Harrisburg, Pennsylvania. It had two pressurized water reactors (PWR). One PWR had an 800 MW capacity and entered service in 1974. It remains one of the best-performing units in the US. The 900 MW Unit 2 was almost brand new.

The accident to Unit 2 happened at 4 a.m. on March 28, 1979 when the reactor was operating at 97% power. It involved a relatively minor malfunction in the secondary cooling circuit which caused the temperature in the primary coolant to rise. This in turn caused the reactor to shut down automatically. Shut down took about one second. At this point a relief valve failed to close, but instrumentation did not reveal the fact, and so much of the primary coolant drained away that the residual decay heat in the reactor core was not removed. The core suffered severe damage as a result. The operators were unable to diagnose or respond properly to the unplanned automatic shutdown of the reactor. Deficient control room instrumentation and inadequate emergency response training proved to be root causes of the accident.


Three Mile Island

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The Chain of Events

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 pressuriser, raising the water level in it. (The pressuriser is a tank which is part of the primary reactor cooling system, maintaining proper pressure in the system. The relief valve is located on the pressuriser. In a PWR like TMI-2, water in the primary cooling system around the core is kept under very high pressure to keep it from boiling.)

Operators responded by reducing the flow of replacement water. Their training told them that the pressuriser water level was the only dependable indication of the amount of cooling water in the system. Because the pressuriser level was increasing, they thought the reactor system was too full of water. Their training told them to do all they could to keep the pressuriser from filling with water. If it filled, they could not control pressure in the cooling system and it might rupture.

Steam then formed in the reactor primary cooling system. Pumping a mixture of steam and water caused the reactor cooling pumps to vibrate. Because the severe vibrations could have damaged the pumps and made them unusable, operators shut them down. This ended the forced cooling of the reactor core. (The operators still believed the system was nearly full of water because the pressuriser level remained high.) 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.

At 6:22 am operators closed a block valve between the relief valve and the pressuriser. 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. 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.

Cooling Restored

By late afternoon, operators began high-pressure injection of water into the reactor cooling system to increase pressure and to collapse steam bubbles. By 7:50 pm on 28 March, they 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 into the environment.

The Hydrogen Bubble

When the reactor's core was uncovered, on the morning of March 28, 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 that same day, the control room instruments showed a sudden rise in reactor building pressure indicating a hydrogen burn had occurred. Hydrogen gas also gathered at the top of the reactor vessel.

From March 30 through April 1,operators removed this hydrogen gas "bubble" by periodically opening the vent valve on the reactor cooling system pressuriser. For a time, regulatory (NRC) officials believed the hydrogen bubble could explode, though such an explosion was never possible since there was not enough oxygen in the system.

Cold Shutdown

On April 27, after an anxious month, 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."

Public Concern and Confusion

When the TMI-2 accident is recalled, it is often in the context of what happened on Friday and Saturday, March 30-31. The height of the TMI-2 accident-induced fear, stress and confusion occurred on those two days. The atmosphere then and the reasons for it are described well in the 1982 book Crisis Contained, The Department of Energy at Three Mile Island, by Philip L Cantelon and Robert C. Williams. This is an official history of the Department of Energy's role during the accident.

Friday appears to have become a turning point in the history of the accident because of two events: the sudden rise in reactor pressure shown by control room instruments on Wednesday afternoon (the "hydrogen burn") which suggested a hydrogen explosion? became known to the Nuclear Regulatory Commission [that day]; and the deliberate venting of radioactive gases from the plant Friday morning which produced a reading of 1,200 millirems (12 mSv) directly above the stack of the auxiliary building.
What made these significant was a series of misunderstandings caused, in part, by problems of communication within various state and federal agencies. Because of confused telephone conversations between people uninformed about the plant's status, officials concluded that the 1,200 millirems (12 mSv) reading was an off-site reading. They also believed that another hydrogen explosion was possible, that the Nuclear Regulatory Commission had ordered evacuation and that a meltdown was conceivable.
Garbled communications reported by the media generated a debate over evacuation. Whether or not there were evacuation plans soon became academic. What happened on Friday was not a planned evacuation but a weekend exodus based not on what was actually happening at Three Mile Island but on what government officials and the media imagined might happen. On Friday confused communications created the politics of fear.(p. 50)

Throughout the book, Cantelon and Williams note that hundreds of environmental samples were taken around TMI during the accident period by the Department of Energy (which had the lead sampling role) or the then-Pennsylvania Department of Environmental Resources. But there were no unusually high readings, except for noble gases, and virtually no iodine. Readings were far below health limits. Yet a political storm was raging based on confusion and misinformation.

No Radiological Health Effects

The TMI-2 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.

Indeed, 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.

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 (mSv), with no more than 1 mSv 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 US residents in a year.

Judge 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 favorable 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.

The TMI-2 Cleanup

The cleanup of the damaged nuclear reactor system at TMI-2 took nearly 12 years and cost approximately US$973 million. The cleanup was uniquely challenging technically and radiologically. Plant surfaces had to be decontaminated. Water used and stored during the cleanup had to be processed. And about 100 tonnes of damaged uranium fuel had to be removed from the reactor vessel -- all without hazard to cleanup workers or the public.

A cleanup plan was developed and carried out safely and successfully by a team of more than 1,000 skilled workers. It began in August 1979, with the first shipments of accident-generated low-level radiological waste to Richland, Washington. In the cleanup's closing phases in 1991, final measurements were taken of the fuel remaining in inaccessible parts of the reactor vessel. Approximately 1% of the fuel and debris remains in the vessel. Also in 1991, the last remaining water was pumped from the TMI-2 reactor. The cleanup ended in December 1993, when Unit 2 received a license from the Nuclear Regulatory Commission (NRC) to enter Post Defueling Monitored Storage (PDMS).

Early in the cleanup, Unit 2 was completely severed from any connection to TMI Unit 1. TMI-2 today is in long-term monitored storage. No further use of the plant is anticipated. Ventilation and rainwater systems are monitored. Equipment necessary to keep the plant in safe long-term storage is maintained.

Defuelling the TMI-2 reactor vessel was the heart of the cleanup. The damaged fuel remained underwater throughout the defuelling. In October 1985, after nearly six years of preparations, workers standing on a platform atop the reactor and manipulating long-handled tools began lifting the fuel into canisters that hung beneath the platform. In all, 342 fuel canisters were shipped safely for long-term storage at the Idaho National Laboratory, a program that was completed in April 1990.

TMI-2 cleanup operations produced over 10.6 megalitres of accident-generated water that was processed, stored and ultimately safely evaporated

In February 1991, the TMI-2 Cleanup Program was named by the National Society of Professional Engineers as one of the top engineering achievements in the U.S. completed during 1990.

The NRC web site has a factsheet on TMI.

Learn More:
Three Mile Island led to 'Sweeping and Permanent' changes in Nuclear Safety
COGnizant-CANDU Owners Group Monthly Newsletter

 

TMI-1: Safe and World-class

From its restart in 1985, Three Mile Island Unit 1 has operated at very high levels of safety and reliability. Application of the lessons of the TMI-2 accident has been a key factor in the plant's outstanding performance.

In 1997, TMI-1 completed the longest operating run of any light water reactor in the history of nuclear power worldwide — 616 days and 23 hours of uninterrupted operation. (That run was also the longest at any steam-driven plant in the US, including plants powered by fossil fuels.) And in October 1998, TMI employees completed three million hours of work without a lost-work day accident.

At the time of the TMI-2 accident, TMI-1 was shut down for refuelling. It was kept shut down during lengthy proceedings by the Nuclear Regulatory Commission. During the shutdown, the plant was modified and training and operating procedures were revamped in light of the lessons of TMI-2.

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.

In 1999, TMI-1 was purchased by AmerGen, a new joint venture between British Energy and PECO Energy.

Training Improvements

Training reforms are among the most significant outcomes of the TMI-2 accident. Training became centered on protecting a plant's cooling capacity, whatever the triggering problem might be. At TMI-2, the operators turned to a book of procedures to pick those that seemed to fit the event. Now operators are taken through a set of "yes-no" questions to ensure, first, that the reactor's fuel core remains covered. Then they determine the specific malfunction. This is known as a "symptom-based" approach for responding to plant events. Underlying it is a style of training that gives operators a foundation for understanding both theoretical and practical aspects of plant operations.

The TMI-2 accident also led to the establishment of the Atlanta-based Institute of Nuclear Power Operations (INPO) and its National Academy for Nuclear Training. These two industry organizations have been effective in promoting excellence in the operation of nuclear plants and accrediting their training programs.

The INPO was formed in 1979. The National Academy for Nuclear Training was established under INPO's auspices in 1985. TMI's operator training program has passed three INPO accreditation reviews since then.

Training has gone well beyond button-pushing. Communications and teamwork, emphasizing effective interaction among crew members, are now part of TMI's training curriculum.

Close to half of the operators' training is in a full-scale electronic simulator of the TMI control room. The $18 million simulator permits operators to learn and be tested on all kinds of accident scenarios.

Increased Safety and Reliability

Disciplines in training, operations and event reporting that grew from the lessons of the TMI-2 accident have made the nuclear power industry demonstrably safer and more reliable. Those trends have been both promoted and tracked by the INPO. To remain in good standing, a nuclear plant must meet the high standards set by INPO as well as the strict regulation of the US Nuclear Regulatory Commission.

A key indicator is the graph of significant plant events, based on data compiled by the Nuclear Regulatory Commission. The number of significant events decreased from 2.38 per reactor unit in 1985 to 0.10 at the end of 1997.

On the reliability front, the median capability factor for nuclear plants — the percentage of maximum energy that a plant is capable of generating — increased from 62.7% in 1980 to almost 90% in 2000. (The goal for the year 2000 was 87%.)

Other indicators for US plants tracked by INPO and its world counterpart, the World Association of Nuclear Operators (WANO) are the unplanned capability loss factor, unplanned automatic scrams, safety system performance, thermal performance, fuel reliability, chemistry performance, collective radiation exposure, volume of solid radioactive waste and industrial safety accident rate. All are reduced, that is, improved substantially, from 1980.

Summary

What happened:

What did not happen:

Longer-term impacts:

Source:
World Nuclear Association www.world-nuclear.org/info/inf36.html
COGnizant-CANDU Owners Group Monthly Newletter