On this day in science history: Chernobyl nuclear plant explosion

In 1986, in Pripet, Russia, one of the four reactors at the Chernobyl nuclear plant exploded in the world's worst civil nuclear catastrophe. It sent a cloud of radioactive dust over Europe. 

The disaster began during a systems test on 26 April 1986 at reactor number four of the Chernobyl plant.  There was a sudden and unexpected power surge, and when an emergency shutdown was attempted, an exponentially larger spike in power output occurred, which led to a reactor vessel rupture and a series of steam explosions. These events exposed the graphite moderator of the reactor to air, causing it to ignite. The resulting fire sent a plume of highly radioactive fallout into the atmosphere and over an extensive geographical area, including Pripyat. The plume drifted over large parts of the western Soviet Union and Europe. From 1986 to 2000, 350,400 people were evacuated and resettled from the most severely contaminated areas of Belarus, Russia, and Ukraine. According to official post-Soviet data, about 60% of the fallout landed in Belarus.

The abandoned city of Pripyat with Chernobyl plant in the distance, by Jason Minshull [Public domain], via Wikimedia Commons

Russia, Ukraine, and Belarus have been burdened with the continuing and substantial decontamination and health care costs of the Chernobyl accident. A report by the International Atomic Energy Agency examines the environmental consequences of the accident. Another UN agency, UNSCEAR, has estimated a global collective dose of radiation exposure from the accident "equivalent on average to 21 additional days of world exposure to natural background radiation"; individual doses were far higher than the global mean among those most exposed, including 530,000 local recovery workers who averaged an effective dose equivalent to an extra 50 years of typical natural background radiation exposure each. Estimates of the number of deaths that will eventually result from the accident vary enormously; disparities reflect both the lack of solid scientific data and the different methodologies used to quantify mortality—whether the discussion is confined to specific geographical areas or extends worldwide, and whether the deaths are immediate, short term, or long term.

Thirty-one deaths are directly attributed to the accident, all among the reactor staff and emergency workers. An UNSCEAR report places the total confirmed deaths from radiation at 64 as of 2008. The Chernobyl Forum predicts the eventual death toll could reach 4,000 among those exposed to the highest levels of radiation (200,000 emergency workers, 116,000 evacuees and 270,000 residents of the most contaminated areas); this figure is a total causal death toll prediction, combining the deaths of approximately 50 emergency workers who died soon after the accident from acute radiation syndrome, nine children who have died of thyroid cancer and a future predicted total of 3940 deaths from radiation-induced cancer and leukemia.

In a peer-reviewed publication in the International Journal of Cancer in 2006, the authors (following a different conclusion methodology to the Chernobyl Forum study, which arrived at the total predicted death toll of 4,000 after cancer survival rates were factored in) stated, without entering into a discussion on deaths, that in terms of total excess cancers attributed to the accident:

The risk projections suggest that by now Chernobyl may have caused about 1000 cases of thyroid cancer and 4000 cases of other cancers in Europe, representing about 0.01% of all incident cancers since the accident. Models predict that by 2065 about 16,000 cases of thyroid cancer and 25,000 cases of other cancers may be expected due to radiation from the accident, whereas several hundred million cancer cases are expected from other causes.

Also based upon extrapolations from the linear no-threshold model of radiation induced damage, down to zero, the Union of Concerned Scientists estimates that, among the hundreds of millions of people living in broader geographical areas, there will be 50,000 excess cancer cases resulting in 25,000 excess cancer deaths.

For this broader group, the 2006 TORCH report, commissioned by the European Greens political party, predicts 30,000 to 60,000 excess cancer deaths. The environmental advocacy group Greenpeace reports the figure at 200,000 or more.

The Russian founder of that region's chapter of Greenpeace also authored a book titled Chernobyl: Consequences of the Catastrophe for People and the Environment, which concludes that among the billions of people worldwide who were exposed to radioactive contamination from the disaster, nearly a million premature cancer deaths occurred between 1986 and 2004. The book, however, has failed the peer review process. Of the five reviews published in the academic press, four considered the book severely flawed and contradictory, and one praised it while noting some shortcomings. The review by M. I. Balonov published by the New York Academy of Sciences concludes that the report is of negative value because it has very little scientific merit while being highly misleading to the lay reader. It characterized the estimate of nearly a million deaths as more in the realm of science fiction than science.

The accident raised concerns about nuclear power worldwide and slowed or reversed the expansion of nuclear power stations. The accident also raised concerns about the safety of the Soviet nuclear power industry, slowing its expansion for a number of years and forcing the Soviet government to become less secretive about its procedures. The government coverup of the Chernobyl disaster was a "catalyst" for glasnost, which "paved the way for reforms leading to the Soviet collapse".

The final shutdown of the undamaged last reactor on the site took place ceremoniously on 15 Dec 2000.

For more information visit:-

https://en.wikipedia.org/wiki/Chernobyl_disaster

http://www.todayinsci.com/4/4_26.htm

http://www.prlabs.co.uk/lab-supplies.php?N=piston-assay-812-200-graphite&Id=66063

Three Nuclear Accidents - An infographic.

Following on from last weeks post the Infographic below details three major nuclear accidents and the outcomes.  Graphic courtesy of LiveScience.

Source:LiveScience

The Chernobyl Nuclear Plant Disaster

28 years ago in the early morning hours of April 26, 1986, the Chernobyl nuclear power plant in Ukraine (formerly part of the Soviet Union) exploded, creating what has been described as the worst nuclear disaster the world has ever seen.

 
Chernobyl is located about 81 miles (130 km) north of the city of Kiev in Ukraine, and about 12 miles (20 km) south of the border with Belarus.  The four reactors at the Chernobyl Nuclear Power Plant were designed and built during the 1970s and 1980s. A manmade reservoir, roughly 8.5 square miles (22 sq. km) in size and fed by the Pripyat River, was created to provide cooling water for the reactor.

On 26 April 1986, at 01:23, reactor four suffered a catastrophic power increase, leading to explosions in its core. This dispersed large quantities of radioactive fuel and core materials into the atmosphere and ignited the combustible graphite moderator. The burning graphite moderator increased the emission of radioactive particles, carried by the smoke, as the reactor had not been encased by any kind of hard containment vessel. The accident occurred during an experiment scheduled to test a potential safety emergency core cooling feature, which took place during a normal shutdown procedure.

In most nuclear reactors, where water is used as a coolant and to moderate the reactivity of the nuclear core, as the core heats up and produces more steam, the increase in steam bubbles or "voids" in the water reduces the reactivity in the nuclear core. This is an important safety feature found in most reactors built in the United States and other Western nations.

 

But not in the RBMK-1000, which used graphite to moderate the core's reactivity and to keep a continuous nuclear reaction occurring in the core. As the nuclear core heated and produced more steam bubbles, the core became more reactive, not less, creating a positive-feedback loop that engineers refer to as a "positive-void coefficient."

Basically, when extremely hot nuclear fuel rods were lowered into cooling water, an immense amount of steam was created, which — because of the RBMK reactors' design flaws — created more reactivity in the nuclear core of reactor number 4. The resultant power surge caused an immense explosion that detached the 1,000-ton plate covering the reactor core, releasing radiation into the atmosphere and cutting off the flow of coolant into the reactor.

A few seconds later, a second explosion of even greater power than the first blew the reactor building apart and spewed burning graphite and other parts of the reactor core around the plant, starting a number of intense fires around the damaged reactor and reactor number 3, which was still operating at the time of the explosions.

The explosions killed two plant workers, who were the first of several workers to die within hours of the accident. For the next several days, as emergency crews tried desperately to contain the fires and radiation leaks, the death toll climbed as plant workers succumbed to acute radiation sickness.

Most of the radiation released from the failed nuclear reactor was from iodine-131, cesium-134 and cesium-137. Iodine-131 has a relatively short half-life of eight days, according to UNSCEAR, but is rapidly ingested through the air and tends to localize in the thyroid gland. Cesium isotopes have longer half-lives (cesium-137 has a half-life of 30 years) and are a concern for years after their release into the environment. 

On April 27, the residents of Pripyat were evacuated — about 36 hours after the accident had occurred. By that time, many were already complaining about vomiting, headaches and other signs of radiation sickness. Officials eventually closed off an 18-mile (30 km) area around the plant; residents were told they would be able to return after a few days, so many left their personal belongings and valuables behind.

Abandoned Pripyat

During the construction of the sarcophagus, a scientific team re-entered the reactor as part of an investigation dubbed "Complex Expedition", to locate and contain nuclear fuel in a way that could not lead to another explosion. These scientists manually collected cold fuel rods, but great heat was still emanating from the core. Rates of radiation in different parts of the building were monitored by drilling holes into the reactor and inserting long metal detector tubes. The scientists were exposed to high levels of radiation and radioactive dust.

After six months of investigation, in December 1986, they discovered with the help of a remote camera an intensely radioactive mass in the basement of Unit Four, more than two metres wide and weighing hundreds of tons, which they called "the elephant's foot" for its wrinkled appearance. The mass was composed of sand, glass and a large amount of nuclear fuel that had escaped from the reactor. The concrete beneath the reactor was steaming hot, and was breached by solidified lava and spectacular unknown crystalline forms termed chernobylite. It was concluded that there was no further risk of explosion.

Contamination from the Chernobyl accident was scattered irregularly depending on weather conditions, much of it deposited on mountainous regions such as the Alps, the Welsh mountains and the Scottish Highlands, where adiabatic cooling caused radioactive rainfall.

For more information visit:-
http://en.wikipedia.org/wiki/Chernobyl_disaster
http://www.livescience.com/39961-chernobyl.html