25th Anniversary of Chornobyl Disaster

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Courtesy of: http://en.wikipedia.org/wiki/Chernobyl_disaster

Backgrounder – Chornobyl Disaster

The Chornobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chornobyl Nuclear Power Plant in Ukraine. It is considered the worst nuclear power plant accident in history, and it is the only one to be classified as a level 7 event on the International Nuclear Event Scale.

The disaster began during a systems test on 26 April 1986 at reactor number four of the Chornobyl plant, which is near the town of Pripyat, Ukraine. There was a sudden power output surge, and when an emergency shutdown was attempted, a more extreme spike in power output occurred, which led to a reactor vessel rupture and a series of explosions. This event exposed the graphite moderator of the reactor to air, causing it to ignite. The resulting fire sent a plume of highly radioactive smoke fallout into the atmosphere and over an extensive geographical area, including Pripyat. The plume drifted over large parts of Ukraine, Belarus 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 accident raised concerns about the safety of the Soviet nuclear power industry, as well as nuclear power in general, slowing its expansion for a number of years and forcing the Soviet government to become less secretive about its procedures.

Russia, Ukraine, and Belarus have been burdened with the continuing and substantial decontamination and health care costs of the Chornobyl accident. Thirty one deaths are directly attributed to the accident, all among the reactor staff and emergency workers, though estimates of the number of deaths potentially resulting from the accident vary enormously; the World Health Organization (WHO) suggest it could reach 4,000 while a Greenpeace report puts this figure at 200,000 or more.

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 the normal shutdown procedure.

Fire containment

Shortly after the accident, firefighters arrived to try to extinguish the fires. First on the scene was a Chornobyl Power Station firefighters. The immediate priority was to extinguish fires on the roof of the station and the area around the building containing Reactor No. 4 to protect No. 3 and keep its core cooling systems intact. The fires were extinguished by 5:00, but many firefighters received high doses of radiation. The fire inside reactor 4 continued to burn until 10 May 1986. The fire was extinguished by a combined effort of helicopters dropping over 5,000 metric tons of sand, lead, clay, and boron onto the burning reactor and injection of liquid nitrogen. The explosion and fire threw hot particles of the nuclear fuel and also far more dangerous fission products, radioactive isotopes such as caesium-137, iodine-131, strontium-90 and other radionuclides, into the air: the residents of the surrounding area observed the radioactive cloud on the night of the explosion.

Evacuation of Pripyat

The nearby city of Pripyat was not immediately evacuated after the incident, as the general population of the Soviet Union was not informed of the disaster until April 29. During that time, all radio broadcasts run by the state were replaced with classical music, which was a common method of preparing the public for an announcement of a tragedy that had taken place. Only after radiation levels set off alarms at the Forsmark Nuclear Power Plant in Sweden, over one thousand kilometers from the Chornobyl Plant, did the Soviet Union admit that an accident had occurred. Nevertheless, authorities attempted to conceal the scale of the disaster.

A government committee was formed and tasked to investigating the accident. It was headed by Valeri Legasov, who arrived at Chornobyl in the evening of 26 April. By the night of 26–27 April — more than 24 hours after the explosion — Legasov’s committee had ample evidence showing extremely high levels of radiation had caused a number of cases of radiation exposure. Based on the evidence at hand, Legasov’s committee acknowledged the destruction of the reactor and ordered the evacuation of Pripyat.

The evacuation began at 14:00 on 27 April. In order to expedite the evacuation, the residents were told to bring only what was necessary, as the authorities had said it would only last approximately three days. As a result, most of the residents left their personal belongings, which are still there today. An exclusion zone of 30 km (19 mi) remains in place today.

Debris removal

The worst of the radioactive debris was collected inside what was left of the reactor, much of it shoveled in by liquidators wearing heavy protective gear (dubbed “bio-robots” by the military).  These workers could only spend a maximum of 40 seconds at a time working on the rooftops of the surrounding buildings because of the extremely high doses of radiation given off by the blocks of graphite and other debris. The reactor itself was covered with bags of sand, lead, and boric acid dropped from helicopters: some 5,000 metric tons of material were dropped during the week that followed the accident. By December 1986, a large concrete sarcophagus had been erected to seal off the reactor and its contents.

International spread of radioactive substances

Four hundred times more radioactive material was released than had been by the atomic bombing of Hiroshima. However, compared to the total amount released by nuclear weapons testing during the 1950s and 1960s, the Chornobyl disaster released 1/100 to 1/1000 the radioactivity. The fallout was detected over all of Europe except for the Iberian Peninsula.

The initial evidence that a major release of radioactive material was affecting other countries came not from Soviet sources, but from Sweden, where on the morning of 28 April workers at the Forsmark Nuclear Power Plant (approximately 1,100 km (680 mi) from the Chornobyl site) were found to have radioactive particles on their clothes. It was Sweden’s search for the source of radioactivity, after they had determined there was no leak at the Swedish plant, that at noon on April 28 led to the first hint of a serious nuclear problem in the western Soviet Union.

Contamination from the Chornobyl accident was scattered irregularly depending on weather conditions. Reports from scientists indicate that Belarus received about 60% of the contamination that fell on the former Soviet Union. Studies in surrounding countries indicate that over one million people could have been affected by radiation.

Radioactive release

Like many other releases of radioactivity into the environment, the Chornobyl release was controlled by the physical and chemical properties of the radioactive elements in the core. While the general population often perceives plutonium as a particularly dangerous nuclear fuel, its effects are almost eclipsed by those of its fission products. Particularly dangerous are highly radioactive compounds that accumulate in the food chain, such as some isotopes of iodine and strontium.

The release of radioisotopes from the nuclear fuel was largely controlled by their boiling points, and the majority of the radioactivity present in the core was retained in the reactor.

In the aftermath of the accident, 237 people suffered from acute radiation sickness, of whom 31 died within the first three months. Most of these were fire and rescue workers trying to bring the accident under control, who were not fully aware of how dangerous exposure to the radiation in the smoke was. Whereas, in the World Health Organization’s 2006 report of the Chornobyl Forum expert group on the 237 emergency workers who were diagnosed with ARS, ARS was identified as the cause of death for 28 of these people within the first few months after the disaster. There were no further deaths identified, in the general population affected by the disaster, as being caused by ARS.

Residual radioactivity in the environment

The Chornobyl nuclear power plant is located next to the Pripyat River, which feeds into the Dnipro River reservoir system, one of the largest surface water systems in Europe. The radioactive contamination of aquatic systems therefore became a major problem in the immediate aftermath of the accident. In the most affected areas of Ukraine, levels of radioactivity in drinking water caused concern during the weeks and months after the accident. After this initial period, however, radioactivity in rivers and reservoirs was generally below guideline limits for safe drinking water.

Bio-accumulation of radioactivity in fish resulted in concentrations (both in western Europe and in the former Soviet Union) that in many cases were significantly above guideline maximum levels for consumption. In the Kiev Reservoir in Ukraine, concentrations in fish were several thousand Bq/kg during the years after the accident.

After the disaster, four square kilometers of pine forest directly downwind of the reactor turned reddish-brown and died, earning the name of the “Red Forest“.

The Chornobyl Shelter Fund

The Chornobyl Shelter Fund was established in 1997 at the Denver 23rd G8 summit to finance the Shelter Implementation Plan (SIP). The plan calls for transforming the site into an ecologically safe condition by means of stabilization of the sarcophagus followed by construction of a New Safe Confinement (NSC). While the original cost estimate for the SIP was US$768 million, the 2006 estimate was $1.2 billion. The SIP is being managed by a consortium of Bechtel, Battelle, and Electricité de France, and conceptual design for the NSC consists of a movable arch, constructed away from the shelter to avoid high radiation, to be slid over the sarcophagus. The NSC is expected to be completed in 2013, and will be the largest movable structure ever built.