The Chernobyl disaster

English: Chernobyl Nuclear Power Plant
English: Chernobyl Nuclear Power Plant 

The Chernobyl disaster (UkrainianЧорнобильська катастрофаChornobylska Katastrofa – Chornobyl Catastrophe) was a catastrophic nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine (then officially Ukrainian SSR), which was under the direct jurisdiction of the central authorities of the Soviet Union. An explosion and fire released large quantities of radioactive particles into the atmosphere, which spread over much of Western USSR and Europe.
The Chernobyl disaster is widely considered to have been the worst nuclear power plant accident in history, and is one of only two classified as a level 7 event on the International Nuclear Event Scale (the other being the Fukushima Daiichi nuclear disaster in 2011).[1] The battle to contain the contamination and avert a greater catastrophe ultimately involved over 500,000 workers and cost an estimated 18 billion rubles.[2] The official Soviet casualty count of 31 deaths has been disputed, and long-term effects such as cancers and deformities are still being accounted for.

History
The disaster began during a systems test on Saturday, 26 April 1986 at reactor number four of the Chernobyl plant, which is near the city of Pripyat and in proximity to the administrative border with Belarus and the Dnieper river. 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.[3] 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.[4][5] According to official post-Soviet data,[6][7] 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.[8][notes 1] The government coverup of the Chernobyl disaster was a “catalyst” for glasnost, which “paved the way for reforms leading to the Soviet collapse”.[9]
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.[7] 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.[10][11][12] 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.[13] An UNSCEAR report places the total confirmed deaths from radiation at 64 as of 2008. The Chernobyl Forum estimates that 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 includes some 50 emergency workers who died of acute radiation syndrome, nine children who died of thyroid cancer and an estimated total of 3940 deaths from radiation-induced cancer and leukemia.[14]
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.[15] For this broader group, the 2006 TORCH report predicts 30,000 to 60,000 excess cancer deaths,[16] and a Greenpeace report puts the figure at 200,000 or more.[17] The Russian publication Chernobyl 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.[18]

Accident   

On 26 April 1986, at 01:23 (UTC+3), 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[19]:73 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.

Steam turbine tests

An inactive nuclear reactor continues to generate a significant amount of residual decay heat. In an initial shut-down state (for example, following an emergency SCRAM) the reactor produces around 7 percent of its total thermal output and requires cooling to avoid core damageRBMK reactors, like those at Chernobyl, use water as coolant.[20][21] Reactor 4 at Chernobyl consisted of about 1,600 individual fuel channels; each required a coolant flow of 28 metric tons (28,000 liters or 7,400 US gallons) per hour.[19]:7
Since cooling pumps require electricity to cool a reactor after a SCRAM, in the event of a power grid failure, Chernobyl’s reactors had three backupdiesel generators; these could start up in 15 seconds, but took 60–75 seconds[19]:15 to attain full speed and reach the 5.5‑megawatt (MW) output required to run one main pump.[19]:30
To solve this one-minute gap, considered an unacceptable safety risk, it had been theorised that rotational energy from the steam turbine (as it wound down under residual steam pressure) could be used to generate the required electrical power. Analysis indicated that this residual momentum and steam pressure might be sufficient to run the coolant pumps for 45 seconds,[19]:16 bridging the gap between an external power failure and the full availability of the emergency generators.[22]
This capability still needed to be confirmed experimentally, and previous tests had ended unsuccessfully. An initial test carried out in 1982 showed that the excitation voltage of the turbine-generator was insufficient; it did not maintain the desired magnetic field after the turbine trip. The system was modified, and the test was repeated in 1984 but again proved unsuccessful. In 1985, the tests were attempted a third time but also yielded negative results. The test procedure was to be repeated again in 1986, and it was scheduled to take place during the maintenance shutdown of Reactor Four.[22]
The test focused on the switching sequences of the electrical supplies for the reactor. The test procedure was to begin with an automatic emergency shutdown. No detrimental effect on the safety of the reactor was anticipated, so the test program was not formally coordinated with either the chief designer of the reactor (NIKIET) or the scientific manager. Instead, it was approved only by the director of the plant (and even this approval was not consistent with established procedures).[23]
According to the test parameters, the thermal output of the reactor should have been no lower than 700 MW at the start of the experiment. If test conditions had been as planned, the procedure would almost certainly have been carried out safely; the eventual disaster resulted from attempts to boost the reactor output once the experiment had been started, which was inconsistent with approved procedure.[23]
The Chernobyl power plant had been in operation for two years without the capability to ride through the first 60–75 seconds of a total loss of electric power, and thus lacked an important safety feature. The station managers presumably wished to correct this at the first opportunity, which may explain why they continued the test even when serious problems arose, and why the requisite approval for the test had not been sought from the Soviet nuclear oversight regulator (even though there was a representative at the complex of 4 reactors).[notes 2]:18–20
The experimental procedure was intended to run as follows:
  1. The reactor was to be running at a low power level, between 700 MW and 800 MW.
  2. The steam-turbine generator was to be run up to full speed.
  3. When these conditions were achieved, the steam supply for the turbine generator was to be closed off.
  4. Turbine generator performance was to be recorded to determine whether it could provide the bridging power for coolant pumps until the emergency diesel generators were sequenced to start and provide power to the cooling pumps automatically.
  5. After the emergency generators reached normal operating speed and voltage, the turbine generator would be allowed to freewheel down.

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