User:Moni3/Chernobyl

thumb|right|300px|View of Unit-Four as it appeared following the explosion on 26 April 1986

The Chernobyl disaster was a cataclysmic explosion on 26 April 1986 at 1.23 am in a nuclear reactor at Unit-Four of the V. I. Lenin Chernobyl Nuclear Power Station located in the Ukrainian Soviet Socialist Republic. It occurred during a routine test in which the supervising engineer neglected to follow protocol, worsened by a fatal flaw in the design of the reactor the plant managers were unaware of. The result was a power surge resulting in a "slow explosion" in which 2,000 fuel rods and control channels were ruptured; intense heat inside the reactor combined with water, creating a steam burst that ripped off the concrete slab roof and sent a shower of steam and sparks an estimated 8 kilometres (5.0 mi) into the night sky.

Initially, Soviet authorities were unaware of the extent of the damage and how much radiation was leaked. Radiation censors near the plant stopped measuring at 200 roentgens an hour and no accurate measurement of how much radiation was released by the explosion is available. Radioactive graphite and other materials expelled from the reactor continued to burn for days following the explosion. The Soviet culture viewed nuclear energy as nearly infallible; other cultural factors such as secrecy and miscommunication through bureaucracy hindered a full understanding of how extensive and dangerous the explosion was. Soviet authorities narrowly defined the deaths resulting from the explosion. International organisations recorded 31 deaths from plant workers, firefighters, and others. Other sources estimate the dead to be as high as 2,000. Hundreds more were hospitalized with radiation poisoning.

As a result of the misunderstanding and miscommunication, nearly 50,000 people in the town of Pripyat, located 3 kilometres (1.9 mi) from the power plant where plant workers lived with their families, were evacuated 36 hours following the explosion after being exposed to dangerous levels of radiation. Within days the "exclusion zone" was widened to 30 kilometres (19 mi) surrounding the plant; eventually, 135,000 people were evacuated in the months following the accident. Winds carried radioactive particles over every part of Europe except the Iberian peninsula, and some parts of the Arabian peninsula and Greenland, forcing many European countries to put embargoes on food products. Over the next two years, about 600,000 people were involved in the clean-up and containment of Chernobyl. When machines failed from radiation contamination, a crew of "bio-robots", or people with makeshift radiation suits, were charged with removing the radioactive debris from the roof of the building. Another crew of miners dug a shaft under the building to install a liquid nitrogen cooling system to prevent another potentially devastating explosion that could have destroyed much of Europe. Unit-Four was eventually fitted with a structure referred to as the "Sarcophagus" that covered and insulated the damaged building. Many of the liquidators, or clean-up crew, suffered prolonged health effects from working so closely to radiation.

In a committed effort to share information about the accident, the Soviet Union provided a report about the details of the accident, but much of the information was distorted or withheld from other countries. Following the breakup of the Soviet Union in 1991, the remaining former Soviet Socialist Republics of the Ukraine and Belarus continue to deal with the effects of the radiation; an estimated 3.5 million people in the Ukraine bear physical effects from Chernobyl, including thyroid cancer and other health problems. As of 2010, Pripyat and several other towns close to the site of the explosion have not been repopulated. The accident at Chernobyl is the most severe to occur at a nuclear power plant as of 2010, and has been a significant factor in reassessing the viability of nuclear power throughout the world.

Background

Chernobyl Nuclear Power Plant

Further information: Chernobyl Nuclear Power Plant

Location of the Chernobyl power plant in relation to Pripyat and the town of Chernobyl

The V. I. Lenin Chernobyl Nuclear Power Station began construction in 1970 and its first reactor was commissioned in 1977, as part of the Soviet Union's nuclear power program, an aggressively ambitious expansion that sought to provide 60% of power in the Ukraine by 2000. Three more reactors were added at Chernobyl, each completed in 1978, 1981, and 1983 when the Unit-Four reactor was commissioned. Two more were designed to be built by 1988. The power station was located near the Ukrainian-Belarus border, along the Pripyat River that flows south into the Kiev Reservoir supplying Kiev, the capital of the Ukraine, where 2.4 million people lived 105 kilometres (65 mi) southeast of the plant. Closer was the town of Pripyat, about 3 kilometres (1.9 mi) from the plant with 45,000 people. The town was new; it was built for the workers. Its stores were well-stocked, it was contemporary and attractive, and people were very eager to live there. The town of Chernobyl, with 12,500 people, was 15 kilometres (9.3 mi) away. Before the power plant was built, about 70 people per km² (80 per sq mi) lived in the 30-kilometre (19 mi) radius surrounding the plant site. By 1986, that number had risen to 110,000. Parallel to the Pripyat River is the Dneiper River, draining a watershed of 106,000 square kilometres (41,000 sq mi) that flows into the Kiev Reservoir. Water passes south through Kiev, into the Black Sea, and eventually to the Mediterranean.

The Chernobyl power station featured four RBMK reactors ("reactor high-power boiling channel type" in Russian or translated as "reactor cooled by water and moderated by graphite"), as opposed to pressurized water reactors (PWRs) more commonly found in the United States and other Western countries. A necessary element in nuclear reactors is a material to slow neutrons that they may efficiently split uranium-235 atoms in turn to create energy. In many PWRs, water or heavy water is used to slow the neutrons. In the RBMK reactors, solid graphite was used in nearly 2,500 block columns interspersed between 1,660 uranium fuel channels surrounded by zirconium and steel. Boron control rods pierced the graphite block columns, acting as throttles when removed, and brakes when inserted as boron carbide absorbs neutrons and decreases heat and energy. Water is added to the bottom of the reactor and as it passes the heated fuel, turns to steam, is sent through pipes to two turbines in another building nearby and turns into electricity.

General function of a RBMK reactor as used in Chernobyl

The graphite in RBMK reactors can reach a temperature of 700 °C (1,292 °F), so must be kept away from air—either by being under water, or inert gases, helium, and nitrogen—or the graphite will burn. In PWRs, when water stops being distributed to the system the neutrons speed up too quickly, and nuclear fission ceases; the system shuts down. The RBMK systems instead led to positive feedback: if any part of the system was not functioning properly, the graphite would continue to absorb neutrons even if water was not present, more heat and energy would be created, and the potential for an uncontrolled chain reaction became high. RBMK reactors were Soviet designed, originally constructed in the 1950s, and favored in the Soviet Union as they were not Western influenced. They were used in the Soviet Union and the West through the 1960s when Western countries began to favor PWRs; although the Chernobyl reactor was relatively new, the design was considered nearing obsolescence.

The entire reactor was contained within a shield composed of water, sand, or concrete so workers could stand next to the reactor and not be exposed to radiation. A building held the reactor and the shields. In the Unit-Four reactor at Chernobyl, the top shield, made of concrete, was attached to all the pressure tubes and control rods. Unlike Western nuclear reactors that are housed in reinforced concrete domes, the ones in the Soviet Union typically had flat concrete roofs that were not reinforced, a difference described as "their most serious shortcoming". Contemporary design of the pipes leading into and out of the Chernobyl reactor ensured that if any of them underneath the reactor ruptured, water or steam contaminated with radiation was contained in pipes beneath the reactor. However, if any of the pipes leading out of the reactor from above were ruptured, the water or steam would not be contained. As the Unit-Four reactor at Chernobyl was about 71 metres (233 ft) tall, containment of the pipes above the reactor was seen at the time as too costly and essentially unnecessary.

Soviet culture

The Soviet Union's communist government promoted a first rate nuclear program, but produced a third-world economy in other ways that in some regions was unable to feed itself. Regimes under different premiers were often unstable and inconsistent. The government was rigidly centralized, resulting in an ineffective bureaucracy that was unable to communicate effectively with the people it governed or establish reasonably achievable goals. This centralized government imposed censorship on any publications that were critical of the state and classified information about nuclear waste disposal, plant safety and health issues, uranium mining, and where future plants were to be located.

Censored news and classified information about nuclear power issues sustained a commonly held belief that nuclear energy was completely safe. More than a dozen nuclear accidents in the Soviet Union before Chernobyl—including an incident at the Unit-One reactor at Chernobyl in September 1982, when radiation was expelled and drifted over Pripyat—were silenced by the government, preventing other plant operators from learning about design flaws or improper procedure. Liubov Kovalevska was a journalist working at Pripyat's only newspaper and given some freedom to write about issues involving the plant. She focused on the shortcuts made in building the Chernobyl power plant, and observed the management system in place there. Nepotism was common; those who were well-connected were quickly promoted in spite of lack of qualifications. Workers who were not well-connected were punished for mistakes, but well-connected workers were not, creating an atmosphere where rules were not uniformly followed. The pay for all workers was quite high for the Soviet Union and they were a privileged set. Criticism of the plant and its construction and management from within was censored. In March 1986, however, Kovalevska published an article about the construction of Chernobyl's fifth reactor: its completion date was decreased from three to two years, more than 2,000 tonnes of metal components were missing, and much of the rest was defective, including sheathing for used nuclear fuel and construction pillars. Upon publication of the article, Soviet authorities ignored it, but Kovalevska was threatened with being terminated from her position by the newspaper.

Shut-down test

In 1984, the Unit-Four reactor became fully operational two months ahead of schedule, during which a series of tests to determine the existence of flaws should have been run. Completing a project early in the Soviet Union was quite rare and brought about large bonuses and other rewards. Several plant managers and nuclear agency officials later admitted that the tests either had failed or had never been run. One of the tests that had been neglected was designed to prevent an uncontrolled chain reaction. Electricity must be supplied to the water pumps at all times, regardless of the type of reactor. If water is not circulated through the reactor core, it can overheat, causing a chain reaction and a meltdown. Most of the time the plant runs on the electricity it produces. To account for emergencies in the form of earthquakes, power blackouts, or a bombing incident, Soviet authorities designed a drill to ensure that backup generators would engage within seconds. A plant in Kursk encountered this problem in 1980 when a blackout caused no water to be pumped through the reactor until its diesel generators engaged 90 seconds later. The Kursk plant's water continued to circulate—albeit more slowly—avoiding a meltdown or other accident occurring.

In Western plants, backup generators are to engage in full capacity in no more than 10 seconds. The Chernobyl generators took 45 seconds to get to full power, so to bridge this 45-second gap, plant management planned to use the energy provided by the turbogenerators that continued to turn although power to them had been cut. Previous testing on a Chernobyl reactor resulted in the voltage from the turbogenerators decreasing too rapidly. A modification to regulate the magnetic field and thus the voltage of turbogenerators in this process was put in place for the test that was to occur between 25 and 26 April, 1986. Further complicating the test was the fact that it was to be run when an estimated 75% of the fuel rods were nearly spent. Mature fuel rods can hold fission products and generate heat, even when the reactor is shut down. The rods made the test conditions unstable; such a test should have taken place when most of the fuel rods were new. Plant management at Chernobyl had previously and successfully performed the emergency shut-down test on other reactors, but not with such a load of spent fuel, and not with an experimental device to modify the voltage on the turbogenerators.

The procedure for the test was submitted to nuclear power authorities by the plant director Viktor Bryukhanov in early 1986. No response came from the various committees and overseeing agencies to alter or halt the test, so Bryukhanov planned to carry on with it. Many of the authorities in various energy agencies had experience in other forms of energy; few of them were familiar with the issues of running a nuclear power plant.

The test began on 25 April at 1.00 am. At full capacity, the Unit-Four reactor produced 3,000 megawatts. By 1.05 pm, the Unit-Four reactor was at 50% producing 1,600 megawatts, ready to decrease further to 30%. The emergency core cooling system was disconnected on the order of deputy chief engineer of operations Nikolai Fomin. An interruption occurred, however, as authorities in Kiev overrode the test to divert electricity for about nine hours. At 11.10 pm the test continued, to be taken over by the midnight shift where Aleksandr Akimov and senior reactor control engineer Leonid Toptunov took over operations. They had not been informed that they were to be participating in the test. Both were young and relatively inexperienced. Only one person in the control room was present who had known about the test and what it was for. The test parameters demanded that the decreased power level should remain between 700 and 1,000 megawatts. The test, however, saw power as low as 30 megawatts—1% of the reactor's power. Tuptunov either forgot to set a controller, or a combination of all the errors and malfunctions in the system caused this to happen.

The low power level created conditions where xenon and iodine began to decay the reactor, a condition called xenon poisoning. Conditions for the test were by this time extremely unfavorable, but the control room crew decided to continue. Grigori Medvedev asserts that the deputy chief engineer on duty, Anatoly Dyatlov, (described as difficult and temperamental by Medvedev) insisted that it take place despite the protest of Tuptunov. The automated computer system did not allow the reactor's energy to be raised, so Tuptunov had to do this manually. Tuptunov both raised the control rods, leaving between six and eight rods in the reactor (30 was the minimum in Soviet standards at the time) and increased the amount of water to the reactor. To ensure the test would be carried out, Tuptunov also disabled the emergency shutdown system. At 1.23 am the test resumed, shutting off the water pumps. Thirty-six seconds later, Tuptunov inserted the emergency rods into the reactor to shut it down. These rods were tipped with graphite, causing a brief surge in power.

Explosion

Aerial view of the damaged core. Roof of the turbine hall is damaged (image center). Roof of the adjacent reactor 3 (image lower left) shows minor fire damage.

At 1.23.44 am on 26 April 1986 the first explosion occurred at Unit-Four reactor. Power in the reactor surged from about 5% to 100 times its normal level in under four seconds. All the water that Tuptunov had directed to the reactor moments before turned to steam, blowing apart the core of the reactor, sending broken fuel rods, burning graphite, and gas upward with the 1,000-tonne (1,100-short-ton) concrete roof of the containment building. Several seconds later, a second explosion occurred, possibly caused by the intense heat separating water molecules into flammable hydrogen and oxygen gases. More debris was cast out of the building onto nearby structures, and a stream of fire and gas was cast 8 kilometres (5.0 mi) high. About 25% of the graphite and fuel channel materials and cladding was expelled in the explosion, 45 tonnes (50 short tons) of which evaporated. Included were the byproducts of nuclear energy: uranium oxide, iodine-131, plutonium-239, neptunium-139, cesium-137, and strontium-90.

Bibliography

• Gale, Robert; Hauser, Thomas (1988). Final Warning: The Legacy of Chernobyl, Warner Books. ISBN 0446514098
• Gould, Peter (1990). Fire in the Rain: The Democratic Consequences of Chernobyl, The Johns Hopkins University Press. ISBN 080184052X
• Marples, David (1988). The Social Impact of the Chernobyl Disaster, St. Martin's Press. ISBN 0312024320
• Medvedev, Grigori (1991) (tr. Evelyn Rossiter). The Truth About Chernobyl, BasicBooks. ISBN 2226040315
• Medvedev, Grigori (1993) (tr. Evelyn Rossiter). No Breathing Room: The Aftermath of Chernobyl, BasicBooks. ISBN 0465051146
• Medvedev, Zhores (1990). The Legacy of Chernobyl, W.W. Norton & Company. ISBN 039302802X
• Mycio, Mary (2005). Wormwood Forest: A Natural History of Chernobyl, Joseph Henry Press. ISBN 0309094305
• Nuclear Energy Agency (1987). Chernobyl and the Safety of Nuclear Reactors, Organisation for Economic Co-operation and Development, Paris. ISBN 9264129758
• Petryna, Adriana (2002). Life Exposed: Biological Citizens after Chernobyl, Princeton University Press. ISBN 0691090181
• Shcherbank, Iurii (1989) (tr. Canadian Institute of Ukranian Studies). Chernobyl: A Documentary Story, Canadian Institute of Ukranian Studies. ISBN 0920862640