User:Moni3/Chernobyl

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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 km 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 km 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 km 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.

Chernobyl Nuclear Power Plant


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 km southeast of the plant. Closer was the town of Pripyat, about 3 km 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 km away. Before the power plant was built, about 70 people per km2 (80 per sq mi) lived in the 30 km 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 106000 km2 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 (in Russian literally "reactor high-power boiling channel type" but translated as "reactor cooled by water and moderated by graphite"), as opposed to the pressurized water reactors (PWRs) more commonly found in the United States and other Western countries. To create the controlled atomic fission conditions found in nuclear reactors, neutrons emitted by the reactor's radioactive fuel must be slowed enough to enable their efficient capture by fissile atoms within the fuel, which themselves then split releasing energy and more neutrons to sustain the reaction. Different reactor designs make use of various methods to "moderate" the nuclear reaction. In many PWRs, water or heavy water (water containing a higher-than-normal proportion of the Deuterium isotope of Hydrogen) is used to slow the neutrons. In the RBMK reactors, solid graphite was used as a moderator material, with nearly 2,500 block columns interspersed between 1,660 fuel channels containing Uranium dioxide-based fuel rods surrounded by a casing (or cladding) made of zirconium alloy. Movable Boron control rods pierced the graphite stack, acting as throttles when withdrawn and brakes when inserted because boron carbide absorbs neutrons and decreases heat and energy. Heated by the fuel rods, the graphite stack in turn heated water pumped in at the bottom of the reactor, turning it to steam. This was piped to two electricity-generating turbines in a nearby building. Additionally, the RBMK design made use of a separate water-cooling system.

The graphite in RBMK reactors can reach a temperature of 700 C, 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. 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 their 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, making employees eager to keep their jobs. 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 2000 t of metal components were missing, and much of the rest was defective, including sheathing for used nuclear fuel and construction pillars. Soviet authorities ignored the article upon publication, 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 on the Unit-Four reactor 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 the water pumps to stall until 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, but this would have delayed the test for another year. 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 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 power-down procedure 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 reactor produced power as low as 30 megawatts—1% of the its full potential. 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 increased, 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. All eight pumps were engaged to send 60000 m3 of water an hour—the normal rate was 45000 m3—putting the pipes at risk of cavitation. 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 of power and an uncontrolled chain reaction.

Explosion
thumb|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 1.24 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. 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 2 km high. The reactor's containment building, at 71 m tall, was demolished, as was the roof of the turbine room. The 2000 t shield atop the reactor was upset and landed on top of the reactor at a 15° angle. About 25% of the graphite, fuel channel materials, and cladding was expelled in the explosion, 45 t of which evaporated, and 64 t of solid nuclear material measuring 10 million curies of radioactivity deposited around and in the buildings. Included were the byproducts of nuclear energy: uranium oxide, iodine-131, plutonium-239, neptunium-139, caesium-137, and strontium-90.

Following a rumbling coming from the floor, plant workers felt two shocks and eyewitnesses reported as many as four. Those in the control room were covered with a fine dust as shock waves buckled the walls around them. They attempted to cool the reactor, not understanding that it had been destroyed. They believed a gas explosion had taken place somewhere, and were confident that they had followed the correct procedures. When workers outside the control room saw the debris and burning graphite, they did not realise what the objects were. Two plant workers climbed the stairs overlooking the reactor to look down and find it open and belching heat, finally confirming that the reactor was demolished and the core exposed.

Burning debris started about 30 fires on the asphalt roofs of several nearby buildings. Within minutes, the Pripyat firefighters arrived and began directing water and held-held extinguishers at the fires. Fire brigades from the town of Chernobyl and Kiev arrived soon after, totaling 300 emergency workers. Almost 450 staff were working that night, considerably less than the regular 2,000 who worked at the plant during the day. Plant manager Viktor Bryukhanov and chief engineer Nikolai Fomin were not in the plant when the reactor exploded, but arrived soon. Despite visual confirmation that the core was exposed, they received other accounts that it was intact and relayed that information to authorities in Moscow. The dosimeter room was destroyed. A held-held dosimeter to measure radiation was obtained from a different part of the plant. Its maximum measurement was 3.6 roentgens an hour, and it was off the scale. A stronger radiometer was obtained, able to measure up to 250 roentgens. Near the destroyed turbines the needle spiked. Bryukhanov refused to believe it and told the civil defense chief of the plant that the radiometer was faulty and the readings impossible.

Radiation is invisible and has no smell or taste. After a few minutes, however, plant workers began to smell ozone, similar to the air after a strong thunderstorm. Exposure to high amounts of radiation similar to what the plant workers, firefighters, and others who were in the vicinity of the reactor explosion encountered resulted in radiation poisoning. Those who were closest to the core died within hours. Others who were farther away—including a pair of fishermen named Pustovoit and Protosov, taking advantage of the fish who were attracted to the warm waters in the plant's cooling pond—experienced various symptoms: nausea, vomiting, nosebleeds, burning eyes, headaches, a metallic taste in the back of their mouths, diarrhea, radiation burns resulting in the darkening of the skin called a "nuclear tan", exhaustion, and disorientation from their 400 REM (4 Grey) doses. In 1986 the maximum levels of radiation exposure set by the World Health Organisation for nuclear plant workers at 5 roentgens a year and 0.5 roentgens for the regular population. No instruments were available to measure the amount of radiation near the exposed reactor core in Unit-Four, but it is estimated that the firemen and plant workers nearest to it received as many as 20,000 roentgens.

None of the firefighters, medical personnel, or plant workers used special clothing or respirators to avoid radiation poisoning. Neither did they have dosimeters to measure for radiation. By 6.30 am, all the fires except the large one eminating from the reactor core had been extinguished, but by that time most of the emergency personnel were taken to the hospital in Pripyat.

Immediate reaction
Officials in Moscow were informed of the disaster within hours and were on scene to supervise. Winds the morning of 26 April blew over nearby forests; radiation readings in Pripyat were initially low. None of the officials volunteered to take credit for the decision to evacuate the town. The fire in the reactor core remained burning for ten days following the explosions. It was so hot that spraying water on it would worsen it by separating the molecules into more flammable elements. Instead, helicopter pilots and members of the Soviet Air Force dropped dolomite and boron carbide over the exposed reactor to neutralize the continuing neutron activity. To extinguish the fire, they dropped clay, sand, and finally lead. More than 4500 t of materials were dropped on the core from low-flying helicopters that were required to hover over a very limited space. Some of the pilots did not disclose what levels of radiation they received, concerned that they would be grounded.

Aside from the Soviet culture that forbade criticism of its policies from its own people, the Soviet Union was entrenched in an ideological conflict with Western countries in the Cold War. Public acknowledgment of mistakes and accidents was generally suppressed so as not to make it seem as if the Soviet side was weak. By pure coincidence, a U.S. satellite passed over the Chernobyl power plant 28 seconds following the accident; images taken of the plant made U.S. authorities conclude that a nuclear missile had possibly exploded in the silo. On 28 April, workers at the Forsmark Nuclear Power Plant in Sweden detected radiation particles on their clothes. After inspecting the Forsmark plant, the workers there deducted from the types of particles and prevailing wind patterns that an explosion occurred somewhere in the Soviet Union. The Soviet Union officially informed the United Kingdom of the accident in a four-sentence statement that day, but had yet to inform the local population surrounding the Chernobyl power plant.