The neutron is a subatomic particle, symbol n or n0, with no net electric charge and a mass larger than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave within the nucleus, each has a mass of one atomic mass unit, they are both referred to as nucleons, their properties and interactions are described by nuclear physics. The chemical and nuclear properties of the nucleus are determined by the number of protons, called the atomic number, the number of neutrons, called the neutron number; the atomic mass number is the total number of nucleons. For example, carbon has atomic number 6, its abundant carbon-12 isotope has 6 neutrons, whereas its rare carbon-13 isotope has 7 neutrons; some elements occur in nature with only one stable isotope, such as fluorine. Other elements occur with many stable isotopes, such as tin with ten stable isotopes. Within the nucleus and neutrons are bound together through the nuclear force. Neutrons are required for the stability of nuclei, with the exception of the single-proton hydrogen atom.
Neutrons are produced copiously in nuclear fusion. They are a primary contributor to the nucleosynthesis of chemical elements within stars through fission and neutron capture processes; the neutron is essential to the production of nuclear power. In the decade after the neutron was discovered by James Chadwick in 1932, neutrons were used to induce many different types of nuclear transmutations. With the discovery of nuclear fission in 1938, it was realized that, if a fission event produced neutrons, each of these neutrons might cause further fission events, etc. in a cascade known as a nuclear chain reaction. These events and findings led to the first self-sustaining nuclear reactor and the first nuclear weapon. Free neutrons, while not directly ionizing atoms, cause ionizing radiation; as such they can be a biological hazard, depending upon dose. A small natural "neutron background" flux of free neutrons exists on Earth, caused by cosmic ray showers, by the natural radioactivity of spontaneously fissionable elements in the Earth's crust.
Dedicated neutron sources like neutron generators, research reactors and spallation sources produce free neutrons for use in irradiation and in neutron scattering experiments. An atomic nucleus is formed by a number of protons, Z, a number of neutrons, N, bound together by the nuclear force; the atomic number defines the chemical properties of the atom, the neutron number determines the isotope or nuclide. The terms isotope and nuclide are used synonymously, but they refer to chemical and nuclear properties, respectively. Speaking, isotopes are two or more nuclides with the same number of protons; the atomic mass number, symbol A, equals Z+N. Nuclides with the same atomic mass number are called isobars; the nucleus of the most common isotope of the hydrogen atom is a lone proton. The nuclei of the heavy hydrogen isotopes deuterium and tritium contain one proton bound to one and two neutrons, respectively. All other types of atomic nuclei are composed of two or more protons and various numbers of neutrons.
The most common nuclide of the common chemical element lead, 208Pb, has 82 protons and 126 neutrons, for example. The table of nuclides comprises all the known nuclides. Though it is not a chemical element, the neutron is included in this table; the free neutron has 1.674927471 × 10 − 27 kg, or 1.00866491588 u. The neutron has a mean square radius of about 0.8×10−15 m, or 0.8 fm, it is a spin-½ fermion. The neutron has no measurable electric charge. With its positive electric charge, the proton is directly influenced by electric fields, whereas the neutron is unaffected by electric fields; the neutron has a magnetic moment, however. The neutron's magnetic moment has a negative value, because its orientation is opposite to the neutron's spin. A free neutron is unstable, decaying to a proton and antineutrino with a mean lifetime of just under 15 minutes; this radioactive decay, known as beta decay, is possible because the mass of the neutron is greater than the proton. The free proton is stable. Neutrons or protons bound in a nucleus can be stable or unstable, depending on the nuclide.
Beta decay, in which neutrons decay to protons, or vice versa, is governed by the weak force, it requires the emission or absorption of electrons and neutrinos, or their antiparticles. Protons and neutrons behave identically under the influence of the nuclear force within the nucleus; the concept of isospin, in which the proton and neutron are viewed as two quantum states of the same particle, is used to model the interactions of nucleons by the nuclear or weak forces. Because of the strength of the nuclear force at short distances, the binding energy of nucleons is more than seven orders of magnitude larger than the electromagnetic energy binding electrons in atoms. Nuclear reactions therefore have an energy density, more than ten million times that of chemical reactions; because of the mass–energy equivalence, nuclear binding energies reduce the mass of nuclei. The ability of the nuclear force to store energy arising from the electromagnetic repulsion of nuclear components is the basis for most of the energy that makes nuclear reactors or bombs possible.
In nuclear fission, the absorption of a neutron by a heavy nuclide causes the nuclide to become unstable and break into light nuclides and additional neu
Control rods are used in nuclear reactors to control the fission rate of uranium and plutonium. They are composed of chemical elements such as boron, silver and cadmium that are capable of absorbing many neutrons without themselves fissioning; because these elements have different capture cross sections for neutrons of varying energies, the composition of the control rods must be designed for the reactor's neutron spectrum. Boiling water reactors, pressurized water reactors and heavy water reactors operate with thermal neutrons, while breeder reactors operate with fast neutrons. Control rods are used in control rod assemblies and inserted into guide tubes within a fuel element. A control rod is removed from or inserted into the central core of a nuclear reactor in order to increase or decrease the neutron flux, which describes the number of neutrons that split further uranium atoms; this in turn affects the thermal power, the amount of steam produced and hence the electricity generated. Control rods stand vertically within the core.
In PWRs they are inserted from above, with the control rod drive mechanisms mounted on the reactor pressure vessel head. In BWRs, due to the necessity of a steam dryer above the core, this design requires insertion of the control rods from beneath; the control rods are removed from the core to allow a chain reaction to occur. The number of control rods inserted and the distance to which they are inserted can be varied to control activity. Typical shutdown time for modern reactors such as the European Pressurized Reactor or Advanced CANDU reactor is 2 seconds for 90% reduction, limited by decay heat. Chemical elements with a sufficiently high neutron capture cross-section include silver and cadmium. Other candidate elements include boron, hafnium, europium, terbium, holmium, thulium and lutetium. Alloys or compounds may be used, such as high-boron steel, silver-indium-cadmium alloy, boron carbide, zirconium diboride, titanium diboride, hafnium diboride, gadolinium nitrate, gadolinium titanate, dysprosium titanate and boron carbide - europium hexaboride composite.
The material choice is influenced by the neutron energy in the reactor, their resistance to neutron-induced swelling and the required mechanical and lifespan properties. The rods may have the form of tubes filled with neutron-absorbing pellets or powder, they can be made out of stainless steel or other neutron window materials such as zirconium, silicon carbide or cubic 11B15N. The burn up of the absorbing isotopes is another limiting lifespan factor, they may be reduced by capturing long isotope rows of the same element or by not using neutron absorbers for trimming. For example, in pebble bed reactors or in possible new type 7lithium-moderated and -cooled reactors that use fuel and absorber pebbles; some rare earth elements are less rare than silver. For example and yttrium, 400 times more common, with middle capturing values, can be found and used together without separation inside minerals like xenotime PO4, or keiviite 2Si2O7, lowering the cost. Xenon is a strong neutron absorber as a gas and can be used for controlling and stopping of helium-cooled reactors, but does not function in cases of pressure loss, or as a burning protection gas together with argon around the vessel part in case of core catching reactors or if filled with sodium or lithium.
Fission-produced xenon can be used after waiting for caesium to precipitate, when no radioactivity is left. Cobalt 59 is used as an absorber for winning of cobalt 60 for x-ray production. Control rods can be constructed as thick turnable rods with a tungsten reflector and absorber side turned to stop by a spring in less than 1 second. Silver-indium-cadmium alloys 80% Ag, 15% In and 5% Cd, are a common control rod material for pressurized water reactors; the somewhat different energy absorption regions of the materials make the alloy an excellent neutron absorber. It has good mechanical strength and can be fabricated, it must be encased in stainless steel to prevent corrosion in hot water. Although indium is less rare than silver, it is more expensive. Boron is another common neutron absorber. Due to the different cross sections of 10B and 11B, materials containing boron enriched in 10B by isotopic separation are used; the wide absorption spectrum of boron makes it suitable as a neutron shield. The mechanical properties of boron in its elementary form are unsuitable, therefore alloys or compounds have to be used instead.
Common choices are boron carbide. The latter is used as a control rod material in both BWRs. 10B/11B separation is done commercially with gas centrifuges over BF3, but can be done over BH3 from borane production or directly with an energy optimized melting centrifuge, using the heat of freshly separated boron for preheating. Hafnium has excellent properties for reactors cooling, it has good mechanical strength, can be fabricated, is resistant to corrosion in hot water. Hafnium can be alloyed with other elements, e.g. with tin and oxygen to increase tensile and creep strength, with iron and niobium for corrosion resistance, with molybdenum for wear resistance and machineability. Such alloys are designated as Hafaloy, Hafaloy-M, Hafaloy-N, Hafaloy-NM; the high cost and low availability of hafnium limit its use in civilian reactors, although it
Advanced Gas-cooled Reactor
An Advanced Gas-cooled Reactor is a type of nuclear reactor designed and operated in the United Kingdom. These are the second generation of British gas-cooled reactors, using graphite as the neutron moderator and carbon dioxide as coolant, they have been the backbone of the UK's nuclear generation fleet since the 1980s. The AGR was developed from the UK's first-generation reactor design; the first Magnox reactors had been optimised for generating plutonium, for this reason it incorporated a number of features that are not the best for economic performance. Primary among these requirements was the Magnox's ability to run on natural uranium, which demanded the use of a coolant with a low neutron cross section, in this case CO2, efficient neutron moderator, graphite. Magnox ran cool compared to other power-producing designs, which made it less efficient extracting power from the reactor core; the AGR retained the Magnox's graphite moderator and CO2 coolant but increased its operating temperature to improve efficiency when converted to steam.
The steam it produced was deliberately identical to that from a coal fired plant, allowing the same turbines and generation equipment to be used. During the initial design stages the system was forced to switch the tubing enclosing the fuel pellets from beryllium to stainless steel. Steel has a higher cross section and this change demanded the switch to enriched uranium fuel to maintain criticality; as part of this change, the new design had higher burnup of 18,000 MWt-days per tonne of fuel, requiring less frequent refuelling. The first prototype AGR became operational in 1962, but the first commercial AGR did not come online until 1976. A total of fourteen AGR reactors at six sites were built between 1976 and 1988. All of these are configured with two reactors in a single building; each reactor has a design thermal power output of 1,500 MWt driving a 660 MWe turbine-alternator set. The various AGR stations produce outputs in the range 555 MWe to 670 MWe though some run at lower than design output due to operational restrictions.
The design of the AGR was such that the final steam conditions at the boiler stop valve were identical to that of conventional coal-fired power stations, thus the same design of turbo-generator plant could be used. The mean temperature of the hot coolant leaving the reactor core was designed to be 648 °C. In order to obtain these high temperatures, yet ensure useful graphite core life a re-entrant flow of coolant at the lower boiler outlet temperature of 278 °C is utilised to cool the graphite, ensuring that the graphite core temperatures do not vary too much from those seen in a Magnox station; the superheater outlet temperature and pressure were designed to be 2,485 psi and 543 °C. The fuel is uranium dioxide pellets, enriched to 2.5-3.5%, in stainless steel tubes. The original design concept of the AGR was to use a beryllium based cladding; when this proved unsuitable due to brittle fracture, the enrichment level of the fuel was raised to allow for the higher neutron capture losses of stainless steel cladding.
This increased the cost of the power produced by an AGR. The carbon dioxide coolant circulates through the core, reaching 640 °C and a pressure of around 40 bar, passes through boiler assemblies outside the core but still within the steel-lined, reinforced concrete pressure vessel. Control rods penetrate the graphite moderator and a secondary system involves injecting nitrogen into the coolant to absorb thermal neutrons to stop the fission process if the control rods fail to enter the core. A tertiary shutdown system which operates by injecting boron beads into the reactor is included in case the reactor has to be depressurized with insufficient control rods lowered; this would mean. The AGR was designed to have a high thermal efficiency of about 41%, better than modern pressurized water reactors which have a typical thermal efficiency of 34%; this is due to the higher coolant outlet temperature of about 640 °C practical with gas cooling, compared to about 325 °C for PWRs. However the reactor core has to be larger for the same power output, the fuel burnup ratio at discharge is lower so the fuel is used less efficiently, countering the thermal efficiency advantage.
Like the Magnox, CANDU and RBMK reactors, in contrast to the light water reactors, AGRs are designed to be refuelled without being shut down first. This on-load refuelling was an important part of the economic case for choosing the AGR over other reactor types, in 1965 allowed the Central Electricity Generating Board and the government to claim that the AGR would produce electricity cheaper than the best coal-fired power stations; however fuel assembly vibration problems arose during on-load refuelling at full power, so in 1988 full power refuelling was suspended until the mid-1990s, when further trials led to a fuel rod becoming stuck in a reactor core. Only refuelling at part load or when shut down is now undertaken at AGRs; the pre-stressed concrete pressure vessel contains the boilers. To minimise the number of penetrations into the vessel the boilers are of the once through design where all boiling and superheating is carried out within the boiler tubes; this necessitates the use of ultra pure water to minimise the buildup of salts in the evaporator and subsequent corrosion problems.
The AGR was intended to be a superior British alternative to American light water reactor designs. It was promoted as a development of the operationally (
Pressurized water reactor
Pressurized water reactors constitute the large majority of the world's nuclear power plants and are one of three types of light water reactor, the other types being boiling water reactors and supercritical water reactors. In a PWR, the primary coolant is pumped under high pressure to the reactor core where it is heated by the energy released by the fission of atoms; the heated water flows to a steam generator where it transfers its thermal energy to a secondary system where steam is generated and flows to turbines which, in turn, spin an electric generator. In contrast to a boiling water reactor, pressure in the primary coolant loop prevents the water from boiling within the reactor. All LWRs use ordinary water as both neutron moderator. PWRs were designed to serve as nuclear marine propulsion for nuclear submarines and were used in the original design of the second commercial power plant at Shippingport Atomic Power Station. PWRs operating in the United States are considered Generation II reactors.
Russia's VVER reactors are similar to U. S. PWRs. France operates many PWRs to generate the bulk of its electricity. Several hundred PWRs are used for marine propulsion in aircraft carriers, nuclear submarines and ice breakers. In the US, they were designed at the Oak Ridge National Laboratory for use as a nuclear submarine power plant with a operational submarine power plant located at the Idaho National Engineering Lab. Follow-on work was conducted by Westinghouse Bettis Atomic Power Laboratory; the first purely commercial nuclear power plant at Shippingport Atomic Power Station was designed as a pressurized water reactor, on insistence from Admiral Hyman G. Rickover that a viable commercial plant would include none of the "crazy thermodynamic cycles that everyone else wants to build."The United States Army Nuclear Power Program operated pressurized water reactors from 1954 to 1974. Three Mile Island Nuclear Generating Station operated two pressurized water reactor plants, TMI-1 and TMI-2; the partial meltdown of TMI-2 in 1979 ended the growth in new construction of nuclear power plants in the United States for two decades.
The pressurized water reactor has three new Generation III reactor evolutionary designs: the AP-1000, VVER-1200, ACPR1000+, APR1400. Nuclear fuel in the reactor pressure vessel is engaged in a fission chain reaction, which produces heat, heating the water in the primary coolant loop by thermal conduction through the fuel cladding; the hot primary coolant is pumped into a heat exchanger called the steam generator, where it flows through hundreds or thousands of small tubes. Heat is transferred through the walls of these tubes to the lower pressure secondary coolant located on the sheet side of the exchanger where the coolant evaporates to pressurized steam; the transfer of heat is accomplished without mixing the two fluids to prevent the secondary coolant from becoming radioactive. Some common steam generator arrangements are single pass heat exchangers. In a nuclear power station, the pressurized steam is fed through a steam turbine which drives an electrical generator connected to the electric grid for transmission.
After passing through the turbine the secondary coolant is cooled condensed in a condenser. The condenser converts the steam to a liquid so that it can be pumped back into the steam generator, maintains a vacuum at the turbine outlet so that the pressure drop across the turbine, hence the energy extracted from the steam, is maximized. Before being fed into the steam generator, the condensed steam is sometimes preheated in order to minimize thermal shock; the steam generated has other uses besides power generation. In nuclear ships and submarines, the steam is fed through a steam turbine connected to a set of speed reduction gears to a shaft used for propulsion. Direct mechanical action by expansion of the steam can be used for a steam-powered aircraft catapult or similar applications. District heating by the steam is used in some countries and direct heating is applied to internal plant applications. Two things are characteristic for the pressurized water reactor when compared with other reactor types: coolant loop separation from the steam system and pressure inside the primary coolant loop.
In a PWR, there are two separate coolant loops, which are both filled with demineralized/deionized water. A boiling water reactor, by contrast, has only one coolant loop, while more exotic designs such as breeder reactors use substances other than water for coolant and moderator; the pressure in the primary coolant loop is 15–16 megapascals, notably higher than in other nuclear reactors, nearly twice that of a boiling water reactor. As an effect of this, only localized boiling occurs and steam will recondense promptly in the bulk fluid. By contrast, in a boiling water reactor the primary coolant is designed to boil. Light water is used as the primary coolant in a PWR. Water enters through the bottom of the reactor's core at about 548 K and is heated as it flows upwards through the reactor core to a temperature of about 588 K; the water remains liquid despite the high temperature due to the high pressure in the primary coolant loop around 155 bar. In water, the critical point occurs at 22.064 MPa.
Pressure in the primary circuit is maintained by a pressurizer, a separate vessel, conne
Lists of nuclear disasters and radioactive incidents
These are lists of nuclear disasters and radioactive incidents. List of attacks on nuclear plants List of Chernobyl-related articles List of civilian nuclear accidents List of civilian radiation accidents List of crimes involving radioactive substances List of criticality accidents and incidents List of nuclear meltdown accidents List of Milestone nuclear explosions List of military nuclear accidents List of nuclear and radiation accidents and incidents List of nuclear and radiation accidents by death toll List of articles about the Three Mile Island accident List of nuclear power accidents by country List of nuclear and radiation fatalities by country List of nuclear power accidents in Canada List of books about nuclear issues List of civilian nuclear ships List of films about nuclear issues Vulnerability of nuclear plants to attack United States military nuclear incident terminology International Nuclear Event Scale Atomic spies Nuclear terrorism Nuclear safety and security Nuclear accident Nuclear power phase-out List of hydroelectric power station failures Radiation exposures in accidents - Annex C of UNSCEAR 2008 Report "The world's worst nuclear power disasters".
Power Technology. 7 October 2013
Uranium is a chemical element with symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 electrons, of which 6 are valence electrons. Uranium is weakly radioactive because all isotopes of uranium are unstable, with half-lives varying between 159,200 years and 4.5 billion years. The most common isotopes in natural uranium are uranium-238 and uranium-235. Uranium has the highest atomic weight of the primordially occurring elements, its density is about 70% higher than that of lead, lower than that of gold or tungsten. It occurs in low concentrations of a few parts per million in soil and water, is commercially extracted from uranium-bearing minerals such as uraninite. In nature, uranium is found as uranium-238, uranium-235, a small amount of uranium-234. Uranium decays by emitting an alpha particle; the half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years, making them useful in dating the age of the Earth.
Many contemporary uses of uranium exploit its unique nuclear properties. Uranium-235 is the only occurring fissile isotope, which makes it used in nuclear power plants and nuclear weapons. However, because of the tiny amounts found in nature, uranium needs to undergo enrichment so that enough uranium-235 is present. Uranium-238 is fissionable by fast neutrons, is fertile, meaning it can be transmuted to fissile plutonium-239 in a nuclear reactor. Another fissile isotope, uranium-233, can be produced from natural thorium and is important in nuclear technology. Uranium-238 has a small probability for spontaneous fission or induced fission with fast neutrons. In sufficient concentration, these isotopes maintain a sustained nuclear chain reaction; this generates the heat in nuclear power reactors, produces the fissile material for nuclear weapons. Depleted uranium is used in kinetic energy penetrators and armor plating. Uranium is used as a colorant in uranium glass. Uranium glass fluoresces green in ultraviolet light.
It was used for tinting and shading in early photography. The 1789 discovery of uranium in the mineral pitchblende is credited to Martin Heinrich Klaproth, who named the new element after the discovered planet Uranus. Eugène-Melchior Péligot was the first person to isolate the metal and its radioactive properties were discovered in 1896 by Henri Becquerel. Research by Otto Hahn, Lise Meitner, Enrico Fermi and others, such as J. Robert Oppenheimer starting in 1934 led to its use as a fuel in the nuclear power industry and in Little Boy, the first nuclear weapon used in war. An ensuing arms race during the Cold War between the United States and the Soviet Union produced tens of thousands of nuclear weapons that used uranium metal and uranium-derived plutonium-239; the security of those weapons and their fissile material following the breakup of the Soviet Union in 1991 is an ongoing concern for public health and safety. See Nuclear proliferation; when refined, uranium is a weakly radioactive metal.
It has a Mohs hardness of 6, sufficient to scratch glass and equal to that of titanium, rhodium and niobium. It is malleable, ductile paramagnetic electropositive and a poor electrical conductor. Uranium metal has a high density of 19.1 g/cm3, denser than lead, but less dense than tungsten and gold. Uranium metal reacts with all non-metal elements and their compounds, with reactivity increasing with temperature. Hydrochloric and nitric acids dissolve uranium, but non-oxidizing acids other than hydrochloric acid attack the element slowly; when finely divided, it can react with cold water. Uranium in ores is extracted chemically and converted into uranium dioxide or other chemical forms usable in industry. Uranium-235 was the first isotope, found to be fissile. Other occurring isotopes are fissionable, but not fissile. On bombardment with slow neutrons, its uranium-235 isotope will most of the time divide into two smaller nuclei, releasing nuclear binding energy and more neutrons. If too many of these neutrons are absorbed by other uranium-235 nuclei, a nuclear chain reaction occurs that results in a burst of heat or an explosion.
In a nuclear reactor, such a chain reaction is slowed and controlled by a neutron poison, absorbing some of the free neutrons. Such neutron absorbent materials are part of reactor control rods; as little as 15 lb of uranium-235 can be used to make an atomic bomb. The first nuclear bomb used in war, Little Boy, relied on uranium fission, but the first nuclear explosive and the bomb that destroyed Nagasaki were both plutonium bombs. Uranium metal has three allotropic forms: α stable up to 668 °C. Orthorhombic, space group No. 63, lattice parameters a = 285.4 pm, b = 587 pm, c = 495.5 pm. Β stable from 668 °C to 775 °C. Tetragonal, space group P42/mnm, P42nm, or P4n2, lattice parameters a = 565.6 pm, b = c = 1075.9 pm. Γ from 775 °C to melting point—this is the most malleable and ductile state. Body-centered cubic, lattice parameter a = 352.4 pm. The major application of uranium in the military sector is
The Chernobyl disaster referred to as the Chernobyl accident, was a catastrophic nuclear accident occurred on 25–26 April 1986 in the No. 4 nuclear reactor at the Chernobyl Nuclear Power Plant, near the now-abandoned town of Pripyat, in northern Soviet Ukraine. The accident occurred during a late-night safety test which simulated a station blackout power-failure, in the course of which safety systems were intentionally turned off. A combination of inherent reactor design flaws and the reactor operators arranging the core in a manner contrary to the checklist for the test resulted in uncontrolled reaction conditions. Water flashed into steam generating a destructive steam explosion and a subsequent open-air graphite fire; this fire produced considerable updrafts for about nine days. These lofted plumes of fission products into the atmosphere; the estimated radioactive inventory, released during this hot fire phase equaled in magnitude the airborne fission products released in the initial destructive explosion.
This radioactive material precipitated onto parts of other European countries. During the accident, steam-blast effects caused two deaths within the facility: one after the explosion, the other compounded by a lethal dose of radiation. Over the coming days and weeks, 134 servicemen were hospitalized with acute radiation syndrome, of which 28 firemen and employees died in the days-to-months afterward. Additionally fourteen radiation induced cancer deaths among this group of 134 hospitalized survivors were to follow within the next ten years. Among the wider population, an excess of 15 childhood thyroid cancer deaths were documented as of 2011, it will take further time and investigation to definitively determine the elevated relative risk of cancer among the surviving employees, those that were hospitalized with ARS, the population at large. The Chernobyl accident is considered the most disastrous nuclear power plant accident in history, both in terms of cost and casualties, it is one of only two nuclear energy accidents classified as a level 7 event on the International Nuclear Event Scale, the other being the Fukushima disaster in Japan in 2011.
The struggle to safeguard against scenarios that were perceived as having the potential for greater catastrophe, together with decontamination efforts of the surroundings involved over 500,000 workers and cost an estimated 18 billion rubles. The remains of the No. 4 reactor building were enclosed in a large cover, named the "Object Shelter" known as "sarcophagus". The purpose of the structure was to reduce the spread of the remaining radioactive dust and debris from the wreckage, thus limiting radioactive contamination, the protection of the wreckage from further weathering; the sarcophagus was finished in December 1986 at a time when what was left of the reactor was entering the cold shutdown phase. The enclosure was not intended as a radiation shield, but was built as occupational safety for the crews of the other undamaged reactors at the power station, with No. 3 continuing to produce electricity up into 2000. The accident motivated safety upgrades on all remaining Soviet-designed RBMK reactors, the same type as Chernobyl No.
4, of which ten continue to power electric grids as of 2019. The disaster began during a systems test on 26 April 1986 at reactor 4 of the Chernobyl plant near Pripyat and in proximity to the administrative border with Belarus and the Dnieper River. There was a unexpected power surge; when operators attempted an emergency shutdown, a much larger spike in power output occurred. This second spike led to a series of steam explosions; these events exposed the graphite moderator of the reactor to air. For the next week, the resulting fire sent long plumes of radioactive fallout into the atmosphere over an extensive geographical area, including Pripyat; the plumes drifted over large parts of Europe. According to official post-Soviet data, about 60% of the fallout landed in Belarus. Thirty-six hours after the accident, Soviet officials enacted a 10-kilometre exclusion zone, which resulted in the rapid evacuation of 49,000 people from Pripyat, the nearest large population centre. Although not communicated at the time, an immediate evacuation of the town following the accident was not advisable as the road leading out of the town had heavy nuclear fallout hotspots deposited on it.
The town itself was comparatively safe due to the favourable wind direction. Until the winds began to change direction, shelter in place was considered the best safety measure for the town. During the accident the wind changed direction; as plumes and subsequent fallout continued to be generated, the evacuation zone was increased from 10 to 30 km about one week after the accident. A further 68,000 persons were evacuated, including from the town of Chernobyl itself; the surveying and detection of isolated fallout hotspots outside this zone over the following year resulted in 135,000 long-term evacuees in total agreeing to be moved. The near tripling in the total number of permanently resettled persons between 1986 and 2000 from the most contaminated areas to 350,000 is regarded as political in nature, with the majority of the rest evacuated in an effort to redeem loss in trust in the government. Many thousands of these evacuees would have been "better off staying home." Risk analysis