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94-453: The RBMK is an early Generation II reactor and the oldest commercial reactor design still in wide operation. Certain aspects of the original RBMK reactor design had several shortcomings, such as the large positive void coefficient , the 'positive scram effect' of the control rods and instability at low power levels—which contributed to the 1986 Chernobyl disaster , in which an RBMK experienced an uncontrolled nuclear chain reaction , leading to

188-420: A burnable nuclear poison to lower the reactivity differences between a new and partially spent fuel assembly. To reduce thermal expansion issues and interaction with the cladding, the pellets have hemispherical indentations. A 2 mm hole through the axis of the pellet serves to reduce the temperature in the center of the pellet and facilitates removal of gaseous fission products. The enrichment level in 1980

282-570: A concentration of atoms that will not necessarily be in the correct alloy proportions. It has been reported that nickel, copper and silicon tend to be enriched at sinks, whereas chromium tends to be depleted. The resulting physical effect is changing chemical composition at grain boundaries or around voids/incoherent precipitates, which also serve as sinks. Voids form due to a clustering of vacancies and generally form more readily at higher temperatures. Bubbles are simply voids filled with gas; they will occur if transmutation reactions are present, meaning

376-422: A degradation in macroscopic properties. As previously mentioned, the chain reaction caused by a PKA often leaves a trail of vacancies and clusters of defects at the edge. This is called a displacement cascade . The vacancy-rich core of a displacement cascade can also collapse into dislocation loops. Due to irradiation, materials tend to develop a higher concentration of defects than is present in typical steels, and

470-472: A diameter and height of 14m × 8m. The maximum allowed temperature of the graphite is up to 730 °C. The reactor has an active core region 11.8 meters in diameter by 7 meters height. There are 1700 tons of graphite blocks in an RBMK-1000 reactor. The pressurized nitrogen in the vessel prevents the escape of the helium-nitrogen mixture used to cool the graphite stack. The reactor vessel has on its outer side an integral cylindrical annular water tank,

564-431: A fuel assembly are held in place with 10 stainless steel spacers separated by 360 mm distance. The two sub-assemblies are joined with a cylinder at the center of the assembly; during the operation of the reactor, this dead space without fuel lowers the neutron flux in the central plane of the reactor. The total mass of uranium in the fuel assembly is 114.7 kg. The fuel burnup is 20 MW·d/kg. The total length of

658-403: A gas is formed due to the breakdown of an atom caused by neutron bombardment. The biggest issue with voids and bubbles is dimensional instability. An example of where this would be very problematic is areas with tight dimensional tolerances, such as threads on a fastener. The creation of defects such as voids or bubbles, precipitates, dislocation loops or lines, and defect clusters can strengthen

752-403: A holding tank for allowing the gaseous radioactive products to decay before being discharged, an aerosol filter to remove solid decay products, and a ventilator stack, the iconic chimney above the space between reactors in second generation RBMKs such as Kursk and Chernobyl 3/4 or some distance away from the reactors in first generation RBMKs such as Kursk and Chernobyl 1/2. The gas is injected to

846-665: A lower enrichment (1.8% enriched uranium instead of considerably more expensive 4% enrichment). This allowed for an extraordinarily large and powerful reactor that could be built rapidly, largely out of parts fabricated on-site instead of by specialized factories. The initial 1000 MWe design also left room for development into yet more powerful reactors. For example, the RBMK reactors at the Ignalina Nuclear Power Plant in Lithuania were rated at 1500 MWe each,

940-582: A material because they block dislocation motion. The movement of dislocations is what leads to plastic deformation. While this hardens the material, the downside is that there is a loss of ductility. Losing ductility, or increasing brittleness, is dangerous in RPVs because it can lead to catastrophic failure without warning. When ductile materials fail, there is substantial deformation before failure, which can be monitored. Brittle materials will crack and explode when under pressure without much prior deformation, so there

1034-515: A number of safety updates. Only two RBMK blocks were started after 1986: Ignalina-2 (located in Lithuania, now decommissioned) and Smolensk-3 . The RBMK was the culmination of the Soviet nuclear power program to produce a water-cooled power reactor with dual-use potential based on their graphite-moderated plutonium production military reactors. The first of these, Obninsk AM-1 ("Атом Мирный", Atom Mirny , Russian for "peaceful atom," analogous to

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1128-445: A redistribution of atoms at sinks such as grain boundaries. The physical effect that can occur is that certain elements will be enriched or depleted in these areas, which often leads to embrittlement of grain boundaries or other detrimental property changes. This is because there is a flux of vacancies towards a sink and a flux of atoms away or toward the sink that may have varying diffusion coefficients. The uneven rates of diffusion cause

1222-467: A simple design, to facilitate its removal and installation by the remotely controlled online refueling machine. The fuel channels may, instead of fuel, contain fixed neutron absorbers, or be filled completely with cooling water. They may also contain silicon-filled tubes in place of a fuel assembly, for the purpose of doping for semiconductors. These channels could be identified by their corresponding servo readers, which would be blocked and replaced with

1316-533: A steam and hydrogen explosion, large fire, and subsequent core meltdown . Radioactive material was released over a large portion of northern and southern Europe—including Sweden—where evidence of the nuclear disaster was first registered outside of the Soviet Union, and before the Chernobyl accident was finally communicated by the Soviet Union to the rest of the world. The disaster prompted worldwide calls for

1410-674: A very large size for the time and even for the early 21st century. For comparison, the EPR has a net electric nameplate capacity of 1600 MW (4500 MW thermal ) and is among the most powerful reactor types ever built. The RBMK-1000's design was finalized in 1968. At that time it was the world's largest nuclear reactor design, surpassing western designs and the VVER (an earlier Soviet PWR reactor design) in power output and physical size, being 20 times larger by volume than contemporary western reactors. Similarly to CANDU reactors it could be produced without

1504-488: A water-filled space between the graphite and the absorber), and a boron carbide neutron absorber section. The role of the graphite section, known as "displacer", is to enhance the difference between the neutron flux attenuation levels of inserted and retracted rods, as the graphite displaces water that would otherwise act as a neutron absorber, although much weaker than boron carbide. A control rod channel filled with graphite absorbs fewer neutrons than when filled with water, so

1598-458: A welded structure with 3 cm thick walls, an inner diameter of 16.6m and an outer diameter of 19m, internally divided to 16 vertical compartments. The water is supplied to the compartments from the bottom and removed from the top; the water can be used for emergency reactor cooling. The tank contains thermocouples for sensing the water temperature and ion chambers for monitoring the reactor power. The tank, along with an annular sand layer between

1692-426: Is cylindrical to fit the round pressure channels. The refueling machine is mounted on a gantry crane and remotely controlled. The fuel assemblies can be replaced without shutting down the reactor, a factor significant for production of weapon-grade plutonium and, in a civilian context, for better reactor uptime. When a fuel assembly has to be replaced, the machine is positioned above the fuel channel: then it mates to

1786-409: Is dependent on the steam production; the steam phase portion is led to the turbines and condensers and returns significantly cooler (155–165 °C (311–329 °F)) than the water returning directly from the steam separator (284 °C). At low reactor power, therefore, the inlet temperature may become dangerously high. The water is kept below the saturation temperature to prevent film boiling and

1880-486: Is greater than 0.1 wt%. Thus, the development of "clean" steels, or ones with very low impurity levels, is important in reducing radiation-induced hardening. Creep occurs when a material is held under levels of stress below their yield stress that causes plastic deformation over time. This is especially prevalent when a material is exposed to high stresses at elevated temperatures, because diffusion and dislocation motion occur more rapidly. Irradiation can cause creep due to

1974-430: Is introducing features to stabilize displaced atoms. This can be done by adding grain boundaries, oversized solutes, or small oxide dispersants to minimize defect movement. By doing this, there would be less radiation-induced segregation of elements, which would in turn lead to more ductile grain boundaries and less intergranular stress corrosion cracking. Blocking dislocation and defect movement would also help to increase

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2068-399: Is not much engineers can do to detect when the material is about to fail. A particularly damaging element in steels that can lead to hardening or embrittlement is copper. Cu-rich precipitates are very small (1-3 nm) so they are effective at pinning dislocations. It has been recognized that copper is the dominant detrimental element in steels used for RPVs, especially if the impurity level

2162-472: Is often the lifetime-limiting component for a nuclear reactor. Understanding the effects radiation has on the microstructure in addition to the physical and mechanical properties will allow scientists to design alloys more resistant to radiation damage. In 2018 Rosatom announced it had developed a thermal annealing technique for RPVs which ameliorates radiation damage and extends service life by between 15 and 30 years. This had been demonstrated on unit 1 of

2256-416: Is removed from the control rod channels by forced gas circulation through the gas circuit. There are 1693 fuel channels and 170 control rod channels in the first generation RBMK reactor cores. Second generation reactor cores (such as Kursk and Chernobyl 3/4) have 1661 fuel channels and 211 control rod channels. The fuel assembly is suspended in the fuel channel on a bracket, with a seal plug. The seal plug has

2350-482: Is sometimes used for modernized generation II designs built post-2000, such as the Chinese CPR-1000 , in competition with more expensive generation III reactor designs. Typically, the modernization includes improved safety systems and a 60-year design life. Generation II reactor designs generally had an original design life of 30 or 40 years. This date was set as the period over which loans taken out for

2444-543: The Balakovo Nuclear Power Plant . Due to the nature of nuclear energy generation, the materials used in the RPV are constantly bombarded by high-energy particles. These particles can either be neutrons or fragments of an atom created by a fission event. When one of these particles collides with an atom in the material, it will transfer some of its kinetic energy and knock the atom out of its position in

2538-581: The Watts Bar Nuclear Generating Station came online and is likely to be the last generation II reactor to become operational in the United States . This article about nuclear power and nuclear reactors for power generation is a stub . You can help Misplaced Pages by expanding it . Reactor pressure vessel A reactor pressure vessel (RPV) in a nuclear power plant is the pressure vessel containing

2632-407: The condensate pumps to deaerators , where remains of gaseous phase and corrosion-inducing gases are removed. The resulting feedwater is led to the steam separators by feedwater pumps and mixed with water from them at their outlets. From the bottom of the steam separators, the feedwater is led by 12 downpipes (from each separator) to the suction headers of the main circulation pumps, and back into

2726-406: The decohesion mechanism, the pressure theory, and the hydrogen attack method . In the decohesion mechanism, it is thought that the accumulation of hydrogen ions reduces the metal-to-metal bond strength, which makes it easier to cleave atoms apart. The pressure theory is the idea that hydrogen can precipitate as a gas at internal defects and create bubbles within the material. The stress caused by

2820-431: The nuclear reactor coolant , core shroud , and the reactor core . Russian Soviet era RBMK reactors have each fuel assembly enclosed in an individual 8 cm diameter pipe rather than having a pressure vessel. Whilst most power reactors do have a pressure vessel, they are generally classified by the type of coolant rather than by the configuration of the vessel used to contain the coolant. The classifications are: Of

2914-455: The 25 cm spacing between channels and thus fuel assemblies. Most of the reactor control rods are inserted from above; 24 shortened rods are inserted from below and are used to augment the axial power distribution control of the core. With the exception of 12 automatic rods, the control rods have a 4.5 m (14 ft 9 in) long graphite section at the end, separated by a 1.25 m (4 ft 1 in) long telescope (which creates

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3008-516: The American Atoms for Peace ) generated 5  MW of electricity from 30 MW thermal power, and supplied Obninsk from 1954 until 1959. Subsequent prototypes were the AMB-100 reactor and AMB-200 reactor both at Beloyarsk Nuclear Power Station . By using a minimalist design that used regular (light) water for cooling and graphite for moderation , it was possible to use fuel with

3102-772: The Kurchatov Institute before the first units were built, but the orders for construction of the first RBMK units, which were at Leningrad, had already been issued in 1966 by the Soviet government by the time their concerns reached the Central Committee of the Communist Party of the Soviet Union and the Soviet Council of Ministers . This prompted a sudden overhaul of the RBMK. Plutonium production in an RBMK would have been achieved by operating

3196-435: The RBMK's flaws such as a lack of protection against no feedwater supply. Leningrad and Chernobyl units 1 both had partial meltdowns that were treated, alongside other nuclear accidents at power plants, as state secrets and so were unknown even to other workers at those same plants. By 1980 NIKIET realized, after completing a confidential study, that accidents with the RBMK were likely even during normal operation, but no action

3290-562: The VVER design was called the "American reactor" due to the pressurized water (PWR) design shared with many Western reactors. A top-secret invention patent for the RBMK design was filed by Anatoly Aleksandrov from the Kurchatov Institute of Atomic Energy, who personally took credit for the design of the reactor, with the Soviet patent office. Because a containment building would have needed to be very large and expensive, doubling

3384-407: The associated drop in heat transfer rate. Generation II reactor A generation II reactor is a design classification for a nuclear reactor , and refers to the class of commercial reactors built until the end of the 1990s. Prototypical and older versions of PWR , CANDU , BWR , AGR , RBMK and VVER are among them. These are contrasted to generation I reactors, which refer to

3478-520: The atomic symbol for silicon. The small clearance between the pressure channel and the graphite block makes the graphite core susceptible to damage. If a pressure channel deforms, e.g. by too high an internal pressure, the deformation can cause significant pressure loads on the graphite blocks and lead to damage. The fuel pellets are made of uranium dioxide powder, sintered with a suitable binder into pellets 11.5 mm in diameter and 15 mm long. The material may contain added europium oxide as

3572-569: The biological shield and for thermal insulation of the reactor space. They consist of serpentinite concrete blocks that cover individual removable steel-graphite plugs, located over the tops of the channels, forming what resembles a circle with a grid pattern. The floor above the reactor is thus known by RBMK plant workers as pyatachok , referring to the five-kopeck coin. There is one cover (lid/block) per plug, and one plug per channel. The fuel channels consist of welded zircaloy pressure tubes 8 cm in inner diameter with 4mm thick walls, led through

3666-420: The bottom, in the spaces between the inner and outer walls. The vessel surrounds the graphite core block stack, which serves as moderator. The graphite stack is kept in a helium-nitrogen mixture, providing an inert atmosphere for the graphite, protecting it from potential fires, and facilitating transfer of excess heat from the graphite to the coolant channels. The moderator blocks are made of nuclear graphite

3760-417: The capacity of 5,500–12,000 m/h and are powered by 6 kV electric motors . The normal coolant flow is 8000 m/h per pump; this is throttled down by control valves to 6,000–7,000 m/h when the reactor power is below 500 MWt. Each pump has a flow control valve and a backflow preventing check valve on the outlet, and shutoff valves on both inlet and outlet. Each of the pressure channels in

3854-510: The center of the LBS and welded to the LBS, supports the LBS and transfers the mechanical load to the building. Above the UBS, there is a space with upper channel piping and instrumentation and control (I&C) or control and monitoring cabling. Above that is Assembly 11, made up of the upper shield cover or channel covers. Their top surfaces form part of the floor of the reactor hall and serve as part of

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3948-399: The channels in the center of the graphite moderator blocks. The top and bottom parts of the tubes are made of stainless steel , and joined with the central zircaloy segment with zirconium-steel alloy couplings. The pressure tube is held in the graphite stack channels with two alternating types of 20mm high split graphite rings. One is in direct contact with the tube and has 1.5mm clearance to

4042-461: The control rod passes that section. This "positive scram" effect was discovered in 1983 at the Ignalina Nuclear Power Plant . The control rod channels are cooled by an independent water circuit and kept at 40–70 °C (104–158 °F). The narrow space between the rod and its channel hinders water flow around the rods during their movement and acts as a fluid damper, which is the primary cause of their slow insertion time (nominally 18–21 seconds for

4136-473: The control rods between manual and emergency protection groups was arbitrary; the rods could be reassigned from one system to another during reactor operation without technical or organizational problems. Additional static boron-based absorbers are inserted into the core when it is loaded with fresh fuel. About 240 absorbers are added during initial core loading. These absorbers are gradually removed with increasing burnup. The reactor's void coefficient depends on

4230-408: The control rods in the fuel assembly. The coolant level measurement probe also enters the vessel through the reactor vessel head. The fuel assembly of nuclear fuel usually consisting of uranium or uranium–plutonium mixes. It is usually a rectangular block of gridded fuel rods. Protecting the inside of the vessel from fast neutrons escaping from the fuel assembly is a cylindrical shield wrapped around

4324-404: The core content; it ranges from negative with all the initial absorbers to positive when they are all removed. The normal reactivity margin is 43–48 control rods. The reactor operates in a helium – nitrogen atmosphere (70–90% He, 10–30% N 2 by volume). The gas circuit is composed of a compressor , aerosol and iodine filters, adsorber for carbon dioxide , carbon monoxide , and ammonia ,

4418-428: The core has its own flow control valve so that the temperature distribution in the reactor core can be optimized. Each channel has a ball type flow meter . The nominal coolant flow through the reactor is 46,000–48,000 m/h. The steam flow at full power is 5,440–5,600 t (6,000–6,170 short tons)/h. The nominal temperature of the coolant at the inlet of the reactor is about 265–270 °C (509–518 °F) and

4512-624: The core stack from the bottom in a low flow rate, and exits from the standpipe of each channel via an individual pipe. The moisture and temperature of the outlet gas is monitored; an increase of them is an indicator of a coolant leak. A single gas circuit serves two RBMK-1000 reactors or a single RBMK-1500; RBMK reactors were always built in pairs. The gas circuit is housed between two reactors in second generation RBMKs such as Chernobyl 3/4, Kursk 3/4 and Smolensk 1–4. The reactor has two independent cooling circuits, each having four main circulating pumps (three operating, one standby) that service one half of

4606-488: The cost of each unit, due to the large size of the RBMK, it was originally omitted from the design. It was argued by its designers that the RBMK's strategy of having each fuel assembly in its own channel with flowing cooling water, was an acceptable alternative for containment. The RBMK was mainly designed at the Kurchatov Institute of Atomic Energy and NIKIET  [ ru ] , headed by Anatoly Aleksandrov and Nikolai Dollezhal respectively, from 1964 to 1966. The RBMK

4700-401: The cylindrical shell of the vessels have evolved over time, but in general they consist of low-alloy ferritic steels clad with 3–10 mm of austenitic stainless steel . The stainless steel cladding is primarily used in locations that come into contact with coolant in order to minimize corrosion. Through the mid-1960, SA-302, Grade B, a molybdenum-manganese plate steel, was used in the body of

4794-468: The decarburization of the steel, which weakens the metal. In addition to hydrogen embrittlement, radiation induced creep can cause the grain boundaries to slide against each other. This destabilizes the grain boundaries even further, making it easier for a crack to propagate along its length. Very aggressive environments require novel materials approaches in order to combat declines in mechanical properties over time. One method researchers have sought to use

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4888-407: The difference between inserted and retracted control rod is increased. When the control rod is fully retracted, the graphite displacer is located in the middle of the core height, with 1.25 m of water at each of its ends. The displacement of water in the lower 1.25 m of the core as the rod moves down could cause a local increase of reactivity in the bottom of the core as the graphite part of

4982-419: The dimensions of which are 25 cm × 25 cm on the plane perpendicular to the channels, and with several longitudinal dimensions of between 20 cm and 60 cm depending on the location in the stack. There are holes of 11.4 cm diameter through the longitudinal axis of the blocks for the fuel and control channels. The blocks are stacked, surrounded by the reactor vessel into a cylindrical core with

5076-606: The early prototype of power reactors, such as Shippingport , Magnox / UNGG , AMB , Fermi 1 , and Dresden 1 . The last commercial Gen I power reactor was located at the Wylfa Nuclear Power Station and ceased operation at the end of 2015. The nomenclature for reactor designs, describing four 'generations', was proposed by the US Department of Energy when it introduced the concept of generation IV reactors . The designation generation II+ reactor

5170-568: The embrittlement of grain boundaries or other defects that can serve as crack initiators, the addition of radiation attack at cracks can cause intergranular stress corrosion cracking. The main environmental stressor that forms due to radiation is hydrogen embrittlement at crack tips. Hydrogen ions are created when radiation splits water molecules, which is present because water is the coolant in PWRs, into OH and H . There are several suspected mechanisms that explain hydrogen embrittlement, three of which are

5264-401: The expanding bubble in addition to the applied stress is what lowers the overall stress required to fracture the material. The hydrogen attack method is similar to the pressure theory, but in this case it is suspected that the hydrogen reacts with carbon in the steel to form methane, which then forms blisters and bubbles at the surface. In this case, the added stress by the bubbles is enhanced by

5358-424: The first place because of this extra size, it has an advantage in not needing annealing to extend its life. Annealing of pressurized water reactor vessels to extend their working life is a complex and high-value technology being actively developed by both nuclear service providers ( AREVA ) and operators of pressurized water reactors. All pressurized water reactor pressure vessels share some features regardless of

5452-426: The floor above the reactor in the central hall, and the steam-water pipes. Below the bottom of the reactor core there is the lower biological shield (LBS), similar to the UBS, but only 2m x 14.5m in size. It is penetrated by the tubes for the lower ends of the pressure channels and carries the weight of the graphite stack and the coolant inlet piping. A steel structure, two heavy plates intersecting in right angle under

5546-421: The fuel assembly is 10.025 m, with 6.862 m of the active region. In addition to the regular fuel assemblies, there are instrumented ones, containing neutron flux detectors in the central carrier. In this case, the rod is replaced with a tube with wall thickness of 2.5 mm; and outer diameter of 15 mm. Unlike the rectangular PWR/BWR fuel assemblies or hexagonal VVER fuel assemblies, the RBMK fuel assembly

5640-487: The fuel assembly. Reflectors send the neutrons back into the fuel assembly to better utilize the fuel. The main purpose though is to protect the vessel from fast neutron induced damage that can make the vessel brittle and reduce its useful life. The RPV provides a critical role in safety of the PWR reactor and the materials used must be able to contain the reactor core at elevated temperatures and pressures. The materials used in

5734-433: The graphite stack deformation), eventually 50 years lifetime was adopted for some units such as Kursk 1-3 and 1-4 (no plans to extend further due to condition of non-replaceable elements). The reactor pit or vault is made of reinforced concrete and has dimensions 21.6m × 21.6m × 25.5m. It houses the vessel of the reactor, which is annular, made of an inner and outer cylindrical wall and top and bottom metal plates that cover

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5828-429: The graphite stack, the other one is directly touching the graphite stack and has 1.3mm clearance to the tube. This assembly reduces transfer of mechanical loads caused by neutron-induced swelling , thermal expansion of the blocks, and other factors to the pressure tube, while facilitating heat transfer from the graphite blocks. The pressure tubes are welded to the top and bottom plates of the reactor vessel. While most of

5922-416: The heat energy from the fission process is generated in the fuel rods, approximately 5.5% is deposited in the graphite blocks as they moderate the fast neutrons formed from fission. This energy must be removed to avoid overheating the graphite. About 80–85% of the energy deposited in the graphite is removed by the fuel rod coolant channels, using conduction via the graphite rings. The rest of the graphite heat

6016-449: The high temperatures of operation induce migration of the defects. This can cause things like recombination of interstitials and vacancies and clustering of like defects, which can either create or dissolve precipitates or voids. Examples of sinks, or thermodynamically favorable places for defects to migrate to, are grain boundaries, voids, incoherent precipitates, and dislocations. Interactions between defects and alloying elements can cause

6110-615: The interaction between stress and the development of the microstructure. In this case, the increase in diffusivities due to high temperatures is not a very strong factor for causing creep. The dimensions of the material are likely to increase in the direction of the applied stress due to the creation of dislocation loops around defects that formed due to radiation damage. Furthermore, applied stress can allow interstitials to be more readily absorbed in dislocation, which assists in dislocation climb. When dislocations are able to climb, excess vacancies are left, which can also lead to swelling. Due to

6204-412: The latter, equalizes pressure within, pulls the rod, and inserts a fresh one. The spent rod is then placed in a cooling pond. The capacity of the refueling machine with the reactor at nominal power level is two fuel assemblies per day, with peak capacity of five per day. The total amount of fuel under stationary conditions is 192 tons. The RBMK core has a relatively low power density at least partly due to

6298-430: The lattice. When this happens, this primary "knock-on" atom (PKA) that was displaced and the energetic particle may rebound and collide with other atoms in the lattice. This creates a chain reaction that can cause many atoms to be displaced from their original positions. This atomic movement leads to the creation of many types of defects. The accumulation of various defects can cause microstructural changes that can lead to

6392-400: The main classes of reactor with a pressure vessel, the pressurized water reactor is unique in that the pressure vessel suffers significant neutron irradiation (called fluence ) during operation, and may become brittle over time as a result. In particular, the larger pressure vessel of the boiling water reactor is better shielded from the neutron flux, so although more expensive to manufacture in

6486-613: The outer side of the tank and inner side of the pit, and the relatively thick concrete of the reactor pit serve as lateral biological shields. The top of the reactor is covered by the upper biological shield (UBS), also called "Schema E", or, after the explosion (of Chernobyl Reactor 4), Elena . The UBS is a cylindrical disc of 3m x 17m in size and 2000 tons in weight. It is penetrated by standpipes for fuel and control channel assemblies. The top and bottom are covered with 4 cm thick steel plates, welded to be helium-tight, and additionally joined by structural supports. The space between

6580-453: The outlet temperature 284 °C (543 °F), at pressure in the drum separator and reactor of 6.9 megapascals (69 bar; 1,000 psi). The pressure and the inlet temperature determine the height at which the boiling begins in the reactor; if the coolant temperature is not sufficiently below its boiling point at the system pressure, the boiling starts at the very bottom part of the reactor instead of its higher parts. With few absorbers in

6674-428: The particular design. The reactor vessel body is the largest component and is designed to contain the fuel assembly, coolant, and fittings to support coolant flow and support structures. It is usually cylindrical in shape and is open at the top to allow the fuel to be loaded. This structure is attached to the top of the reactor vessel body. It contains penetrations to allow the control rod driving mechanism to attach to

6768-714: The particular reactor was built and brought online: Nine RBMK blocks under construction were cancelled after the Chernobyl disaster , and the last of three remaining RBMK blocks at the Chernobyl Nuclear Power Plant was shut down in 2000. As of April 2024, there are still seven RBMK reactors ( Leningrad units 3 & 4; Smolensk units 1,2,3; Kursk units 3 & 4), and three small EGP-6 graphite moderated light-water reactors ( Bilibino units 2,3,4) operating in Russia. All have been retrofitted with

6862-472: The pellet to the tube. The pellets are axially held in place by a spring . Each rod contains 3.5 kg of fuel pellets. The fuel rods are 3.64 m long, with 3.4 m of that being the active length. The maximum allowed temperature of a fuel rod is 600 °C. The fuel assemblies consist of two sets ("sub-assemblies") with 18 fuel rods and 1 carrier rod. The fuel rods are arranged along the central carrier rod, which has an outer diameter of 1.3 cm. All rods of

6956-569: The plant would be paid off. However, many generation II reactors are being life-extended to 50 or 60 years, and a second life-extension to 80 years may also be economical in many cases. By 2013 about 75% of still operating U.S. reactors had been granted life extension licenses to 60 years. Chernobyl 's No.4 reactor that exploded was a generation II reactor, specifically RBMK-1000 . Fukushima Daiichi 's three destroyed reactors were generation II reactors; specifically Mark I Boiling water reactors (BWR) designed by General Electric . In 2016, unit 2 at

7050-399: The plates and pipes is filled with serpentinite , a rock containing significant amounts of bound water . The serpentinite provides the radiation shielding of the biological shield and was applied as a special concrete mixture. The disk is supported on 16 rollers, located on the upper side of the reinforced cylindrical water tank. The structure of the UBS supports the fuel and control channels,

7144-485: The reactor at all times in order to protect against an accident, as loosely articulated by the Operational Reactivity Margin (ORM) parameter. An ORM chart recorder and display were added to RBMK control rooms after the Chernobyl disaster. Initially the service life was expected to be 30 years, later it was extended to a 45-year lifetime with mid-life refurbishments (such as fixing the issue of

7238-477: The reactor control and protection system rods, or about 0.4 m/s). After the Chernobyl disaster, the control rod servos on other RBMK reactors were exchanged to allow faster rod movements, and even faster movement was achieved by cooling of the control rod channels by a thin layer of water between an inner jacket and the Zircaloy tube of the channel while letting the rods themselves move in gas. The division of

7332-454: The reactor core, such as during the Chernobyl accident, the positive void coefficient of the reactor makes the reactor very sensitive to the feedwater temperature. Bubbles of boiling water lead to increased power, which in turn increases the formation of bubbles. If the coolant temperature is too close to its boiling point, cavitation can occur in the pumps and their operation can become erratic or even stop entirely. The feedwater temperature

7426-460: The reactor top; each has 2.8 m (9 ft 2 in) diameter, 31 m (101 ft 8 in) length, wall thickness of 10 cm (3.9 in), and weighs 240  t (260 short tons ). Steam, with steam quality of about 15%, is taken from the top of the separators by two steam collectors per separator, combined, and led to two turbogenerators in the turbine hall, then to condensers , reheated to 165 °C (329 °F), and pumped by

7520-467: The reactor under special thermal parameters, but this capability was abandoned early on. This was the design that was finalized in 1968. The redesign did not solve further flaws that were not discovered until years later. Construction of the first RBMK, which was at Leningrad Nuclear Power Plant , began in 1970. Leningrad unit 1 opened in 1973. At Leningrad it was discovered that the RBMK, due to its high positive void coefficient, became harder to control as

7614-477: The reactor. The cooling water is fed to the reactor through lower water lines to a common pressure header (one for each cooling circuit), which is split to 22 group distribution headers, each feeding 38–41 pressure channels through the core, where the coolant boils. The mixture of steam and water is led by the upper steam lines, one for each pressure channel, from the reactor top to the steam separators , pairs of thick horizontal drums located in side compartments above

7708-402: The reactor. There is an ion exchange system included in the loop to remove impurities from the feedwater. The turbine consists of one high-pressure rotor (cylinder) and four low-pressure ones. Five low-pressure separators-preheaters are used to heat steam with fresh steam before being fed to the next stage of the turbine. The uncondensed steam is fed into a condenser, mixed with condensate from

7802-432: The reactors to be completely decommissioned; however, there is still considerable reliance on RBMK facilities for power in Russia. Most of the flaws in the design of RBMK-1000 reactors were corrected after the Chernobyl accident and a dozen reactors have since been operating without any serious incidents for over thirty years. RBMK reactors may be classified as belonging to one of three distinct generations, according to when

7896-422: The reference also have >0.04 wt% sulfur. Low-alloyed NiMoMn ferritic steels are attractive for this purpose due to their high thermal conductivity and low thermal expansion, properties that make them resistant to thermal shock. However, when considering the properties of these steels, one must take into account the response it will have to radiation damage. Due to harsh conditions, the RPV cylinder shell material

7990-466: The resistance to radiation assisted creep. Attempts have been reported of instituting yttrium oxides to block dislocation motion, but it was found that technological implementation posed a greater challenge than expected. Further research is required to continue improving the radiation damage resistance of structural materials used in nuclear power plants. Because of the extreme requirements needed to build large state-of-the-art reactor pressure vessels and

8084-401: The separators, fed by the first-stage condensate pump to a chemical (ion-exchange) purifier, then by a second-stage condensate pump to four deaerators where dissolved and entrained gases are removed; deaerators also serve as storage tanks for feedwater. From the deaerators, the water is pumped through filters and into the bottom parts of the steam separator drums. The main circulating pumps have

8178-412: The space between the inner and outer walls, without covering the space surrounded by the vessel. The reactor vessel is an annular steel cylinder with hollow walls and pressurized with nitrogen gas, with an inner diameter and height of 14.52m × 9.7m, and a wall thickness of 16mm. In order to absorb axial thermal expansion loads, it is equipped with two bellows compensators , one on the top and another on

8272-473: The specialized industry required by the large and thick-walled reactor pressure vessels such as those used by VVER reactors, thus increasing the number of factories capable of manufacturing RBMK reactor components. No prototypes of the RBMK were built; it was put directly into mass production. The RBMK was proclaimed by some as the national reactor of the Soviet Union, probably due to nationalism because of its unique design, large size and power output. Meanwhile

8366-436: The uranium fuel was consumed or burned up, becoming unpredictable by the time it was shut down after three years for maintenance. This made controlling the RBMK a very laborious, mentally and physically demanding task requiring the timely adjustment of dozens of parameters every minute, around the clock, constantly wearing out switches such as those used for the control rods and causing operators to sweat. The enrichment percentage

8460-414: The vessel. As changing designs required larger pressure vessels, the addition of nickel to this alloy by roughly 0.4-0.7 wt% was required to increase the yield strength. Other common steel alloys include SA-533 Grade B Class 1 and SA-508 Class 2. Both materials have main alloying elements of nickel, manganese, molybdenum, and silicon, but the latter also includes 0.25-0.45 wt% chromium. All alloys listed in

8554-399: Was 2% (0.4% for the end pellets of the assemblies). Maximum allowable temperature of the fuel pellet is 2100 °C. The fuel rods are zircaloy (1% niobium ) tubes 13.6 mm in outer diameter, 0.825 mm thick. The rods are filled with helium at 0.5 MPa and hermetically sealed. Retaining rings help to seat the pellets in the center of the tube and facilitate heat transfer from

8648-410: Was favored over the VVER by the Soviet Union due to its ease of manufacture, due to a lack of a large and thick-walled reactor pressure vessel and relatively complex associated steam generators, and its large power output, which would allow the Soviet government to easily meet their central economic planning targets. The flaws in the original RBMK design were recognized by others, including from within

8742-499: Was increased to 2.0%, up from 1.8% to alleviate these issues. The RBMK was considered by some in the Soviet Union to be already obsolete shortly after the commissioning of Chernobyl unit 1. Aleksandrov and Dollezhal did not investigate further or even deeply understand the problems in the RBMK, and the void coefficient was not analyzed in the manuals for the reactor. Engineers at Chernobyl unit 1 had to create solutions to many of

8836-428: Was taken to correct the RBMK's flaws. Instead, manuals were revised, which was believed to be enough to ensure safe operation as long as they were followed closely. However, the manuals were vague and Soviet power plant staff already had a habit of bending the rules in order to meet economic targets, despite inadequate or malfunctioning equipment. Crucially, it was not made clear that a number of control rods had to stay in

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