RBMK is an acronym for the Russian reaktor bolshoy moshchnosti kanalniy (Russian: Реактор Большой Мощности Канальный) which means "reactor (of) high power (of the) channel (type)", and describes a now obsolete class of graphite-moderated nuclear power reactor which was built only in the Soviet Union. The RBMK reactor was the type involved in the Chernobyl accident. In 2004, several were still operating, but there were no plans to build any more and there is international pressure to close those that remain. There are several types of graphite moderated nuclear reactors that have been used in commercial electricity generation: Gas-cooled reactors Magnox Advanced gas-cooled reactor (AGR) Water-cooled reactors RBMK High temperature gas-cooled reactors (past) Dragon reactor AVR Peach Bottom Atomic Power Station, Unit 1 THTR-300 Fort St. ... Core of a small nuclear reactor used for research. ... The nuclear power plant at Chernobyl prior to the completion of the sarcophagus. ...

Control room of the world's first nuclear power plant, named the Peaceful Atom, at Obninsk, Russia

The RBMK was the culmination of the Soviet program to produce a water-cooled power reactor based on their graphite-moderated plutonium production military reactors. The first of these, AM-1 ("Атом Мирный", Russian for Atom Mirny, "peaceful atom") produced 5 MW electric (30 MW thermal) and delivered power to Obninsk from 1954 until 1959. Image File history File links Obninsk_npp. ... Image File history File links Obninsk_npp. ... Obninsk (Russian: ) is a city in Kaluga Oblast, Russia, located 102 km southwest of Moscow, on the main rail line between Moscow and Kiev. ... General Name, Symbol, Number plutonium, Pu, 94 Chemical series actinides Group, Period, Block n/a, 7, f Appearance silvery white Standard atomic weight (244) g·mol−1 Electron configuration [Rn] 5f6 7s2 Electrons per shell 2, 8, 18, 32, 24, 8, 2 Physical properties Phase solid Density (near r. ... The megawatt (symbol: MW) is a unit for measuring power corresponding to one million (106) watts. ... Obninsk (Russian: ) is a city in Kaluga Oblast, Russia, located 102 km southwest of Moscow, on the main rail line between Moscow and Kiev. ...

Using light water for cooling and graphite for moderation, it is possible to use natural uranium for fuel. Thus, a large power reactor (RBMK reactors at the Ignalina Nuclear Power Plant in Lithuania were rated at 1500 MWe each, a very large size for the time and even for today) can be built that requires no separated isotopes, such as enriched uranium or heavy water. Unfortunately, such a configuration is also unstable. Light water, in the terminology of nuclear reactors, is ordinary water. ... This does not adequately cite its references or sources. ... General Name, Symbol, Number uranium, U, 92 Chemical series actinides Group, Period, Block n/a, 7, f Appearance silvery gray metallic; corrodes to a spalling black oxide coat in air Standard atomic weight 238. ... Ignalina Nuclear Power Plant is a two-unit RBMK-1500 nuclear power station in Visaginas, Lithuania. ... Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes, for example separating natural uranium into enriched uranium and depleted uranium. ... Isotopes are any of the several different forms of an element each having different atomic mass (mass number). ... These pie-graphs showing the relative proportions of uranium-238 (blue) and uranium-235 (red) at different levels of enrichment. ... Heavy water is dideuterium oxide, or D2O or 2H2O. It is chemically the same as normal water, H2O, but the hydrogen atoms are of the heavy isotope deuterium, in which the nucleus contains a neutron in addition to the proton found in the nucleus of any hydrogen atom. ... Look up stability in Wiktionary, the free dictionary. ...

Design

An RBMK employs long (7 meter) vertical pressure tubes running through a graphite moderator and cooled by water, which is allowed to boil in the core at 290 °C, much as in a boiling water reactor. Fuel is low-enriched uranium oxide made up into fuel assemblies 3.5 meters long. With moderation largely due to the fixed graphite, excess boiling simply reduces the cooling and neutron absorption without inhibiting the fission reaction, so the reactor can have a large positive void coefficient, and a positive feedback problem can arise, such with the disaster at Chernobyl, involving the 4th reactor, which was an RBMK reactor. Steel Pressure Vessel A pressure vessel is a closed, rigid container designed to hold gases or liquids at a pressure different from the ambient pressure. ... Graphite (named by Abraham Gottlob Werner in 1789 from the Greek γραφειν (graphein): to draw/write, for its use in pencils) is one of the allotropes of carbon. ... This does not adequately cite its references or sources. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... These pie-graphs showing the relative proportions of uranium-238 (blue) and uranium-235 (red) at different levels of enrichment. ... This article or section does not adequately cite its references or sources. ... In nuclear engineering, the void coefficient (more properly called void coefficient of reactivity) is a number that can be used to estimate how much the thermal output of a nuclear reactor increases (or decreases, if negative) as voids (steam bubbles) form in the reactor moderator or coolant. ... Positive feedback is a feedback system in which the system responds to the perturbation in the same direction as the perturbation (It is sometimes referred to as cumulative causation). ... The nuclear power plant at Chernobyl prior to the completion of the sarcophagus. ...

Because the water used to remove heat from the core in a light-water reactor absorbs some of the free neutrons normally generated during operation of the reactor, the concentration of the naturally fissionable U-235 isotope in uranium used to fuel light-water reactors must be increased above the level of natural uranium to assist in sustaining the nuclear chain reaction in the reactor core: the remainder of the uranium in the fuel is U-238. Increasing the concentration of U-235 in nuclear fuel uranium above the level that occurs in natural uranium is accomplished through the process of enrichment. A free neutron is a neutron that exists outside of an atomic nucleus. ... Uranium-235 is an isotope of uranium that differs from the elements other common isotope, uranium-238, by its ability to cause a rapidly expanding fission chain reaction. ... Isotopes are any of the several different forms of an element each having different atomic mass (mass number). ... A schematic nuclear fission chain reaction. ... There are two objects with this name: Unterseeboot 238 Uranium-238, the most common isotope of uranium This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ... Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes, for example separating natural uranium into enriched uranium and depleted uranium. ...

The fuel core for a light water reactor can have up to 3,000 fuel assemblies. An assembly consists of a group of sealed fuel rods, each filled with uranium oxide (UO₂) pellets, held in place by end plates and supported by metal spacer-grids to brace the rods and maintain the proper distances between them. The fuel core can be thought of as a reservoir from which heat energy can be extracted through the nuclear chain reaction process. During the operation of the reactor, the concentration of U-235 in the fuel is decreased as those atoms undergo nuclear fission which creates heat energy. Some U-238 atoms are converted to atoms of fissile Pu-239, some of which will, in turn, undergo fission and produce energy. The products created by the nuclear fission reactions are retained within the fuel pellets and these become neutron-absorbing products, also called nuclear poisons, that act to slow the rate of nuclear fission and heat production. As the reactor operation is continued, a point is reached at which the declining concentration of fissile nuclei in the fuel and the increasing concentration of poisons result in lower than optimal heat energy generation. The RBMK has a refueling machine that can change the fuel on-load, while the reactor is still producing power. A light water reactor or LWR is a thermal nuclear reactor that uses ordinary water, also called light water, as its neutron moderator. ... Nuclear power station at Leibstadt, Switzerland. ... General Name, Symbol, Number plutonium, Pu, 94 Chemical series actinides Group, Period, Block ?, 7, f Appearance silvery white Atomic mass (244) g/mol Electron configuration [Rn] 5f6 7s2 Electrons per shell 2, 8, 18, 32, 24, 8, 2 Physical properties Phase solid Density (near r. ... For the generation of electrical power by fission, see Nuclear power plant An induced nuclear fission event. ... For information on radioactive toxins see Radiation poisoning A nuclear poison is a substances with a large neutron absorption cross-section in applications, such as nuclear reactors, when absorbing neutrons is an undesirable effect. ... This article or section should include material from Fissile material In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. ...

High Positive Void Coefficient

Light water (the ordinary H₂O) is both a neutron moderator and a neutron absorber. This means that it can slow down neutrons to velocities in equilibrium with surrounding molecules ("thermalize" them), but also absorb them outright. Heavy water is also a good neutron moderator, but does not absorb neutrons as easily. This does not adequately cite its references or sources. ... The process of neutron capture can proceed in two ways - as a rapid process (an r-process) or a slow process (an s-process). ... A chart displaying the speed probability density functions of the speeds of a few noble gases at a temperature of 298. ... Heavy water is dideuterium oxide, or D2O or 2H2O. It is chemically the same as normal water, H2O, but the hydrogen atoms are of the heavy isotope deuterium, in which the nucleus contains a neutron in addition to the proton found in the nucleus of any hydrogen atom. ...

In RBMK's, light water was used as a coolant; moderation was instead carried out by graphite. As graphite already moderated neutrons, light water had not much business in slowing them down, but could still absorb them. This means that the reactor's moderation level (adjustable by appropriate neutron-absorbing rods) had to account for the neutrons absorbed by light water. Graphite (named by Abraham Gottlob Werner in 1789 from the Greek γραφειν (graphein): to draw/write, for its use in pencils) is one of the allotropes of carbon. ...

In the case of evaporation of water to steam, the place occupied by water would be occupied by water vapor, which has a density hundreds of times smaller than that of liquid water (the exact number depends on pressure and temperature; at standard conditions, steam is about 1350 times lighter than liquid water). Because of this lower density (of mass, and consequently of atom nuclei able to absorb neutron), light water's capability of absorbing neutrons would practically disappear. It has been suggested that this article be split into articles entitled steam and water vapor, accessible from a disambiguation page. ...

In RBMK's, generation of steam in the coolant water would then in practice create void, a bubble that does not absorb neutrons; the reduction in moderation by light water is irrelevant, as graphite is still moderating the neutrons. This event would dramatically alter the balance of neutron production, causing a runaway condition in which more and more neutrons are produced, and their density grows exponentially fast. Such a condition is called a positive void coefficient, and it is particularly high for RBMK reactors. Look up Void in Wiktionary, the free dictionary Void can refer to: Aether as the source of all elements, the quintessence. ...

A high void coefficient does not automatically make a reactor unsafe. Some RBMK designs did include control rods on electromagnetic grappels, thus controlling the reaction speed and, if necessary, stopped the reaction completely. The RBMK at Chernobyl, however, had manual control rods. This diagram demonstrates the defense in depth quality of nuclear power plants. ...

After the Chernobyl disaster, all RBMKs in operation underwent significant changes, lowering their void coefficients to +0.7 b. This new number precludes the possibility of a low-coolant meltdown. Chernobyl reactor number four after the disaster, showing the extensive damage to the main reactor hall (image center) and turbine building (image lower left) The Chernobyl disaster was a major accident that took place at the Chernobyl Nuclear Power Plant on April 26, 1986 at 01:23 a. ... The neutron cross section of an element is the effective cross sectional area that an atom of that element presents to a neutron. ... Three Mile Island Nuclear Generating Station consisted of two pressurized water reactors manufactured by Babcock & Wilcox each inside its own containment building and connected cooling towers. ...

Containment

The RBMK design includes several kinds of containment needed for normal operation. There is a sealed metal containment structure filled with inert gases surrounding the reactor to keep oxygen away from the graphite (which is normally at about 700 degrees Celsius). There is also a large amount of shielding to absorb radiation from the reactor core. This includes a concrete slab on the bottom, sand and concrete around the sides, and a large concrete slab on top of the reactor. Much of the reactor's internal machinery is attached to this top slab, including the water pipes. General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Standard atomic weight 15. ... Graphite (named by Abraham Gottlob Werner in 1789 from the Greek γραφειν (graphein): to draw/write, for its use in pencils) is one of the allotropes of carbon. ... Celsius is, or relates to, the Celsius temperature scale (previously known as the centigrade scale). ... Radiation as used in physics, is energy in the form of waves or moving subatomic particles. ...

Initially, the RBMK design focused solely on accident prevention and mitigation, not on containment of severe accidents. However, since the Three Mile Island incident, RBMK design also includes a partial containment structure (not a full containment building) for dealing with emergencies. The pipes underneath the reactor are sealed inside leak-tight boxes filled with a large amount of water. If these pipes leak or burst, the radioactive material is trapped by the water inside these boxes. However, RBMK reactors were designed to allow fuel rods to be changed without shutting down (as in the pressurized heavy water CANDU reactor), both for refueling and for plutonium production (for nuclear weapons). This required large cranes above the core. As the RBMK reactor is very tall (about 7 meters), the cost and difficulty of building a heavy containment structure prevented building of additional emergency containment structure for pipes on top of the reactor. Unfortunately, in the Chernobyl accident, the pressure rose to levels high enough to blow the top off of the reactor, breaking open these pipes in the process. Three Mile Island Nuclear Generating Station consists of two nuclear reactors, each with its own containment building and cooling towers. ... A containment building, in its most common usage, is a steel or concrete structure enclosing a nuclear reactor. ... The CANDU reactor is a pressurized-heavy water, natural-uranium power reactor designed in the 1960s by a partnership between Atomic Energy of Canada Limited and the Hydro-Electric Power Commission of Ontario as well as several private industry participants. ... General Name, Symbol, Number plutonium, Pu, 94 Chemical series actinides Group, Period, Block n/a, 7, f Appearance silvery white Standard atomic weight (244) g·mol−1 Electron configuration [Rn] 5f6 7s2 Electrons per shell 2, 8, 18, 32, 24, 8, 2 Physical properties Phase solid Density (near r. ... The mushroom cloud of the atomic bombing of Nagasaki, Japan, 1945, rose some 18 kilometers (11 mi) above the hypocenter. ...

Improvements since the Chernobyl accident

Since the Chernobyl accident, all remaining RBMKs have been retrofitted with a number of updates for safety. The largest of these updates fixes the RBMK control rod design. Previously the control rods were designed with graphite tips, which when initially inserted into the reactor sped up the reaction, instead of slowing or stopping it. This design flaw caused the first explosion of the Chernobyl accident, when the emergency button was pressed to stop the reactor. The updates are Chernobyl reactor number four after the disaster, showing the extensive damage to the main reactor hall (image center) and turbine building (image lower left) The Chernobyl disaster was a major accident that took place at the Chernobyl Nuclear Power Plant on April 26, 1986 at 01:23 a. ... This diagram demonstrates the defense in depth quality of nuclear power plants. ...

• An increase in fuel enrichment from 2% to 2.4%. This difference improves neutron absorption, reducing the reliance on cooling water for reactor control.
• Manual control rod count increased from 30 to 45.
• 80 additional absorbers inhibit operation at low power, where the RBMK design is most dangerous.
• SCRAM (rapid shut down) sequence reduced from 18 to 12 seconds.
• Precautions against unauthorised access to emergency safety systems.

A SCRAM is an emergency shutdown of a nuclear reactor - though the term has been extended to cover shutdowns of other complex operations, such as server farms and even large model railroads (see Tech Model Railroad Club). ...

Closures

Of the 13 RBMKs built (one is still under construction at Kursk), all three surviving reactors at the Chernobyl plant have now been closed (the fourth having been destroyed in the accident) and one of the two reactors at Ignalina in Lithuania has shut down with the second due to close by 2009. [1] Kursk (Russian: ; pronunciation: koorsk; IPA: ) is a city in the western part of Central Russia, at the confluence of Kur, Tuskar, and Seym rivers. ... Ignalina Nuclear Power Plant is a two-unit RBMK-1500 nuclear power station in Visaginas, Lithuania. ...

References

• Technical data on RBMK-1500 reactor at Ignalina nuclear power plant - one of the remaining working RBMK reactors
• Chernobyl - A Canadian Perspective (PDF 405KB) - A brochure describing nuclear reactors in general and the RBMK design in particular, focusing on the safety differences between them and CANDU reactors. Published by the CANDU organization.
• The Chernobyl Disaster - how the RBMK's design made it possible.