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RBMK is an acronym for the Russian reaktor bolshoy moshchnosti kanalniy (Russian: Реактор Большой Мощности Канальный) which means "High Power Channel-type Reactor", and describes a class of graphite-moderated nuclear power reactor which was built in the Soviet Union for use in nuclear power plants to produce nuclear power from nuclear fuel. The RBMK reactor was the type involved in the Chernobyl disaster. In 2010, there were at least 11 RBMK reactors operating in Russia,[1] but there are no plans to build new RBMK type reactors (the RBMK technology was developed in the 1950s and is now considered obsolete) and there is international pressure to close those that remain.

Schematic diagram of an RBMK

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", Atom Mirny, Russian for "peaceful atom") produced 5 MW of electricity (30 MW thermal) and delivered power to Obninsk from 1954 until 1959.

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.

Contents

Design

Reactor hall of the RBMK-1500 at Ignalina Nuclear Power Plant, Lithuania

An RBMK employs long (7 metre) 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 3.65 metres 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, as with the disaster at Chernobyl.

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.

Safety

The RBMK design has several types of safety systems needed for normal operation and emergency situations. In-core feedback sensors monitor the amount of reactivity during operation; if they detect an increase they can automatically insert control rods to reduce power, if they detect a decrease in power they raise controls rods to increase power. If the sensors detect a sharp increase in output they can insert all 211 boron control rods to stop the reaction altogether. There is also a separate control system, the Reactor Protection System. This system is automatically activated when needed or can be manually activated by the operators. RBMK reactors also have a radiation monitoring station that monitors radiation from the plant and the nearby environment. A large amount of shielding is provided to absorb radiation produced under both normal operation and emergency situations. The RBMK reactor also has an Accident Localization System which serves as a containment but this system can only handle minor pipe breaks. The Accident Localization System's ineffectiveness was shown in the Chernobyl incident.

High Positive Void Coefficient

Light water (the ordinary H2O) is both a neutron moderator and a neutron absorber. This means that not only can it slow down neutrons to velocities in equilibrium with surrounding molecules ("thermalize" them and turn them into low-energy neutrons that are far more likely to interact with the Uranium nuclei than the fast neutrons produced by fission initially), but it can also absorb some of them outright. Heavy water is also a good neutron moderator, but does not absorb neutrons as easily.

In RBMKs, light water was used as a coolant; moderation was instead carried out by graphite. As graphite already moderated neutrons, light water had a lesser effect 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.

In the case of evaporation of water to steam, the place occupied by water would be occupied by water vapor, which has a density vastly lower than that of liquid water (the exact number depends on pressure and temperature; at standard conditions, steam is about 1/1350th as dense as liquid water). Because of this lower density (of mass, and consequently of atom nuclei able to absorb neutrons), light water's neutron-absorption capability practically disappears when it boils. This allows more neutrons to fission more U-235 nuclei and thereby increase the reactor power, which leads to higher temperatures that boil even more water, creating a thermal feedback loop.

In RBMKs, generation of steam in the coolant water would then in practice create a void, a bubble that does not absorb neutrons; the reduction in moderation by light water is irrelevant, as graphite is still moderating the neutrons, enabling them to be absorbed more easily to continue the reaction. 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.

A high void coefficient does not automatically make a reactor unsafe, as some of the fission neutrons are emitted with a delay of seconds or even minutes (post-fission neutron emission from daughter nuclei), so steps can be taken to reduce the fission rate before it gets too high, but it does make it much harder to control the reactor and makes it imperative that the control systems be very reliable. Some RBMK designs did include control rods on electromagnetic grapples, thus controlling the reaction speed and, if necessary, stopped the reaction completely. The RBMK at Chernobyl, however, had manual control rods.

After the Chernobyl disaster, all RBMKs in operation underwent significant changes, lowering their void coefficients to +0.7 β. This new number precludes the possibility of a low-coolant meltdown.

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.

Initially, the RBMK design focused solely on accident prevention and mitigation, not on containment of severe accidents. However, since the Three Mile Island accident, 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 metres), the cost and difficulty of building a heavy containment structure prevented building of additional emergency containment structure for pipes on top of the reactor. In the Chernobyl accident, the pressure rose to levels high enough to blow the top off the reactor, breaking open these pipes in the process.

Improvements since the Chernobyl accident

In his posthumously published memoirs, Valeri Legasov, the First Deputy Director of the Kurchatov Institute of Atomic Energy, revealed that the Institute's scientists had long known that the RBMK reactor had significant design flaws.[2][3] Legasov's death from suicide, apparently as a result of becoming bitterly disillusioned with the failure of the authorities to confront the flaws, caused shockwaves throughout the Soviet nuclear industry and the problems with the RBMK design were rapidly accepted.[4][5]

Following Legasov's death[4] all remaining RBMKs were 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 first speed up the reaction and after that begin slowing or stopping it. This design flaw contributed to the first explosion of the Chernobyl accident.

The updates are:

  • An increase in fuel enrichment from 2% to 2.4% to compensate for control rod modifications and the introduction of additional absorbers.
  • 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 unauthorized access to emergency safety systems.

Development

A development of the RBMK is the MKER (Russian: МКЭР, Многопетлевой Канальный Энергетический Реактор [Mnogopetlevoy Kanalnyi Energeticheskiy Reaktor] which means Multi-loop pressure tube power reactor), with improved safety and containment.[6][7] The physical prototype of the MKER-1000 is the 5th unit of the Kursk nuclear power plant. The construction of Kursk 5 is still uncertain.[8] A MKER-800, MKER-1000 and MKER-1500 planned for the Leningrad nuclear power plant.[9][10][11]

Closures

Of the 17 RBMKs built (one was still under construction at the Kursk Nuclear Power Plant), all three surviving reactors at the Chernobyl plant have now been closed (the fourth having been destroyed in the accident). Chernobyl 5 and 6 were under construction at the time of the mishap at Chernobyl, but further construction was stopped due to the high level of contamination at the site limiting its longer term future. Both reactors at Ignalina in Lithuania were also shut down. [12]. The others built are still operational at Saint Petersburg (4 RBMK-1000), Smolensk (3 RBMK-1000) and Kursk (4 RBMK-1000) [13].

Status

Reactors which are in operation marked in green, closed reactors red, reactors under construction yellow and abandoned plants marked blue.

Location[14] Reactor type Status Net
Capacity (MW)
Gross
Capacity (MW)
Chernobyl-1 RBMK-1000 shut down in 1996 740 800
Chernobyl-2 RBMK-1000 shut down (1991 after accident) 925 1,000
Chernobyl-3 RBMK-1000 shut down in 2000 925 1,000
Chernobyl-4 RBMK-1000 shut down (1986 after accident) 925 1,000
Chernobyl-5 RBMK-1000 construction cancelled in 1988 950 1,000
Chernobyl-6 RBMK-1000 construction cancelled in 1988 950 1,000
Ignalina-1 RBMK-1500 shut down in 2004 1,185 1,300
Ignalina-2 RBMK-1500 shut down in 2009 1,185 1,300
Ignalina-3 RBMK-1500 construction cancelled in 1988 1,380 1,500
Ignalina-4 RBMK-1500 plan cancelled in 1988 1,380 1,500
Kostroma-1 RBMK-1500 construction cancelled in 1980s 1,380 1,500
Kostroma-2 RBMK-1500 construction cancelled in 1980s 1,380 1,500
Kursk-1 RBMK-1000 operational 925 1,000
Kursk-2 RBMK-1000 operational 925 1,000
Kursk-3 RBMK-1000 operational 925 1,000
Kursk-4 RBMK-1000 operational 925 1,000
Kursk-5 RBMK-1000 under construction since 1980 925 1,000
Kursk-6 RBMK-1000 construction cancelled in 1993 925 1,000
Leningrad-1 RBMK-1000 operational 925 1,000
Leningrad-2 RBMK-1000 operational 925 1,000
Leningrad-3 RBMK-1000 operational 925 1,000
Leningrad-4 RBMK-1000 operational 925 1,000
Smolensk-1 RBMK-1000 operational 925 1,000
Smolensk-2 RBMK-1000 operational 925 1,000
Smolensk-3 RBMK-1000 operational 925 1,000
Smolensk-4 RBMK-1000 construction cancelled in 1993 925 1,000

References

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