RBMK

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RBMK
Developing country: Soviet UnionSoviet Union Soviet Union
Reactor data
Reactor type: Boiling water reactor
Design type: Pressure tube reactor
Moderator: Graphite and (to a small extent) light water
Cooling: light water
Fuel: 235 uranium
Degree of enrichment: 1.8% to 2.4%
Vapor bubble coefficient: positive
Power classes in MW (gross): 1000, 1500, 2400 MW
Containment: Unavailable

An RBMK ( Russian Реактор Большой Мощности Канальный , transcribed Reaktor Bolshoi Moschtschnosti Kanalny , in German about high-performance reactor with channels ) is a graphite-moderated , water-cooled boiling water - pressure tube type reactor of the Soviet type.

The reactor type became known worldwide through the Chernobyl disaster , which occurred with a reactor of the type RBMK-1000. A total of 26 of these reactors were to be built, of which nine were not completed. Of the 17 RBMK reactors commissioned, 10 are still in operation (as of July 2019). Safety improvements were made after the Chernobyl accident. The last RBMK is to be shut down in 2050.

history

The RBMK reactor type was developed in the Soviet Union in the mid-1960s under the leadership of academician Nikolai Antonowitsch Dolleschal . Thereby one could fall back on experiences with the first Soviet nuclear power plants Obninsk and Belojarsk . The aim was to build a larger number of power reactors in a relatively short time and without major investments in the development of new technologies. The first RBMK reactor was Unit 1 of the Leningrad Nuclear Power Plant , which went into operation in 1973.

The largest reactors of this type, the RBMK-1500, are located in the now decommissioned Lithuanian nuclear power plant Ignalina near Visaginas . The two units, which went into commercial operation in 1984 and 1987, were the largest reactors ever built in the Soviet Union . In terms of electrical performance, it wasn't until 2018 that the RBMK-1500 was surpassed by the EPR , which went into operation in the People's Republic of China . In terms of thermal performance, the RBMK remains the record holder even after that.

construction

Simplified sectional view of the RBMK

The RBMK is a graphite-moderated boiling water pressure tube reactor. Instead of a reactor pressure vessel, it contains numerous pressure tubes with a diameter of 8 cm, in which the fuel elements are located. The chain reaction in the reactor is controlled by control rods . The heat generated by the nuclear fission is absorbed by water and its evaporation . The resulting saturated steam is passed through steam separators in order to return liquid water to the reactor and then used in steam turbines that drive generators and thus provide electricity.

In order to improve the heat transfer between the graphite blocks within the reactor , a gas mixture of helium and nitrogen circulates in the gaps between the graphite blocks. The control rods contain boron carbide (B 4 C) and can be inserted into the reactor core partly from above and partly from below. The control rods immersed from above are used to regulate the output during operation; the retractable bottom bars are used to set a uniform power distribution in the reactor core. The control rods are normally controlled by neutron detectors in the automatic control system in the reactor core. The reactor has two separate cooling systems with four pumps each, which each cool one half of the reactor core. In normal operation, three pumps are in operation, while another pump is ready to operate as a reserve. If the core overheats or the power supply is interrupted, a core emergency cooling system is started automatically.

Fuel assemblies

RBMK BE component: 1 - spacer; 2 - zircalloy shell; 3 - fuel tablet

The fuel of the RBMK initially consisted of uranium enriched to 2% 235 U. Since Chernobyl, uranium enriched to 2.4% has been used, sometimes also 2.6% or 2.8%, since higher degrees of enrichment make reactor operation more stable. The fuel is in the form of small fuel tablets made of sintered uranium dioxide with an axially central hole. They are housed in rods made of zircalloy with a diameter of 13.6 mm and a length of 3.65 meters. A fuel assembly consists of two components with 18 rods each, which are arranged cylindrically. Two of the BE components are located on top of each other in the seven-meter-long pressure tube. They can be exchanged while the reactor is in operation, as each individual pressure tube can be separated from the water circuit by valves. One fuel element contains 114.7 kg of uranium; the entire reactor contains up to 192 tons if all channels are occupied.

Control rods

RBMK control rods have a graphite displacement body below the absorber material, also known as the “graphite tip”. This detail serves to reduce xenon poisoning . Xenon-135, which acts as a neutron poison, is inevitably produced during reactor operation and is broken down by neutron capture at constant reactor power at the same rate . It becomes a problem if the control rods are partially retracted and the power should be increased again later. The neutron flux and thus the rate of degradation of xenon-135 are reduced when the power is reduced, but its generation (by radioactive decay of a fission product) initially still takes place at the previous rate, so that its concentration increases temporarily. The graphite component on the control rods now causes the empty channel to only partially fill with water when it is pulled out. Carbon absorbs neutrons much weaker than water. The graphite body therefore increases the neutron flux locally so that the Xe-135 is degraded more quickly.

Channel scheme of an RBMK reactor core (number in brackets):
_ Neutron source (12)
_ Manual control rod (167)
_ Short control rod, acting from below (32)
_ Automatic control rod (12)
_Fuel element (1661)
The numbers on the control rods indicate the depth in cm at the moment of the Chernobyl explosion.

The control and protection system of a 2nd generation RBMK can control 211 control rods. They are embedded in selected channels that are connected to a special cooling circuit. They are divided into 4 classes.

  • Manual control rods to control the radial neutron flux
  • Short control rods for controlling the axial neutron flux, which are retracted from below
  • Automatic control rods that are regulated by the control system
  • Emergency control rods

The control rods consist of boron carbide elements, each 967.5 mm long. The short control rods consist of 3 such elements, they have a total length of 3.05 m. The other rod types consist of 5 elements and are 5.12 m long. Except for the automatic control rods, all control rods are equipped with the graphite displacement bodies described above.

Reactor protection systems

On the website of the Leningrad Nuclear Power Plant (LNPP ) several automatic safety systems for the local RBMK reactors are listed. However, the description is kept general and does not contain any information about the type of measuring equipment used, redundancy and possibilities or need for human intervention.

Confinement

A containment , that is a pressure-tight containment around the reactor and radioactive ancillaries, not RBMK reactors. The confinement of light water reactors is a protection system that is intended to prevent radioactive materials, such as escaping cooling water in the event of a broken pipeline, from escaping from a secure zone. All RBMK from the second generation onwards have such a confinement.

To protect against radiation, the reactor is surrounded by thick reinforced concrete walls ( biological shield ) and several cavities that are intended as confinement. The steam separators each have their own radiation shield.

Technical specifications

Technical specifications RBMK-1000 RBMK-1500 RBMKP-2400
Thermal performance 3200 MW th 4800 MW th 6500 MW th
Electrical power 1000 MW 1500 MW 2400 MW
Coolant pressure 6.9 to 6.2 MPa 7.5 to 7.0 MPa -
Coolant flow rate 37,440 t / h - 39,300 t / h
Coolant temperature 284 ° C 277 to 290 ° C -
Steam production capacity 5,600 t / h - 8,580 t / h
Fuel enrichment 2.0% to 2.4% 2.0% 1.8% to 2.3%
Number of fuel assemblies 1,550 to 1,580 - -
Number of pressure tubes 1661 to 1693 1661 1920 (960 for steam overheating)
Number of control rods 191 to 211 235 -
Height of the reactor 7 meters 7 meters 7 meters
Size of the base of the reactor Diameter 11.8 meters Diameter 11.8 meters 7.5 × 27 meters

Advantages and disadvantages

This article or the following section is not adequately provided with supporting documents ( e.g. individual evidence ). Information without sufficient evidence could be removed soon. Please help Wikipedia by researching the information and including good evidence.

The RBMK line has some special features compared to other reactor types.

advantages

  • The systems can be built in modular design.
    • There are no large forgings like a pressure vessel.
    • This makes the construction less dependent on local conditions and the existing infrastructure.
    • By adding more compatible elements to the design, it is possible to increase the overall output up to reactor outputs that (with RBMK-1500 and 2400) are higher than with western nuclear reactors.
  • Utilization and availability are said to have been above the average for other reactors in the Soviet Union. In this respect, the systems should have proven themselves in practical operation.
  • Graphite as a moderator allows the use of fissile materials that cannot be used in light water moderated reactors.
  • The change of fuel elements is possible during operation; the reactor does not have to be switched off for this.
    • A longer annual break in operation to reload the reactor core can thus be dispensed with.
    • The reactor does not have to be equipped with a fuel supply for e.g. B. be loaded for an entire year of operation; this is a security benefit.
    • The ongoing change of fuel elements enables the production of weapons plutonium with a low 240 Pu content.

disadvantage

  • A cooling disruption can lead to an increase in the heat output. The reason for this is the positive coolant loss coefficient of this type of reactor. This is a fundamental deficit of the reactor design.
  • RBMK have a significantly increased need for inspection due to the use of pressure tubes that have many welded joints .
  • RBMK release significantly more radioactivity during normal operation compared to other constructions. The emissions lead to equivalent doses of up to 2.0  mSv per year. For comparison: an average western nuclear power plant causes doses of 0.001 mSv to 0.01 mSv in the area per year. The radiation exposure from natural sources amounts to an average of 2.4 mSv per year. For computed tomography , the radiation dose is 2 mSv to 10 mSv.
  • The reactor has no containment , but instead a so-called confinement (see above). The system becomes more complex and more prone to failure by linking several systems, especially the emergency cooling system. In addition, many safety systems at the RBMK are not available or have insufficient redundancy .
  • The reactor contains a lot of graphite . Graphite is flammable and forms flammable gases on contact with water vapor at temperatures above 900 ° C.
  • There is no real quick shutdown system , because in an emergency, the control rods need 12 to 18 seconds to go from fully extended to fully retracted and thus suppress the nuclear chain reaction, during this time it can happen in a supercritical reactor due to the rapidly rising temperature come to a meltdown. The resulting hydrogen can cause the reactor to explode.
  • One of the basics of Soviet reactor construction was to assign more skills to the human operator than to the automatic control, although this usually makes fewer mistakes.
  • The large size of the reactor favors an inhomogeneous power distribution. This places special demands on the control, especially at low power.
  • The control rods are moved electrically, which can have serious consequences in the event of a power failure.
  • There are displacement bodies made of graphite at the tips of the control rods, which increases the reactivity when the fully extended control rods are inserted into the water channels .
  • So far there is no solution for the dismantling and final storage of the radioactive graphite core.

Improvement of the facilities

Smolensk nuclear power plant with three RBMK-1000
Block 1 of the Ignalina nuclear power plant

After the Chernobyl accident , improvements were made to numerous RBMK reactors to make a repetition of the nuclear power excursion less likely. Among other things, the operating mode was changed so that instead of an operational reactivity reserve of 30 control rod equivalents, at least 45 rods must now be run in. This was achieved by using uranium with a higher fuel enrichment of 2.4% instead of 2.0% and this is compensated for with permanently installed absorber rods that cannot be moved in 80 pressure tubes. Some RBMK even use uranium enriched to 2.8%. The reason for this is that the secondary production of other substances during the fission process is supposed to cause increased neutron absorption. This makes the reactivity less dependent on the steam content of the cooling water. In addition to these changes, the void coefficient of the RBMK was reduced from + 4.5% beta to + 0.7% beta, so that a neutron excursion can be prevented more easily.

A total of 179 of the 211 control rods on the inlet side of the reactor had graphite tips, which displaced the cooling water. As a preliminary measure, the extension of the rods was limited so that they always protrude at least 1.2 m into the reactor core and thus the displacement body below the absorber material covers the lower reactor area so that no more water is replaced by graphite in the reactor core when it is retracted. Later the control rods were replaced by those with a longer holding rod between the absorber and the graphite body, so that the displacement body hangs lower when the control rod is fully extended and no increase in reactivity is possible when moving in by displacing water.

The dynamics of the control technology of the reactor were also improved by replacing the drives of the control rods. The time it takes for the control rods to move completely into the reactor core as part of an emergency shutdown has thus been reduced from 18 to 12 seconds. To improve the effectiveness of the rods, new rods made of boron carbide were installed. In addition, an emergency shutdown system was installed that no longer allows 24 emergency staffs to enter water-filled pressure tubes, but rather gas-filled tubes. These channels are wetted with a thin film of water for cooling. With this new mechanic, it takes less than 2.5 s to decrease reactivity by 2β.

In 1995, Unit 1 of the Chernobyl nuclear power plant was shut down in order to carry out major maintenance work. Some pressure tubes were removed from the reactor core in order to examine them for material strength and wear. The inspection showed that the tubes were brittle and worn. To reduce these aging effects, a new type of pressure tube has been developed. Exchanging these is one of the long-term projects, including in Smolensk 3 , as well as the replacement of old valves, the use of new safety valves and the improvement of the existing core emergency cooling system. In order to optimize the radiation shielding, improvements to the reactor building are being considered.

Further development

The RBMKP-2000 as well as the RBMKP-2400 are further developments of the RBMK with an electrical output of over 2000 MW, which were developed in the 1970s. However, the design was not completed. In the 1980s there were plans to build an RBMK-2400.

A further development of the RBMK that has not yet been used is the MKER . This type is based on the same basic principle, but has an improved security system and is enclosed by a containment. RBMK reactors can be retrofitted with the safety and computer systems that have already been developed. This was done to increase the safety standard for all RBMK reactors in Russia. The upgrade is not only intended to increase safety standards, but also to extend the operating time of existing RBMK systems. The upgrade allows a total term of 45 years. In 2006, Rosatom considered upgrading all RBMK reactors in Russia in order to extend their service life by 15 years.

Use in the USSR

Reactor hall of the RBMK-1500 in the Ignalina nuclear power plant (Block 1) from the inside with the cover stones removed
Chernobyl nuclear power plant

Nuclear power plants of the type RBMK were only built on the territory of the former Soviet Union. Today they are located in Lithuania ( Ignalina ), Russia ( Kursk , Smolensk , Leningrad , Belojarsk ) and Ukraine ( Chernobyl ). In 1980, construction of the Kostroma nuclear power plant with two RBMK-1500 reactors began in the Kostroma Oblast in what is now Russia, ten kilometers south of the city of Bui . However, the project was abandoned due to protests. In 2006 the decision was taken in Russia, the construction of the RBMK-1000 of Block 5 nuclear power station Kursk resume, this decision was in 2012 in favor of the new building of the power station Kursk II type VVER withdrawn and set construction activity.

The reactors were used in the USSR because they corresponded to the initial manufacturing capabilities of Soviet industry (no large pressure vessels required) and were relatively inexpensive to build in a short time. During the Cold War, the possibility of extracting relatively pure plutonium- 239 suitable for nuclear weapons at the same time as generating electricity was interesting ; it is based on the fact that, with this type of reactor, individual fuel elements can be continuously replaced after a short dwell time without having to shut down the reactor. However, it is not known whether these reactors were actually used for this purpose.

There are three generations of RBMK reactors in total. The first generation reactors (OPB-72) were built until around the mid-1970s. The second generation is called the OPB-82 and was built from the late 1970s to the early 1980s. The name OPB-82 comes from the fact that the reactor met the safety standards of 1982. After the Chernobyl disaster , the third generation of RBMK reactors, named OPB-88, which met the safety standards of 1988, was developed. There are a total of six first-generation reactors, three of which have been decommissioned. There are also eight OPB-82 generation reactors, one of which was damaged in the Chernobyl accident, another was shut down and construction of two units was suspended. There is a completed reactor of the generation OPB-88. The construction of three more OPB-88s was discontinued.

In the USSR itself, the technology became a showcase for the then new nuclear technology of the Soviet Union. Until 1986, the Chernobyl nuclear power plant , the largest in Ukraine , with its four RBMK-1000 was a model plant. The German specialist magazine Atomwirtschaft also wrote in December 1983: "The reliability of Chernobyl is very high". At the time of the accident, Unit 4 was the newest reactor on site, and with it the power plant had an output of 4  GW . The expansion to 6 GW was already underway in 1986. The nuclear power plant was thus one of the youngest in the Soviet Union. In 1986 an attempt to test improvements to the emergency power system in Unit 4 triggered a catastrophic accident . Since then, the RBMK reactor type has been criticized for safety concerns. For this reason, many construction projects were terminated and plans abandoned.

See also

literature

Web links

Commons : RBMK  - collection of images, videos and audio files

Individual evidence

  1. ↑ RBMK Reactors | reactor bolshoy moshchnosty kanalny | Positive void coefficient - World Nuclear Association. Retrieved August 4, 2019 .
  2. ↑ Medvedev, Grigori: Burned Souls - The Chernobyl Catastrophe . Hanser Verlag, 1991 ISBN 3-446-16116-3
  3. ↑ Chernobyl - Ten Years After. The accident and the safety of the RBMK plants, Society for Plant and Reactor Safety, Cologne, February 1996, (GRS; Vol. 121), ISBN 3-923875-74-6 , page 36
  4. ↑ a b c d e f world-nuclear.org
  5. ↑ Mikhail V. MALKO: The Chernobyl Reactor: Design Features and Reasons for Accident. In: Recent Research Activities about the Chernobyl NPP Accident in Belarus, Ukraine and Russia. Kyoto University, July 2002, accessed January 13, 2020 .
  6. ↑ LNPP - Emergency reactor protection system (English)
  7. ↑ a b LNPP - Confinement (English)
  8. ↑ AECL - Chernobyl - A Canadian Perspective ( Memento of the original from February 4, 2012 in the Internet Archive ) Info: The archive link was automatically inserted and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF). @1@ 2Template: Webachiv / IABot / canteach.candu.org
  9. ↑ LNPP - Main characteristics of RBMK-1000 (English)
  10. ↑ Rosatom-Volgodonsk-Generation  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (English).@1@ 2Template: Dead Link / vnpp.rosenergoatom.ru  
  11. ↑ LNPP - Design and main characteristics (English).
  12. ↑ a b AECL - Russian Nuclear Power Program (past, present, and future) Dr. IgorPioro, Senior Scientist, CRL AECL  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (English).@1@ 2Template: Dead Link / www.cns-snc.ca  
  13. ↑ Handbook about the Ignalina NPP (English; PDF; 382 kB).
  14. ↑ I. S. Zheludev, LV Konstantinov: Nuclear power in the USSR. In: IAEA Bulletin. Volume 22, No. 2, Vienna 1980. pp. 34–45, iaea.org (PDF; 372 kB).
  15. ↑ Basic knowledge of nuclear energy (brochure on nuclear energy)
  16. ↑ a b INSP - The RBMK (English)
  17. ↑ a b c d The reactor accident in Chernobyl (PDF) on kernfragen.de.
  18. ↑ (PDF; 657 kB) p. 10 Study on Russian nuclear power plants ( Memento from April 21, 2014 in the Internet Archive )
  19. ↑ Emissions of the RBMK  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Dead Link / www.rosenergoatom.ru  
  20. ↑ Brochure - Emissions from Nuclear Power Plants and Radiation Exposure; May 2008 ( Memento of the original dated February 6, 2009 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF). @1@ 2Template: Webachiv / IABot / www.kernenergie.de
  21. ↑ Radiation exposure by CT
  22. ↑ Anatoly Dyatlov: How it was: an operator's perspective. In: Nuclear Engineering International. Global Trade Media, November 1991, accessed January 13, 2020 .
  23. ↑ A nuclear power plant for the energy island. In: FAZ , July 15, 2011. Retrieved July 15, 2011.
  24. ↑ a b c INSAG-7: The Chernobyl Accident: Updating of INSAG-1. In: Safety Report Series. IAEA , November 1992, accessed January 13, 2020 .
  25. ↑ IAEA - Performance analysis of WWER-440/230 nuclear power plants (PDF; 9.2 MB), p. 25 (English).
  26. ↑ Technology and Soviet Energy Availability - November 1981 - NTIS order # PB82-133455 (PDF; 5.8 MB), p. 122 (English).
  27. ↑ Gabaraev, Cherkashov et al. a .: Multiloop Pressure Tube Power Reactors (MKER) - Consolidation of Expertise in Design of Domestic Pressure Tube Reactors ( Memento of the original from September 27, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.nikiet.ru
  28. ↑ Rosenergoatom "Directorate for Construction of Kostroma NPP" ( Memento of the original of September 27, 2007 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (English). @1@ 2Template: Webachiv / IABot / www.rosenergoatom.ru
  29. ↑ Nuclear Energy: World Report 2006; atw 52nd year (2007) Issue 4 - April ( Memento of the original from September 29, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF). @1@ 2Template: Webachiv / IABot / www.ktg.org
  30. ↑ Vladimir Slivyak: COMMENT: Rosatom scraps ancient Chernobyl reactor project at Kursk: Right decision, wrong message , Bellona, ​​March 6, 2012. Accessed September 30, 2016.