Boiling water reactor

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The boiling water reactor ( BWR ) is a light water - nuclear reactor for power generation in power plants , in which water as a moderator is used and coolant. After the pressurized water reactor (PWR), which is also usually operated with light water, it is the most common type of nuclear reactor (20% of the world's nuclear energy generation). In contrast to the PWR with primary and secondary circuits, the BWR only has a single steam-water circuit. The cycle of the radioactively contaminated coolant is therefore not limited to the safety container (containment). The achievable efficiency of a BWR power plant is slightly higher than the value of PWR power plants, since the water evaporates in the reactor itself and the power loss of the additional heat transfer in the evaporator is eliminated. The pressure and temperature in the reactor pressure vessel are lower than in the PWR, but harsher corrosion conditions prevail in the reactor due to the formation of vapor bubbles ( two-phase flow ).

The boiling water reactor was developed by the Argonne National Laboratory and General Electric in the mid-1950s under the direction of Samuel Untermyer II . The main current manufacturer is GE Hitachi Nuclear Energy , a company headquartered in Wilmington, North Carolina specializing in the design and construction of this type of reactor.

Scheme of a nuclear power plant with a boiling water reactor

Mode of action

The reactor model of the SWR Leibstadt nuclear power plant in its information center

The preheated feed water is pumped into the reactor pressure vessel , which is isolated from the rest of the structure by the safety vessel . The fuel elements are located in the pressure vessel , mostly with uranium dioxide enriched to around 4% as fuel. The reactor pressure vessel is about two thirds full with water . Due to the heat generated during nuclear fission , water evaporates (evaporative cooling ) at z. B. 71 bar and 286  ° C in the reactor pressure vessel; this steam drives the turbine . A generator converts the energy supplied by the turbine into electricity . The relaxed water vapor is liquefied by cooling water in the condenser and fed back into the circuit. The amount of steam in a boiling water reactor is typically around 7,000 tons per hour.

The reactor output can be regulated via circulation pumps within the reactor pressure vessel in the range between about 50 and 100% for load adjustment . In addition, it can be regulated via the neutron flux using control rods made of boron carbide , hafnium or cadmium . Since the average moderator density in the upper area is lower due to the vapor bubbles, the control rods in the BWR are retracted from below so that the power density remains as homogeneously distributed as possible. When all circulating pumps are switched off, the output drops to 30 to 40% of the nominal output in the so-called natural circulation point. The (potential) efficiency of a boiling water reactor is insignificantly greater than that of the pressurized water reactor (≈ 33%); the net efficiency of a BWR is around 35%, as lower temperature and pressure are used. In practice, however, the differences in efficiency play only a subordinate role, since the fuel costs for electricity generation are only around 20%.

Security and Contamination

Assembly work on the control rod drives in Gundremmingen

The steam turbine in the boiling water reactor - in contrast to the pressurized water reactor - is operated directly by the water vapor generated in the reactor pressure vessel, so that the radioactive substances transported with the steam reach the turbine building ( machine house ). In Germany, this is therefore - unlike the pressurized water reactor - part of the controlled area . This results in the following three main differences:

  • A large part of the nacelle can only be accessed to a limited extent during operation; However, this is possible shortly after switching off (see below).
  • The turbine shaft must be equipped with an elaborate system for sealing and for sucking off any leaking steam.
  • A complex system is required for suctioning off and treating the gases carried along with the steam.

The radioactive substances contaminating the steam can be divided into three groups:

Waterborne fabrics
These are activated ions (e.g. 24 Na ), metal particles from the pipe and container materials (e.g. 60 Co ) and water-soluble fission products (e.g. 137 Cs , 99m Tc ).
In the BWR nuclear power plants currently in operation, a combination of water separator and steam dryer is built into the reactor pressure vessel . The waterborne contamination therefore remains for the most part within the reactor pressure vessel together with the separated water.
Gaseous substances
The gaseous substances are practically completely discharged from the reactor pressure vessel with the steam and pass through the turbine. During the subsequent condensation of the steam, the gases are sucked out of the condenser and fed to an exhaust gas treatment system.
The dominant part of the radioactivity in the steam consists of the nitrogen isotope 16 N, which by activation of the oxygen isotope 16 O is produced. 16 N has a half-life of 7 seconds. After the reactor has finished operating, the machine house can therefore be accessed again after a few minutes. Furthermore, gaseous fission products occur in the steam , mainly radioactive isotopes of the noble gases krypton and xenon .
iodine
The transfer of iodine isotopes from the reactor water to the steam is determined on the one hand by the water solubility and on the other hand by the volatility of the iodine or its chemical compounds. The concentration of radioactive iodine in the steam is generally higher than that of the water-borne isotopes.

The radioactive substances in the steam and their decay products contaminate the pipes and parts of the turbines on the surface. If such parts are replaced, the old materials must be decontaminated by removing the surface, for example by sandblasting , before scrapping . Lines carrying reactor water are decontaminated using chemical processes prior to inspection or replacement.

The control rods in the German and generally newer boiling water reactors are adjusted by electric drives . Independently of this, a hydraulic system is available for the emergency shutdown , in which water under high pressure pushes the control rods into the reactor. The emergency shutdown system is based on the fail-safe principle , i.e. H. Errors in the system lead to the automatic triggering of the emergency shutdown. In addition, there is a system for feeding in a boron salt solution, i.e. neutralized boric acid , which has a high cross-section for neutron capture and can therefore make the reactor subcritical .

Regardless of the reactor type, the decay heat must be dissipated after shutdown . In the case of a boiling water reactor, this can be done by discharging steam into the turbine condenser or into a condensation tank. Despite the high energy dissipation via the heat of evaporation , the boiling water reactor requires a sustained and sufficient water make-up. In many boiling water systems, a high-pressure pump is available for this purpose, which is driven by a small steam turbine. This simultaneously removes energy from the reactor and replenishes water. This unit can also be fed from batteries so that core cooling is possible for a limited time even without emergency power generators.

A difference to the pressurized water reactor is that if the coolant is lost to below the upper edge of the reactor core, the upper part of the fuel assemblies is still cooled to a limited extent by steam flowing past. The nuclear accidents at Fukushima I showed that this did not prevent damage to the fuel elements in the earlier boiling water reactor series due to overheating.

Cooling failure

Failure to cool the reactor out of operation leads to overheating and subsequent melting of the fuel rods ( core meltdown ). The fuel rod claddings , which are usually made of zircalloy , react chemically with water at high temperatures. In this case, hydrogen is formed. When mixed with air, an explosive mixture is created which can lead to violent oxyhydrogen gas explosions in the reactor building.

The classic German safety philosophy for nuclear power plants assumed that the biggest accident to be assumed ( GAU ) would be a break in the main coolant line with complete loss of the cooling water. This so-called design-basis accident should be able to be controlled as a permit requirement without massive pollution of the environment. If there is a partial or complete core melt, a radioactive melt with a temperature of up to 2400 ° C collects at the bottom of the reactor pressure vessel and can cause the bottom of the vessel to melt through. When the radioactive melt has penetrated the reactor pressure vessel as well as the containment, a large part of the radioactivity of the reactor is released into the environment. This event is known as a worst-case scenario because it goes beyond the worst-case scenario for which the nuclear power plants are designed in terms of safety. If the radioactive melt, the so-called Corium , z. B. in the form of external cooling water, a water vapor explosion can take place, in which considerable amounts of the material are released into the atmosphere. The hot radioactive material can also melt into the ground and radiate the groundwater . The presence of a core catcher could prevent this melting into the ground. At the moment, however, only a few nuclear power plants are equipped with core catchers.

Construction lines used in Germany

Mark I type SWR

In Germany, a boiling water reactor is only in operation at the Gundremmingen site (until 2021). Boiling water reactors from the US company General Electric are widely used internationally . The reactor cores of the series 1–4 (BWR / 1 to BWR / 4) called Boiling Water Reactor (BWR) were installed in a containment of the type Mark I or from the reactor core series BWR / 5 of the type Mark II . The first generation of boiling water reactors built in Germany also goes back to a cooperation with General Electric.

First generation (GE-AEG)

KRB Block A in Gundremmingen, August 1966

With the boiling water reactors in Germany (and partly in other countries) a distinction is made between different construction lines. A typical feature of the types of the first building lines was the dome-shaped building with a containment under the concrete shell. These reactors were designed by AEG in collaboration with General Electric in the 1950s and 1960s . German power plants of this construction line were Kahl , Gundremmingen A and Lingen . All three reactors have now been shut down and dismantled , or are in the dismantling phase. In the neighboring countries of Germany, later generations of boiling water reactors built by General Electric are still in operation. B. the Swiss plant in Leibstadt .
A special design of the aforementioned reactor type was the Großwelzheim superheated steam reactor in Karlstein am Main, right next to the Kahl nuclear power plant.

Building line 69 (KWU)

The second construction line is construction line 69. This type of reactor was designed in 1969 by what was then Kraftwerk Union . A typical feature of these power plants are the box-shaped structures and the separate spherical containment inside the building. A direct forerunner of the type 69 was the decommissioned and dismantling Würgassen nuclear power plant .

The ARD political magazine " Fakt " reported on March 14th, 2011 that an Austrian study on the construction line 69 identified a serious design flaw: the weld seam of the reactor pressure vessel can develop hairline cracks that could lead to breakage. According to the study, this risk also exists in the series 69 power plants used in Germany. According to the report, there is a risk that checking the endangered weld seams will be difficult or even impossible. This construction error cannot be remedied by modifications.

The nuclear power plants were still in operation until 2011

The latter plant was the most powerful boiling water reactor in the world until Oskarshamn 3 was increased in output in 2010/11.

After the nuclear moratorium imposed by the federal government in March 2011 as a result of the reactor disaster in Fukushima, the federal and state governments decided at the end of May 2011 to shut down the aforementioned reactors (and four others).

Building line 72 (KWU)

The last building line to be realized in Germany so far is building line 72, also named after the year it was designed. The reactors of these power plants are housed in cylindrical buildings. Inside the reinforced concrete shell there is a cylindrical containment. Units B and C of the Gundremmingen nuclear power plant were the only nuclear power plant in the world to be built with reactors of this construction line. Building line 72 is a technical further development of the 69 building line, with a revised safety concept and a new building concept and design.

After the shutdown of Gundremmingen B on December 31, 2017, Gundremmingen C is the only boiling water reactor still in operation in Germany.

According to the Helmholtz Association , compared to the General Electric boiling water reactors in Fukushima , the series has better safety equipment, including a 6-fold redundant emergency power supply, passive cooling systems, a stronger containment building, a pressure relief chimney and the option of compensating for coolant losses from outside.

Further development

Under the name KERENA (until March 2009 SWR 1000), Areva NP is developing the successor to the 72 series in cooperation with E.ON , a boiling water reactor with an electrical output of 1250 MW. AREVA NP and the Canadian province of New Brunswick signed a letter of intent in July 2010, which includes the construction of a KERENA as an option. Advanced American BWR versions are the ABWR and the ESBWR .

Scope and locations

Boiling water reactors are less common than pressurized water reactors, although both types of reactors are similar in efficiency . Their advantage over pressurized water reactors is the lower structural effort (there is only one water circuit instead of two, operating pressure and temperature are significantly lower) as well as theoretically simpler accident control. A major disadvantage is the restricted accessibility of parts of the machine house during power operation due to the radiation prevailing there (primarily because of 16 N activity). The performance of the boiling water reactor is regulated between about 50 and 100 percent by changing the rate of circulation of the water and thus the vapor bubble content in the reactor. Because of its higher control speed , the boiling water reactor can be used to generate medium loads.

So that the distribution of the vapor bubbles in the reactor water is largely uniform, the BWR must be vertical. In the usual construction with internal boiling, it can therefore not be used as a ship reactor.

A variant of the boiling water reactor is the boiling water pressure tube reactor , the best-known type of which is the RBMK , a Soviet-style reactor that was used in the exploded Chernobyl nuclear power plant .

Locations in Germany:

Locations in Switzerland:

Location in Austria:

Further plants with SWR in Europe:

Load following operation

For most German nuclear power plants (KKW), the ability to operate in a load sequence was a design criterion that determined the concept. For this reason, the core monitoring and the reactor control have already been designed when the reactors are designed so that no subsequent upgrading of the systems for load-following operation is necessary. The Bavarian state government replied to the request that all Bavarian NPPs are designed for load-following operation. German SWR that were (or will be) run in load-following mode are e.g. E.g .: Gundremmingen Blocks B and C , Isar 1 and Philippsburg 1 .

For German SWR, the minimum output is sometimes 35, sometimes 60% of the nominal output, and the output gradients are 3.8 to 5.2% of the nominal output per minute. Power gradients of up to 100 MW per minute can be achieved relatively easily with SWR in the range between 60 and 100% nominal power by changing the speed of the forced circulation pumps. With the SWR, however, certain operating states may require limited power change rates and reduce the load following capability to around 1% of the nominal power per minute.

The output regulation of the BWR is done either by varying the core throughput (coolant throughput) or by moving the control rods.

Isar NPP 1

At the Isar 1 nuclear power plant , load changes were carried out during ongoing operation by varying the core throughput.

Control rods

Load change adjustments with control rods were mainly made when starting up the reactor with a low core throughput and low reactor power. Control by means of control rods is not suitable for load following operation for the following reasons:

  • The rate of load change is relatively slow and depends on the extension length and the position of the control rods in the reactor.
  • The local fuel load is very high because the power only changes in the control rod cells concerned.
  • The power distribution in the core is greatly changed, which has a negative effect on the local fuel load.

Core throughput

By changing the core throughput by means of the forced circulation pumps, the mean vapor bubble content in the core and thus the moderation changes (see negative vapor bubble coefficient ). This is the usual method of carrying out load following operation on the SWR. The advantages of this procedure are:

  • The power distribution in the core remains almost unchanged. The load change is thus evenly distributed over the reactor core.
  • The load change can theoretically be carried out with a load change rate of 10% per second.

restrictions

The following restrictions must be observed:

  • In the case of longer periods of partial load, the control rods must be retracted due to changes in the concentration of 135 xenon in order not to reach the limits of the operating map (see xenon poisoning ).
  • If the forced circulation pumps fail, attention must be paid to neutron flux oscillations (core stability).
  • If the use of the control rods in load following operation is to be avoided, the maximum permissible load stroke must be restricted.

See also

literature

  • Hans Michaelis: Manual of nuclear energy . Deutscher Taschenbuch-Verlag, Munich 1982, ISBN 978-3-423-04367-0 .
  • Richard Zahoransky: Energy technology. Energy conversion systems. Compact knowledge for studies and work with 44 tables , 5th, revised. and exp. Edition, Vieweg Teubner, Wiesbaden 2010, ISBN 978-3-8348-1207-0 .
  • Hanno Krieger: Fundamentals of radiation physics and radiation protection , 3rd, revised. and exp. Edition, Vieweg Teubner, Wiesbaden 2009, ISBN 978-3-8348-0801-1 .
  • Markus Borlein: Kerntechnik , 1st ed. Edition, Vogel Industrie Medien, Würzburg 2009, ISBN 978-3-8343-3131-1 .
  • Ulrich Kilian: How does it work? Die Technik , 6th, updated edition. Edition, Meyers, Mannheim 2011, ISBN 978-3-411-08856-0 .

Web links

Wiktionary: Boiling water reactor  - explanations of meanings, word origins, synonyms, translations
Commons : Schematic drawings of boiling water reactors  - collection of images, videos and audio files

Individual evidence

  1. ^ Nuclear power plants, world-wide, reactor types; European Nuclear Society, 2015
  2. ^ Karl-Heinz Neeb: The radiochemistry of nuclear power plants with light water reactors, page 235
  3. ↑ On the volatility of borates in boiling water reactors (PDF; 741 kB)
  4. ↑ Johann Bienlein and Roland Wiesendanger : Introduction to the structure of matter , p. 205. BG Teubner Verlag, Leipzig, 2003.
  5. ↑ Wolfgang Kromp et al .: Weakness Report Boiling Water Reactors, Building Line 69 (PDF file; 1.4 MB), ISR Report 2010 / 2a, October 2010.
  6. ↑ Brochure: The Krümmel nuclear power plant goes into operation, special print from "Atomtechnik 29 (1984)", publisher Kraftwerk Union AG
  7. ↑ Sweden: Oskarshamn-3 with fully increased performance  ( 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.nuklearforum.ch  
  8. ↑ Brochure: Start in 4 Phases, special print from " Energiewirtschaftliche Tagesfragen 36 (1986)", publisher Kraftwerk Union AG
  9. ↑ What is the difference between the German boiling water reactors and the reactors in Fukushima? , Report by the Helmholtz Association , Helmholtz-Zentrum Dresden-Rossendorf , Forschungszentrum Jülich , Karlsruhe Institute of Technology , 2011, accessed on July 30, 2015
  10. ↑ Handelszeitung [1]
  11. ↑ Focus on the energy market - nuclear energy - special edition for the 2010 annual edition (PDF; 2.1 MB; p. 10) BWK DAS ENERGIE-FACHMAGAZIN, May 2010, accessed on May 24, 2015 .
  12. ↑ a b c d Holger Ludwig, Tatiana Salnikova and Ulrich Waas: Load changing capabilities of German NPPs. (PDF 2.4 MB pp. 5–6) (No longer available online.) Internationale Zeitschrift für Kernenergie, atw Volume 55 (2010), Issue 8/9 August / September, archived from the original on July 10, 2015 ; accessed on May 24, 2016 . 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 / de.areva.com
  13. ↑ a b c d e Matthias Hundt, Rüdiger Barth, Ninghong Sun, Steffen Wissel, Alfred Voß: Compatibility of renewable energies and nuclear energy in the generation portfolio - technical and economic aspects. (PDF 291 KB, pp. 6–7) University of Stuttgart - Institute for Energy Economics and Rational Energy Use, October 2009, accessed on May 24, 2016 .
  14. ↑ a b c Written question from the Member of Parliament Ludwig Wörner SPD from July 16, 2013 - regulability of Bavarian nuclear power plants. (PDF; 15.1 KB) (No longer available online.) Www.ludwig-woerner.de, July 16, 2013, archived from the original on May 24, 2016 ; Retrieved May 7, 2016 . 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.ludwig-woerner.de
  15. ↑ a b c d e f Martin Frank: LOAD SEQUENCE OPERATION AND PRIMARY CONTROL - EXPERIENCE WITH THE BEHAVIOR OF THE REACTOR. (PDF 92.6 KB pp. 1–2) E.ON Kernkraft - GmbH Kernkraftwerk Isar, accessed on May 24, 2016 .