Pressurized water reactor

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Construction of a nuclear power plant with pressurized water reactor, visible the primary circuit (red in the containment ), the secondary circuit to the machine house and the tertiary circuit to the river and cooling tower

The pressurized water reactor ( PWR ; English Pressurized Water Reactor , PWR ) is a nuclear reactor type, in which water as a moderator is used and coolant. In contrast to the boiling water reactor, the operating pressure of the water is selected to be so high that it does not boil at the intended operating temperature. The fuel rods are therefore evenly wetted, the heat distribution on their surface is balanced, and the vapor phase with its special corrosive effect is eliminated. The even heat distribution causes a smooth control behavior with good utilization of the released energy.

The water heated in the reactor core ( primary circuit ) releases its heat in a steam generator to a separate water-steam circuit, the secondary circuit . The secondary circuit is free of radioactivity from abrasion and corrosion products, which z. B. the maintenance of the steam turbine is much easier.

Mostly light water (H 2 O) is used as a cooling medium for the fuel rods , i.e. as a transport medium for the heat energy gained. These reactors therefore belong to the light water reactors . According to the International Atomic Energy Agency, there are around 279 of these reactors worldwide (as of 2015). The use of heavy water (D 2 O) is also possible, but is only used in around 10 percent of all reactors worldwide (see heavy water reactor ). Overall, pressurized water reactors are the most common type of reactor worldwide; they have a share of 68% of the total nuclear power generation.

history

Alvin Weinberg was the inventor of the pressurized water reactor (abbreviation PWR) in the early 1950s. The first partially commercially operated pressurized water reactor was located in the Shippingport nuclear power plant in the USA. It started operations in 1957. The development was based on preliminary work by the US Navy for ship propulsion.

technical description

Primary circuit

Reactor vessel with indicated core of a PWR

A variable amount of boric acid is added to the coolant water . Boron is an effective neutron absorber ; the output of the reactor can therefore be slowly regulated by the boric acid concentration and adapted to the gradual burn-up of the fuel. The control rods are used for fast power control and load adjustment . An automatic power stabilization results from the physical dependence of the reactivity on the fuel and coolant temperature, because a temperature increase in the reactor means:

  • increased fuel temperature: This increases the tendency of the uranium isotope 238, which cannot be fissioned by thermal neutrons , to absorb these neutrons (see Doppler coefficient ).
  • Increased coolant temperature, lower density: This reduces the moderation effect of the coolant, so that fewer thermal neutrons are available for splitting uranium-235 nuclei.

These effects reduce the reactivity and thus the performance of the reactor.

In the primary circuit, the coolant is passed through the reactor core under increased pressure of up to 160 bar , where it absorbs the heat generated by nuclear fission and heats up to 330 ° C. From there it flows into the steam generator, which is designed as a tube bundle heat exchanger . After the heat has been transferred, the coolant is pumped back into the reactor core by centrifugal pumps. This has the advantage over the boiling water reactor that the coolant, which is always somewhat radioactive, is always inside the containment . Therefore, no radiation protection measures are necessary in the machine house .

In order to achieve a radial temperature distribution that is as uniform as possible, the initial loading is carried out with fuel assemblies with an enrichment level that increases from the inside out. After the end of the first fuel cycle (around 1 year), only the outer third of the inventory is replaced by new fuel elements, which are moved from the outside to the inside in the course of the following cycles. In addition to this aim of uniform radial power density distribution, other core loads can either increase the burn-up of the fuel elements or a lower neutron flux near the wall of the reactor pressure vessel can be achieved.

Secondary circuit

The water in the secondary circuit is under a pressure of around 70 bar, which is why it only evaporates on the heating pipes of the steam generator at 280 ° C. In a nuclear power plant block with an electrical output of 1400 MW, which is common in Germany, the amount of steam produced for all steam generators together is around 7000 tons per hour. The water vapor is fed into a steam turbine via pipes , which generates electricity via the connected generator . The steam is then condensed in a condenser and fed back to the steam generators as water with the feed pump .

The condenser, in turn, is cooled with cooling water, usually from a river. Depending on the initial temperature and the flow of the river, this cooling water has to be cooled down again before it is returned to the river. For this purpose, part of the cooling water is made to evaporate in a cooling tower. This creates white clouds over the cooling towers in some weather conditions .

Pressurized water reactors have an efficiency of 32–36% (if the uranium enrichment is included), so very similar values ​​to a NPP of the boiling water reactor type. The efficiency could be increased by a few percentage points if the steam temperature could be increased to over 500 ° C, as in coal-fired power plants . The maximum temperature of the primary coolant is limited by the principle of supercooled boiling used to temperatures below the critical point and thus live steam temperatures of this type cannot be achieved in a conventional pressurized water reactor.

Examples of the pressurized water reactor are the convoy built by Siemens in Germany in the 1980s , the N4 built by Framatome in France and the Soviet VVER . Areva NP is currently building a European Pressurized Water Reactor (EPR) in Olkiluoto ( Finland ) , a further development of the convoy and N4 nuclear reactors .

Pressurized water reactors already have a long technical development behind them. This type of reactor was initially built in large numbers to drive warships such as the Nimitz class . The first application for peaceful purposes was the Shippingport nuclear power station , USA, completed in 1957 , with an output of 68 MW.

Security container

The reactor pressure vessel of a pressurized water reactor is surrounded by one or more nested safety containers (containments). The containment does not have an operational function, but serves to close off various operational areas from one another and from the outside.

In the normal or special operating conditions considered in the design (see design basis accident ), the inner containment limits the escape of radioactive vapor or radioactive gas to the smallest possible quantities. The outer containment is intended to prevent outside influences on the reactor. The safety containers are designed according to theoretical models for the respective operating conditions. Each containment is dimensioned for a certain maximum pressure from the inside and for a certain maximum effect (impulse load) from the outside.

Older NPPs only had an operating building that prevents the weather from affecting the plant, but does not offer any protection against the escape of steam, or protection against explosively increased pressure or against the impact of a missile. Such systems are no longer in operation in Western Europe today (2016).

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 PWRs that were (or will be) run in load-following mode are e.g. E.g .: Emsland , Grafenrheinfeld , and Isar 2 .

For German PWR, the minimum output is 20, 45 or 50% of the nominal output, and the output gradients are 3.8 to 5.2 or 10% of the nominal output per minute. With power increases and power reductions, load changes of 50% of the nominal power are possible within a maximum of a quarter of an hour. An even higher load following capability exists above 80% of the nominal power with power gradients of up to 10% of the nominal power per minute.

For the Isar 2 nuclear power plant , the following performance gradients were specified in the operating manual: 2% per minute for changes in output in the range from 20 to 100% of the nominal output, 5% per minute in the range from 50 to 100% of the nominal output and 10% per minute in the range of 80 up to 100% of the nominal power.

The PWR output is regulated by extending and retracting control rods . The PWR has two types of control rods for this purpose: control rods that are used for power control ( D-Bank ) and control rods that always remain at the highest possible position in the core during power operation and thus serve as a switch-off reserve ( L-Bank ). For a power increase, the power gradient is limited, among other things, by the permissible power density in the reactor core. The power can be reduced at practically any desired speed.

In the PWR, the control rods are inserted into the reactor core from above, while in the boiling water reactor this is done from below. They are electromagnetically held in a position above the reactor core. In the event of a reactor shutdown , the control rods of the PWR fall into the core due to gravity.

The behavior of the reactor core during load changes is influenced by various factors such as B. Fuel temperature, coolant temperature, coolant density, concentration of 135 xenon (see Xenon poisoning ) and other parameters are determined.

literature

  • A. Ziegler, H.-J. Allelein (Hrsg.): Reaktortechnik : Physical-technical basics . 2nd edition, Springer-Vieweg, Berlin, Heidelberg 2013, ISBN 978-3-642-33845-8 .
  • Dieter Smidt: Reactor technology . 2 volumes, Karlsruhe 1976, ISBN 3-7650-2018-4
  • Günter Kessler: Sustainable and safe nuclear fission energy. Technology and safety of fast and thermal nuclear reactors . Springer 2012, ISBN 978-3-642-11989-7
  • Richard Zahoransky: Energy technology systems for energy conversion Compact knowledge for studies and work with 44 tables , 5th, revised. and exp. Edition, Vieweg Teubner, Wiesbaden 2010, ISBN 978-3-8348-1207-0 .

See also

Web links

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

Individual evidence

  1. ↑ The phase diagram of water is shown in the lower part of the following figure, from which the line between the triple point and the critical point results in the much lower boiling pressure associated with the operating temperature. See phase diagrams . The difference between pressurized water and boiling water reactors gives an example of what is known as Gibbs' phase rule : In the pressurized water reactor, the number of degrees of freedom is f = 2 ; Operating pressure and operating temperature can be set independently of one another and are entirely within the liquid range of the phase diagram. In the case of the boiling water reactor, on the other hand, the boiling pressure and the boiling temperature are mutually fixed, and the operation moves exactly on the boundary line given above between the liquid and the vapor phase. In this case f = 1 .
  2. ↑ a b Statistics of the IAEA on reactors worldwide , accessed on 10th 2015 (English)
  3. ↑ ENSI description of the mode of operation of various nuclear reactors. (PDF; 21 kB) p. 6 , archived from the original on July 14, 2011 ; Retrieved December 22, 2013 .
  4. ↑ Light water reactors. Retrieved July 7, 2011 . Information from the Austrian Nuclear Society
  5. ↑ 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 27, 2015 .
  6. ↑ a b c Holger Ludwig, Tatiana Salnikova and Ulrich Waas: Load changing capabilities of German NPPs. (PDF 2.4 MB pp. 2–3) (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 ; Retrieved October 26, 2014 . 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
  7. ^ A b c d 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, p. 3 (iii), 10) University of Stuttgart - Institute for Energy Economics and Rational Energy Use, October 2009, accessed on July 23, 2015 .
  8. ↑ a b c d Written question from the MP 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 ; accessed on May 27, 2015 . 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
  9. ↑ a b nuclear energy. RWE , accessed on May 27, 2015 .
  10. ↑ Great flexibility makes Emsland nuclear power plant a reliable partner for renewable energies. RWE, August 15, 2014, accessed on May 28, 2015 .
  11. ↑ Isar 2 nuclear power plant for the 10th time world class. (No longer available online.) E.ON , May 5, 2014, archived from the original on September 24, 2015 ; accessed on July 27, 2015 . 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.eon.com
  12. ↑ a b c LOAD FOLLOWING OPERATION AND PRIMARY CONTROL - EXPERIENCE WITH THE BEHAVIOR OF THE REACTOR - Isar nuclear power plant. (PDF; 743 kB; pp. 1, 7–8) E.ON, accessed on August 5, 2015 .
  13. ↑ M. Hundt, R. Barth, N. Sun, S. Wissel, A. Voß: Does an extension of the service life of nuclear power plants slow down the expansion of renewable energies? (PDF 1.8 MB, p. 25) (No longer available online.) University of Stuttgart - Institute for Energy Economics and Rational Energy Use, February 16, 2010, archived from the original on September 23, 2015 ; accessed on July 23, 2015 . 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.bdi.eu
  14. ↑ Pressurized water reactor (PWR). GRS , accessed August 3, 2015 .