Pressurized water reactor
The pressurized water reactor (PWR ) (English: PWR = Pressurized Water Reactor - Moderated ) is a design of a nuclear reactor, is used in the water as moderator and coolant. The operating pressure of the water is herein different from the boiling water reactor, so high that it does not boil at the intended operating temperature. The fuel rods are thus evenly wetted, the heat distribution on the surface thereof is compensated for and the vapor phase is omitted with their particular corrosion. The uniform heat distribution causes a calm control response with good use of the energy released.
The heated water in the reactor core (primary circuit ) are in a steam generator transfers its heat to a separate water - steam circuit from the secondary circuit. The secondary circuit is free of radioactivity from abrasion and corrosion products, which considerably facilitates, for example, the maintenance of the steam turbine.
Most light water (H2O ) is used as a cooling medium for the fuel rods and as a transport medium for energy. Therefore, these reactors are among the light-water reactors. Worldwide, there are according to the International Atomic Energy Agency about 270 of these reactors. The use of heavy water ( D2O ) is also possible, but is used worldwide in only about 10 percent of all reactors ( see heavy water reactor). In total there are pressurized water reactors, the most common types of reactor.
The coolant water is added to a variable amount of boric acid. Boron is an effective absorber of neutrons can thus be controlled by the concentration of boric acid the performance of the reactor. A further automatic power control is derived from the physical function of the reactivity of the fuel and coolant temperature. An increase in temperature in the reactor means:
- Increased fuel temperature: Thus, the slope of the thermal neutron bad fissionable uranium isotope 238 to absorb neutrons increases.
- Increased coolant temperature, lower density: This reduces the moderation property of the coolant, so fewer are thermal neutron fission of uranium -235 nuclei are available.
Due to these effects, the reactivity and thus the power of the reactor is reduced.
The coolant is passed in the primary circuit at an elevated pressure of from 154 to 160 bar through the reactor core, where it picks up the heat generated by nuclear fission, and heated to about 325 ° C. From there it flows into the steam generator, which are designed as a tube bundle heat exchanger. After the transfer of heat with the refrigerant centrifugal pump is pumped back into the reactor core. This results in an advantage over the boiling water reactor, the coolant, which is always something radioactively contaminated, ever is inside the containment. Therefore, no radiation protection measures are necessary in the turbine building.
To achieve a very uniform radial temperature distribution of the initial loading is carried out with fuel from the inside to the outside with an increasing degree of enrichment. After the end of the first fuel cycle ( about 1 year ) only the outer third of the inventory will be replaced by new fuel, which will be implemented from outside to inside during the following cycles, respectively. In addition to this goal of uniform radial power density distribution can be increased or a lower neutron flux near the wall of the reactor pressure vessel be achieved through other core loadings either the combustion of the fuel.
The water in the secondary circuit is under a pressure of about 70 bar, and therefore it vaporizes in the heating tubes of the steam generator, first at 280 ° C. In a nuclear power plant unit is customary in Germany the electric power of 1400 MW, the resulting amount of steam is for all steam generators together about 7,000 tons per hour. The water vapor is directed through conduits to a steam turbine that generates on the coupled generator electricity. After the steam is condensed in a condenser and fed back to the water with the feed pump to the steam generators.
The capacitor is again with cooling water, usually from a river, cooled. Depending on the initial temperature and water flow of the river has this cooling water before it is returned to the river, in its turn be cooled down again. For this purpose, a part of the cooling water is brought into a cooling tower to evaporate. This results in some weather conditions, white clouds above the cooling towers.
Pressurized water reactors have an efficiency of 32-36 % ( if you count uranium enrichment ), ie very similar values as a nuclear power plant of the type boiling water reactor. The efficiency could be increased by several percentage points if one could increase the steam temperature as in coal-fired power plants beyond 500 ° C. The maximum temperature of the primary coolant is limited by the used principle of subcooled boiling at temperatures below the critical point and thus such steam temperatures can not be achieved with a conventional pressurized water reactor.
Versions of the pressurized water reactor, for example in the 1980s, built by Siemens in Germany convoy, which was built by Framatome in France N4 and the Soviet VVER. Areva NP is currently building at Olkiluoto (Finland ) a European Pressurized Water Reactor (EPR ), a development of the convoy and N4- core reactors.
Pressurized water reactors have a long technical development behind it. This type of reactor was initially built in large numbers for the propulsion of warships, such as the Nimitz class. The first application for peaceful purposes was finished in 1957, nuclear power plant, Shippingport, USA, with a capacity of 68 MW.
The reactor pressure vessel of a pressurized water reactor is surrounded by one or more nested safety contained ( containment ). The security containers have no operational function, but serve the completion of various operating areas from each other and to the outside.
The inner containment have taken into account in the in the design (see design basis accident ) normal or special operating conditions, the safety function to limit the escape of radioactive vapor or radioactive gas to small amounts as possible. The outer containment should also prevent outside influences from outside the reactor.
The design of containment is based on models for the respective operating states. The security container is dimensioned for a certain maximum pressure on the inside and a predetermined maximum exposure ( pulse exposure) from the outside.
Older NPPs have only an operating building, prevents the weathering effect on the plant, but did not graduate from steam outlet, no protection at explosively increased pressure and no protection at impact of a missile.