A reactor pressure vessel (RPV) in a nuclear power plant is a pressure vessel containing a nuclear reactor coolant, a core housing, and a reactor core.
Video Reactor pressure vessel
Classification of nuclear power reactors
Not all power plants have reactor pressure vessels. The power plant is generally classified by the type of refrigerant rather than by the reactor vessel configuration used to contain the refrigerant. The classification is:
- Light water reactors - Including pressurized water reactors and boiling water reactors. Most nuclear power plants are of this type.
- Graphite-moderated reactors - Including the Chernobyl reactor (RBMK), which has a very unusual reactor configuration compared to most nuclear power plants in Russia and around the world.
- Gas-cooled gas reactors - Including Advanced Gas Refrigerated Reactors, gas-cooled fast-burning reactors, and high temperature gas cooled reactors. An example of a gas-cooled reactor is the British Magnox.
- Heavy-pressure water reactors - utilizing heavy water, or water with a higher proportion of the higher than normal isotope deuterium hydrogen, in some way. However, D 2 O (heavy water) is more expensive and can be used as the main component, but not necessarily as a coolant in this case. Examples of heavy water reactors are Canadian CANDU reactors. Liquid refrigerated reactors - using molten metal, such as sodium or tin-bismuth alloys to cool the reactor core.
- Liquid salt reactors - salts, usually fluorides of alkali metals and of alkaline earth metals, are used as coolants. Operation is similar to a metal-cooled reactor with high temperature and low pressure, reducing the pressure applied to the reactor vessel compared to the water or steam-cooled design.
From the main class of the reactor to the pressure vessel, the pressurized water reactor is unique because the pressure vessel experiences significant neutron irradiation (called fluence) during operation, and may become brittle over time as a result. In particular, larger pressure vessels from boiling water reactors are better protected from neutron flux, so even though it is more expensive to make in the first place because of this extra measure, it has the advantage of not needing annealing to prolong its life.
Various pressured water reactor vessels to extend their service life are complex and valuable technologies that are actively developed by nuclear service providers (AREVA) and pressurized water reactor operators.
Maps Reactor pressure vessel
Components of pressure water pressure reactor vessel
All pressure vessels of pressurized water reactors have some features regardless of special design.
Reactor vessel body
The reactor vessel body is the largest component and is designed to contain fuel, cooling, and fitting assemblies to support coolant flow and support structures. Usually cylindrical and open at the top to allow fuel to be loaded.
Head of reactor vessel
This structure is attached to the top of the body of the reactor vessel. It contains penetration to allow the driving mechanism of the control rod to be attached to the control rod in the fuel assembly. The coolant level measurement probe also enters the vessel through the head of the reactor vessel.
Fuel assembly
Nuclear fuel fuel assemblies typically consist of a mixture of uranium or uranium-plutonium. This is usually a rectangular grid fuel rectangle block.
Neutron reflector or absorber
Protecting the inside of the vessel from the rapid neutrons released from the fuel assembly is the cylindrical protector that is wrapped around the fuel assembly. The reflector sends the neutrons back to the fuel assembly to better utilize the fuel. The ultimate goal is to protect the ship from damage caused by rapid neutrons that can make the ship fragile and reduce its useful life.
Material for reactor pressure vessel
RPV provides an important role in the safety of the PWR reactor and the materials used should be able to contain the reactor core at high temperature and pressure. The materials used in cylindrical shells of vessels have evolved over time, but generally they consist of low ferritic steel coated with austenitic stainless steel 3-10 mm. The stainless steel cladding is mainly used in locations that come into contact with the coolant to minimize corrosion. Through the mid-1960s, SA-302, Grade B, molybdenum-magnesium steel plates, were used in ship bodies. Since design changes require larger pressure vessels, the addition of nickel in this alloy of about 0.4-0.7 weight% is required to increase yield strength. Other common steel alloys include SA-533 Class B Class 1 and Class SA-508 2. Both materials have the main alloy elements of nickel, manganese, molybdenum, and silicon, but the latter also includes 0.25-0.45% by weight of chromium. All the alloys listed in the reference also have & gt; 0.04% by weight of sulfur. The low ferritic NiMoMn alloy steels are particularly attractive for this purpose because of their high thermal conductivity and low thermal expansion, properties that make them resistant to thermal shock. However, when considering the properties of this steel, one must take into account its response to radiation damage. Due to harsh conditions, RPV cylindrical shell material is often a lifetime barrier component for nuclear reactors. Understanding the effects of radiation on microstructures in addition to physical and mechanical properties will allow scientists to design alloys more resistant to radiation damage.
Radiation damage to metals and alloys
Due to the nature of the generation of nuclear energy, the materials used in RPV are constantly being bombarded by high-energy particles. These particles can be neutrons or fragments of atoms created by fission events. When one of these particles collides with the atom in the material, it will transfer some of its kinetic energy and drop the atom from its position in the lattice. When this happens, these primary "knock-on" atoms (PKA) are displaced and energetic particles can rebound and collide with other atoms in the lattice. This creates a chain reaction that can cause many atoms to be moved from their original position. This atomic movement leads to the creation of many types of defects. The accumulation of various defects can lead to microstructural changes that may cause degradation in macroscopic properties. As mentioned earlier, chain reactions caused by PKA often leave traces of emptiness and a group of defects at the edges. This is called a displacement cascade. The gap core of the displacement cascade can also collapse into the dislocation loop. Because of irradiation, the material tends to develop a higher concentration of defects than that of ordinary steel, and high operating temperatures cause defective migration. This can cause things like interstitial recombination and emptiness and groupings such as defects, which can make or dissolve deposits or voids. Examples of sinks, or beneficial thermodynamic places for defects to migrate to, are grain boundaries, voids, incoherent precipitates, and dislocations.
Radiation-induced segregation
The interaction between defects and alloying elements can lead to redistribution of atoms on the sink such as grain boundaries. The physical effect that can occur is that certain elements will be enriched or emptied in these areas, which often lead to grain boundary embrittlement or other adverse property changes. This is because there are fluxes from the vacuum to the sink and the atomic flux away or toward the sink which may have varying diffusion coefficients. An uneven diffusion level causes the concentration of atoms not necessarily in the correct proportion of alloys. It has been reported that nickel, copper and silicon tend to be enriched in the sink, whereas chromium tends to run out. The resulting physical effect is to change the chemical composition at the grain boundaries or around the coherent voids/precipitates, which also act as sinks.
Formation of voids and bubbles
Voids are formed due to vacancy clusters and are generally more easily formed at higher temperatures. The bubble is just a void filled with gas; they will occur if the transmutation reaction is present, which means the gas is formed by the breakdown of atoms caused by neutron bombing. The biggest problem with voids and bubbles is dimensional instability. An example of where this would be particularly problematic is areas with tight dimensional tolerances, such as threads on binders.
Irradiation harden
Creation of defects such as voids or bubbles, precipitates, loops or dislocation lines, and disabled groups can strengthen the material as they block the movement of dislocations. This dislocation movement causes plastic deformation. While this hardens the material, the downside is that there is loss of tenacity. Losing ductility, or increasing fragility, is dangerous in RPV because it can cause catastrophic failure without warning. When the ductile material fails, there is substantial deformation before failure, which can be monitored. The brittle material will crack and explode when under pressure without much previous deformation, so not many engineers can perform to detect when the material will fail. The most destructive element of steel that can cause hardening or embrittlement is copper. Cu-rich precipitation is very small (1-3 nm) so they are effective in clamping the dislocations. It has been recognized that copper is the dominant destructive element in steels used for RPV, especially if the impurity rate is greater than 0.1 wt.%. Thus, the development of "clean" steels, or those with very low levels of impurities, is important in reducing radiation-induced hardening.
Creep
Creep occurs when the material is held under stress levels under the resulting stress that causes plastic deformation over time. This is especially true when a material is exposed to high pressure at high temperatures, because diffusion and dislocation movements occur more rapidly. Irradiation can cause creep because of the interaction between stress and microstructure development. In this case, the increase in diffusivity due to high temperature is not a very strong factor to cause creep. The dimensions of the material tend to increase toward the applied stress due to the formation of dislocation loops around the defects that are formed due to radiation damage. In addition, the stress applied can allow the interstitial to be more easily absorbed in the dislocation, which helps to climb the dislocation. When dislocations can climb, the remaining void left, which can also cause swelling.
Irradiation assisted corrosion of crack corrosion
Because of grain boundary embrittlement or other defects that may serve as initiators of cracks, the addition of radiation attacks to cracks may lead to corrosion of intergranular stress cracking. The main environmental stressor formed by radiation is a hydrogen coating on the crack tip. Hydrogen ions are created when the radiation separates the water molecules, which are present because water is the coolant in PWR, to OH - and H . There are several suspected mechanisms that describe hydrogen embrittlement, three of which are the mechanisms of decohesion, pressure theorists, and the methods of hydrogen attack . In the decohesion mechanism, it is thought that the accumulation of hydrogen ions reduces the strength of metal-to-metal bonds, which makes it easier to separate atoms. The pressure theory is the idea that hydrogen can precipitate as gas on internal defects and create bubbles inside the material. The stress caused by the bubble that extends in addition to the stress used is what decreases the overall stress required to break the material. The method of hydrogen attack is similar to pressure theory, but in this case it is suspected that hydrogen reacts with carbon in the steel to form methane, which then forms blisters and bubbles on the surface. In this case, the additional pressure by the bubbles is enhanced by the decarburization of the steel, which weakens the metal. In addition to hydrogen embrittlement, radiated induced creep can cause grain boundaries to shift to each other. This further destabilizes the grain boundaries, making it easier to crack spreading along its length.
Designing radiation-resistant materials for reactor pressure vessels
A highly aggressive environment requires a new material approach to combat the deterioration of mechanical properties over time. One method that researchers use is to introduce features to stabilize the displaced atoms. This can be done by adding grain boundaries, too large solvents, or small oxide dispersants to minimize defect movement. By doing this, there will be a separation of radiation-induced elements, which in turn will lead to more brittle grain boundaries and reduce corrosion of intergranular stress cracks. Blocking dislocations and flaw movements will also help improve resistance to radiation-assisted creep. Experiments have been reported to institutionalize yttrium oxidation to block the movement of dislocations, but it was found that the implementation of the technology presented a greater than expected challenge. Further research is needed to continuously improve the durability of radiation damage from structural materials used in nuclear power plants.
See also
- Nuclear physics
- Nuclear reactor
- The physics of a nuclear reactor
- Nuclear reactor vessels
- Radiation damage
References
Source of the article : Wikipedia
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