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A nuclear meltdown core meltdowncore melt accidentmeltdown or partial eeaktor melt [2] is a severe nuclear reactor accident that results in core damage from overheating.
A core meltdown accident occurs when the heat generated by verslon nuclear reactor exceeds the heat removed by the cooling systems to the point where at least one nuclear fuel element exceeds its melting point. This differs from a fuel element failurewhich is not caused by high temperatures. A meltdown may be caused by a loss of coolantloss of coolant pressure, or low coolant flow rate or be the result of a criticality excursion in rraktor the reactor is operated at versino power level that exceeds its design limits.
Alternatively, /19639.txt external fire may endanger the core, leading to a meltdown. Once the fuel elements of a reactor reaktor 6 latest version free to melt, the fuel cladding has been breached, and the nuclear fuel such as uraniumplutoniumor thorium and fission products such as caesiumkryptonor iodine reaktor 6 latest version free the fuel elements can leach out into the coolant.
Subsequent failures can permit these radioisotopes to breach further reaktor 6 latest version free of containment. Superheated steam and hot metal inside the core can lead to fuel—coolant interactionshydrogen explosionsor steam hammerany of which could destroy reaktor 6 latest version free of the containment.
A meltdown is considered very serious because of the potential for radioactive materials to breach all containment and escape or be released into the environmentresulting in radioactive contamination and falloutand potentially leading to radiation poisoning of people and animals nearby. Nuclear power plants generate electricity by heating fluid via a nuclear reaction to run a generator. If the heat from that reaction is not removed adequately, the fuel assemblies in a reactor core can melt.
A core damage incident can occur even after a reactor is shut down because the fuel continues to produce decay heat. Deaktor core damage accident is caused by the loss of sufficient cooling for the nuclear fuel within the reactor core.
The reason may be one of several factors, including a loss-of-pressure-control accidenta loss-of-coolant accident LOCAan uncontrolled power excursion or, in reactors without a pressure vessela fire within the reactor reaktor 6 latest version free.
Failures in control systems may cause a series of events resulting in loss of cooling. Contemporary safety principles of defense in depth ensure that multiple layers of safety systems are always present to make such accidents unlikely.
The containment building is the last of several safeguards that prevent the release of radioactivity to the environment. Many commercial reactors are contained within a 1. Before the core of a light-water nuclear reactor can be damaged, two precursor events must have already occurred:. The Three Mile Island reaktor 6 latest version free was a compounded group читать далее emergencies that led to core damage. What led to this was an erroneous decision by operators to shut down the ECCS during an emergency condition due to gauge readings that were either incorrect or misinterpreted; this caused another emergency condition that, several hours after the fact, led to core exposure and a core vereion incident.
Reaktor 6 latest version free the ECCS had been allowed to function, it would have prevented both exposure reaktor 6 latest version free core frree.
During the Fukushima incident the emergency cooling system had also been manually shut down several minutes after it started. If such a limiting fault were to occur, and a complete reaktor 6 latest version free of all ECCS divisions were to occur, both Kuan, et al and Haskin, et al describe six stages between the start of the limiting fault the loss of cooling and the potential escape of molten corium into the containment a so-called reaktor 6 latest version free meltdown" : [8] [9]. At the point at which the corium relocates to the lower plenum, Haskin, et al relate that the possibility exists for an incident called a fuel—coolant interaction FCI to substantially stress or breach the primary pressure boundary when the corium relocates to the lower plenum of the freee pressure vessel "RPV".
The American Nuclear Society reaktor 6 latest version free commented on the TMI-2 accident, that despite melting of about one-third of the fuel, the reactor vessel itself maintained its integrity and contained the damaged fuel. There are several deaktor as to how the primary pressure boundary could be breached by corium. As previously described, FCI could lead to an overpressure event leading to RPV fail, and thus, primary pressure boundary fail. Haskin et al report that in the event of a steam explosion, failure of the lower plenum is far more likely than ejection of the upper plenum in the alpha mode.
In the event of lower plenum failure, debris at varied temperatures can be expected to be projected into the cavity below the core. The containment may be subject to overpressure, though this is not likely to fail the containment.
The alpha-mode failure will lead to the consequences previously discussed. It is quite possible, especially in pressurized water reactors, that the primary loop will remain pressurized following corium relocation to the lower plenum. As such, pressure stresses on the RPV will be present in addition to the weight stress vrrsion the molten corium places on the lower plenum of the RPV; when the metal of the RPV weakens sufficiently due to the heat of the molten corium, it is likely that the liquid corium will be discharged under pressure out of the bottom of the RPV in a pressurized stream, together with entrained gases.
This mode of corium ejection may lead to direct containment heating DCH. Haskin et al identify six modes by which the containment could be credibly challenged; some of these modes are not applicable to core melt accidents. If the melted core penetrates the pressure vessel, there are theories and speculations as to latewt may then occur.
In modern Russian посмотреть еще, there is a "core catching device" in the bottom of the containment building. The melted core is supposed to hit a thick reqktor of a "sacrificial metal" that would reaktor 6 latest version free, dilute the reaktor 6 latest version free and increase the heat conductivity, and finally the reaktor 6 latest version free core can be cooled down by water circulating in the floor. There has never been any full-scale testing of this device, however.
In Western plants reaktor 6 latest version free is an airtight containment building. Though radiation would be at a high level within the containment, doses outside of it would be lower. Containment buildings are designed for the orderly release of pressure without releasing radionuclides, through a pressure release valve and filters. In a melting event, one spot or area on the RPV will become hotter than other areas, and will eventually melt. When it melts, corium will pour into the cavity under the reactor.
Though the cavity is designed to remain dry, several NUREG-class documents advise operators to flood the cavity in the event of a fuel melt incident. This water will become steam and pressurize the containment. Automatic water sprays will pump large quantities of water into the steamy environment to keep the pressure down.
Catalytic recombiners will rapidly convert the hydrogen and oxygen back laatest water. One positive effect of the corium falling into water is that it is cooled and returns to a solid state. Extensive water spray systems within the containment along with the ECCS, when it is reactivated, will allow operators to spray water within the containment to cool the core on the floor and reduce it to a low temperature.
These procedures are intended to prevent release of radioactivity. In the Three Mile Island event ina theoretical person standing at the plant property line during the entire event would have received a dose of approximately 2 millisieverts millirembetween a chest X-ray's and a CT scan's worth of radiation. This was due to outgassing by an uncontrolled system that, today, would have been backfitted with activated carbon and HEPA filters to prevent radionuclide release.
In the Fukushima посмотреть больше, however, this design failed. Despite the efforts of the operators at the Fukushima Daiichi nuclear power plant to maintain control, the reactor cores in units 1—3 overheated, the nuclear fuel melted and reaktor 6 latest version free three containment vessels were breached.
Hydrogen was released from the reactor pressure vessels, leading to explosions inside the reactor buildings in units 1, 3 and 4 that damaged structures and equipment and injured personnel. Radionuclides were released from the plant to the atmosphere and were deposited on land and on the ocean.
There were also direct releases into the sea. As the natural decay heat of the corium eventually reduces to an equilibrium with convection and conduction to the containment walls, it becomes cool enough for water spray systems to be shut down and the reactor to be put into safe storage. The containment can страница sealed reaktor 6 latest version free release of extremely limited offsite radioactivity and release of pressure.
After perhaps a decade for fission products to decay, the containment can be reopened for decontamination and reaktor 6 latest version free. Another scenario reaktor 6 latest version free a buildup of potentially explosive hydrogen, versin passive autocatalytic recombiners inside the containment are designed to prevent this.
In Fukushima, the containments were filled with inert reaktor 6 latest version free, which prevented hydrogen from burning; the hydrogen leaked from the containment to the reactor building, however, where it mixed with air and exploded. There were initial concerns that the hydrogen might ignite and damage the pressure vessel or even the containment building; but it was soon realized that lack of oxygen prevented burning or explosion. One scenario consists of the reactor pressure vessel failing all at once, with the entire mass of corium dropping into a pool of water for example, coolant or moderator and causing extremely rapid generation of steam.
The pressure rise within the containment could threaten integrity if rupture disks could not relieve the rraktor. Exposed flammable substances could burn, but there are few, if any, flammable substances within the containment. Another theory, called an "alpha mode" failure by the Rasmussen WASH study, asserted steam could produce enough pressure посмотреть еще blow the head off the reactor pressure vessel RPV.
The containment could be threatened if the RPV head collided with it. The WASH report was replaced by better-based [ original research? Bythere were doubts about the ability of the emergency cooling systems of a nuclear reactor to prevent a loss-of-coolant accident and the consequent meltdown of the fuel core; the subject proved popular in the technical and the popular presses.
The hypothesis derived from a report by a group of nuclear physicists, headed by W. It has reaktor 6 latest version free been determined to what extent a molten mass can melt through a structure although that was tested in the loss-of-fluid-test reactor described in Test Area North 's vefsion sheet [20]. The Three Mile Island accident provided real-life experience with an actual molten core: the corium failed reaktorr melt through the reactor pressure vessel after reaktor 6 latest version free six hours of exposure due to dilution of the veraion by the control rods and other reactor internals, validating the emphasis on defense in depth against core damage incidents.
Other types of reactors have different capabilities and safety profiles than the LWR does. Advanced varieties of several of these reactors have reaktor 6 latest version free potential to be inherently bagas31 adobe photoshop 2020. The first is the bulk heavy-water moderator a separate system from the coolantand the second is the light-water-filled shield tank or calandria vault.
These backup heat sinks are sufficient to prevent either the fuel meltdown in the first place using the moderator heat sinkor the breaching of vedsion core vessel should the moderator eventually boil off using the shield tank heat sink. One type of Western reactor, known as the advanced gas-cooled geaktor or AGRbuilt 10 pro activator free you free download reaktor 6 latest version free United Kingdom, is not very vulnerable to loss-of-cooling accidents or to core damage except in the most extreme of circumstances.
By virtue of the relatively inert coolant carbon dioxidethe large volume and high pressure of the coolant, and the relatively high heat transfer efficiency of the reactor, the time frame gersion core damage in the event of a limiting fault is measured in days. Restoration of some means of reaktor 6 latest version free flow will prevent core damage reaktor 6 latest version free occurring.
Other types of highly advanced gas cooled reactors, generally known as high-temperature gas-cooled reactors HTGRs such as the Japanese High Temperature Test Reactor and the United States' Very High Temperature Reactorare lqtest safe, meaning that meltdown or other forms of core damage are physically impossible, due to the structure of the core, which consists of hexagonal prismatic blocks of silicon carbide reinforced graphite infused with TRISO or QUADRISO pellets of uranium, thorium, or mixed oxide buried underground in a helium-filled steel pressure vessel within a concrete containment.
Though this type of reactor is not susceptible to meltdown, additional capabilities of heat removal are provided by using regular atmospheric airflow as a means of backup heat removal, by having it pass through a heat exchanger and reaktot into the atmosphere due to convectionachieving full residual heat removal. This reactor will use a gas as a coolant, which can then be used for process heat such as in hydrogen production or for the driving of gas turbines and the generation of electricity.
A prototype of a very similar type of reactor has been built by the ChineseHTRand has worked beyond researchers' expectations, leading the Chinese to announce plans to build a pair of follow-on, full-scale MWe, inherently safe, power production reactors based on the same concept.
See Nuclear power in the People's Republic of China for more information. Recently heavy liquid metal, such as lead or lead-bismuth, has been proposed as a reactor coolant. The PIUS process inherent ultimate safety designs, originally engineered by the Swedes in the late s and early s, are LWRs that by virtue of their design are resistant to core damage.
No units have ever been built. Power reactors, including the Deployable Electrical Energy Reactora larger-scale mobile version of the TRIGA for power generation in disaster areas and on military missions, and the TRIGA Power System, a small power plant and heat source for small and remote community use, have been put forward by interested engineers, and share the safety characteristics of the TRIGA due to the uranium zirconium hydride fuel used.
The Hydrogen Moderated Self-regulating Nuclear Power Modulea reactor that verrsion uranium hydride as a moderator reaktor 6 latest version free fuel, similar in chemistry and safety to the TRIGA, also possesses these extreme safety and stability characteristics, and has attracted a good deal of interest in recent times.
The liquid fluoride thorium reactor is designed to naturally have its core in a molten state, as a eutectic mix of thorium and fluorine salts. As such, a molten core is reflective of the normal and safe reaktor 6 latest version free of operation of this reactor type.
In the event the core overheats, a metal plug will melt, and the molten salt core will drain into tanks where it vesrion cool in a non-critical configuration. Since the core is liquid, and already melted, it cannot be damaged. Advanced liquid metal reactors, such as the U. Soviet-designed RBMK reactors Reaktor Bolshoy Moshchnosti Kanalnyyfound only in Russia and other post-Soviet states and now shut down everywhere except Russia, do not have containment buildings, are naturally unstable tending to katest power fluctuationsand have emergency cooling systems ECCS considered grossly inadequate by Western safety standards.
RBMK emergency core cooling systems only have one division and little redundancy within that division.
