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daym2015

HowStuffWorks "Nuclear Catastrophe and Reactor Shutdown" - 0 views

science.howstuffworks.com/nuclear-power5.htm

shut down

shared by daym2015 on 05 Jun 14 - Cached
  • Plants such as Japan's Fukushima-Daiichi facility, Russia's Chernobyl and the United States' Three Mile Island remain a black eye for the nuclear power industry, often overshadowing some of the environmental advantages the technology has to offer. You can read more about exactly what happened in How Japan's Nuclear Crisis Works.
wizardbrown

Nuclear meltdown - Wikipedia, the free encyclopedia - 0 views

en.wikipedia.org/...Nuclear_meltdown

nuclear-meltdown

shared by wizardbrown on 06 Jun 14 - No Cached
  • A core melt accident occurs when the heat generated by a 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 failure, which is not caused by high temperatures. A meltdown may be caused by a loss of coolant, loss of coolant pressure, or low coolant flow rate or be the result of a criticality excursion in which the reactor is operated at a power level that exceeds its design limits
  • Once the fuel elements of a reactor begin to melt, the fuel cladding has been breached, and the nuclear fuel (such as uranium, plutonium, or thorium) and fission products (such as cesium-137, krypton-85, or iodine-131) within the fuel elements can leach out into the coolant. Subsequent failures can permit these radioisotopes to breach further layers of containment. Superheated steam and hot metal inside the core can lead to fuel-coolant interactions, hydrogen explosions, or water hammer, any of which could destroy parts 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 environment, resulting in radioactive contamination and fallout, and potentially leading to radiation poisoning of people and animals nearby.
  • In a loss-of-coolant accident, either the physical loss of coolant (which is typically deionized water, an inert gas, NaK, or liquid sodium) or the loss of a method to ensure a sufficient flow rate of the coolant occurs. A loss-of-coolant accident and a loss-of-pressure-control accident are closely related in some reactors. In a pressurized water reactor, a LOCA can also cause a "steam bubble" to form in the core due to excessive heating of stalled coolant or by the subsequent loss-of-pressure-control accident caused by a rapid loss of coolant. In a loss-of-forced-circulation accident, a gas cooled reactor's circulators (generally motor or steam driven turbines) fail to circulate the gas coolant within the core, and heat transfer is impeded by this loss of forced circulation, though natural circulation through convection will keep the fuel cool as long as the reactor is not depressurized.[6]
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  • 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. A 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 accident, a loss-of-coolant accident (LOCA), an uncontrolled power excursion or, in reactors without a pressure vessel, a fire within the reactor core. 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 1.2-to-2.4-metre (3.9 to 7.9 ft) thick pre-stressed, steel-reinforced, air-tight concrete structure that can withstand hurricane-force winds and severe earthquakes.
  • A core melt accident occurs when the heat generated by a 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 failure, which is not caused by high temperatures. A meltdown may be caused by a loss of coolant, loss of coolant pressure, or low coolant flow rate or be the result of a criticality excursion in which the reactor is operated at a power level that exceeds its design limits. Alternately, in a reactor plant such as the RBMK-1000, an external fire may endanger the core, leading to a meltdown. Once the fuel elements of a reactor begin to melt, the fuel cladding has been breached, and the nuclear fuel (such as uranium, plutonium, or thorium) and fission products (such as cesium-137, krypton-85, or iodine-131) within the fuel elements can leach out into the coolant. Subsequent failures can permit these radioisotopes to breach further layers of containment. Superheated steam and hot metal inside the core can lead to fuel-coolant interactions, hydrogen explosions, or water hammer, any of which could destroy parts 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 environment, resulting in radioactive contamination and fallout, and potentially leading to radiation poisoning of people and animals nearby.
  • In a loss-of-pressure-control accident, the pressure of the confined coolant falls below specification without the means to restore it. In some cases this may reduce the heat transfer efficiency (when using an inert gas as a coolant) and in others may form an insulating "bubble" of steam surrounding the fuel assemblies (for pressurized water reactors). In the latter case, due to localized heating of the "steam bubble" due to decay heat, the pressure required to collapse the "steam bubble" may exceed reactor design specifications until the reactor has had time to cool down. (This event is less likely to occur in boiling water reactors, where the core may be deliberately depressurized so that the Emergency Core Cooling System may be turned on). In a depressurization fault, a gas-cooled reactor loses gas pressure within the core, reducing heat transfer efficiency and posing a challenge to the cooling of fuel; however, as long as at least one gas circulator is available, the fuel will be kept cool.[6]
  • In an uncontrolled power excursion accident, a sudden power spike in the reactor exceeds reactor design specifications due to a sudden increase in reactor reactivity. An uncontrolled power excursion occurs due to significantly altering a parameter that affects the neutron multiplication rate of a chain reaction (examples include ejecting a control rod or significantly altering the nuclear characteristics of the moderator, such as by rapid cooling). In extreme cases the reactor may proceed to a condition known as prompt critical. This is especially a problem in reactors that have a positive void coefficient of reactivity, a positive temperature coefficient, are overmoderated, or can trap excess quantities of deleterious fission products within their fuel or moderators. Many of these characteristics are present in the RBMK design, and the Chernobyl disaster was caused by such deficiencies as well as by severe operator negligence. Western light water reactors are not subject to very large uncontrolled power excursions because loss of coolant decreases, rather than increases, core reactivity (a negative void coefficient of reactivity); "transients," as the minor power fluctuations within Western light water reactors are called, are limited to momentary increases in reactivity that will rapidly decrease with time (approximately 200% - 250% of maximum neutronic power for a few seconds in the event of a complete rapid shutdown failure combined with a transient).
  • Core-based fires endanger the core and can cause the fuel assemblies to melt. A fire may be caused by air entering a graphite moderated reactor, or a liquid-sodium cooled reactor. Graphite is also subject to accumulation of Wigner energy, which can overheat the graphite (as happened at the Windscale fire). Light water reactors do not have flammable cores or moderators and are not subject to core fires. Gas-cooled civilian reactors, such as the Magnox, UNGG, and AGCR type reactors, keep their cores blanketed with non reactive carbon dioxide gas, which cannot support a fire. Modern gas-cooled civilian reactors use helium, which cannot burn, and have fuel that can withstand high temperatures without melting (such as the High Temperature Gas Cooled Reactor and the Pebble Bed Modular Reactor).
  • Byzantine faults and cascading failures within instrumentation and control systems may cause severe problems in reactor operation, potentially leading to core damage if not mitigated. For example, the Browns Ferry fire damaged control cables and required the plant operators to manually activate cooling systems. The Three Mile Island accident was caused by a stuck-open pilot-operated pressure relief valve combined with a deceptive water level gauge that misled reactor operators, which resulted in core damage.
  • wizardbrown on 06 Jun 14
     
    "A core melt accident occurs when the heat generated by a 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 failure, which is not caused by high temperatures. A meltdown may be caused by a loss of coolant, loss of coolant pressure, or low coolant flow rate or be the result of a criticality excursion in which the reactor is operated at a power level that exceeds its design limits."
laurenh468

Everyday Exposure - 0 views

people.chem.duke.edu/...exposure.html

Exposure

shared by laurenh468 on 03 Jun 14 - No Cached
  • Radioactive elements found in rock, soil, water, air, and in food from the earth make there way in our bodies when we drink water, breath air or eat foods which contain them. These naturally occurring radioisotopes such as carbon-14, potassium-40, thorium-223, uranium-238, polonium-218, and tritium(hydrogen-3) expose us to radiation from within our bodies.
  • Radioactivity in nature comes from two main sources, terrestrial and cosmic. Terrestrial radioisotopes are found on the earth that came into existence with the creation of the planet.
  • Terrestrial
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  • . In areas where surface rocks contain a high concentration of uranium, radon gas could enter a home through a crack in the foundation. A concern for homeowners is the possibility that radon gas could accumulate to dangerous levels. This is especially a problem during the winter months when windows and doors are tightly shut.
  • interaction of cosmic rays with the earth's upper atmophere. Cosmic rays permeate all of space and are composed of highly energized, positively charged particles as well as high energy photons.
  • Approaching the earth at near the speed of light, most cosmic rays are blocked by the earth's protective atmosphere and magnetic field. As a byproduct of the interaction between cosmic rays (i.e. particles) and the atmosphere, many radioactive isotopes are formed such as carbon-14.
  • Cosmic rays are also composed of high energy photons, and not all are prevented from reaching the earth's surface. It makes sense that the higher you are in altitude, the more you are exposed to cosmic radiation. In fact, the average amount of exposure to cosmic radiation that a person gets in the Unites States roughly doubles for every 6,000 foot increase in elevation.
  • lying can indeed add a few extra units of exposure to one's daily exposure. Of course, the amount of extra exposure you get depends on how high the plane flies and how long you are in the air.
  • Think about this : Estimate how much cosmic radiation that astronauts are exposed to during their flights. Recall that astronauts fly at heights of about 160 miles
  • The human production of tobacco products introduces another way for us to get exposure to radiation. Smokers recieve a dose of radiation from polonium-210 which is naturally present in tobacco. Smokers also recieve an additional dose of radiation from the decay product of radon gas, polonium-218. Polonium-218 clings to aerosols such as tobacco smoke, and eventually winds up in the lungs. Once in the lungs, polonium decays by alpha particle emission and in the process may damage cells.
  • For examples, the bricks, stones, cements and drywalls that we use for the building of our homes, schools, offices frequently contain uranium ores and are thus sources of radon.
  • This exposure results from the attempt to diagnose fractures or cavities using x-rays, or to diagnose or treat cancer using injected radioisotopes. Patients are exposed to nuclear radiation in the diagnosis and treatment of cancer. Additionally, radiologists routinely use radioisotopes of technetium or thorium to diagnose heart disease.
  • These higher risk occupations include underground miners, radiologists, medical technologists, nuclear plant operators, research scientists and pilots.
  • Any amount of radiation can be dangerous because of the potential effect that it has on living cells. Radiation can disrupt normal chemical processes of the cells, causing them to grow abnormally or to die. Cells that are altered by the radiation may go on to produce more abnormal cells - a process that could eventually lead to cancer. At low doses, such as have been described here, cells are able to repair any damage rapidly. Any cells that die due to exposure can be replaced by the body. If one receives a very high dose, unlike any exposure mentioned here, the cells may not be able to be replaced fast enough and tissues or organs may fail to function properly.
wizardbrown

Nuclear reactor - Wikipedia, the free encyclopedia - 0 views

en.wikipedia.org/...Nuclear_reactor

nuclear-reactor

shared by wizardbrown on 05 Jun 14 - Cached
  • A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Nuclear reactors are used at nuclear power plants for electricity generation and in propulsion of ships. Heat from nuclear fission is passed to a working fluid (water or gas), which runs through turbines. These either drive a ship's propellers or turn electrical generators.
  • When a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission. The heavy nucleus splits into two or more lighter nuclei, (the fission products), releasing kinetic energy, gamma radiation, and free neutrons. A portion of these neutrons may later be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on. This is known as a nuclear chain reaction. To control such a nuclear chain reaction, neutron poisons and neutron moderators can change the portion of neutrons that will go on to cause more fission.[2] Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if monitoring detects unsafe conditions.[3] Commonly-used moderators include regular (light) water (in 74.8% of the world's reactors), solid graphite (20% of reactors) and heavy water (5% of reactors). Some experimental types of reactor have used beryllium, and hydrocarbons have been suggested as another possibility.[2][not in citation given]
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