Nuclear Meltdown and its Effects.Video.

A nuclear meltdown is an informal term for a severe nuclear reactor accident that results in core damage from overheating. The term is not officially defined by theInternational Atomic Energy Agency^[1] or by the U.S. Nuclear Regulatory Commission.^[2]

A meltdown occurs when a severe failure of a nuclear power plant system prevents proper cooling of the reactor core, to the extent that the nuclear fuel assembliesoverheat and melt, either partially or completely. A meltdown is considered very serious because of the potential that radioactive materials could be released into the environment. A core meltdown will also render the reactor unstable until it is repaired. The scrapping and disposal of the reactor core will incur substantial costs for the operator.

The fuel assemblies in a reactor core can melt if heat is not removed. A nuclear reactor does not have to remain critical for a core damage incident to occur, because decay heat continues to heat the reactor fuel assemblies after the reactor has shut down, though this heat decreases with time.

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 some types, 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 intended to prevent the release of radioactivity to the environment. This is due to the reactor being contained within a 1.2-to-2.4-metre (3.9 to 7.9 ft) thick pre-stressed, steel-reinforced, air-tight concrete dome.

• In a loss of coolant accident, either the physical loss of coolant (which is typically deionized water, an inert gas, 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 loss of coolant accident 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]
• 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 undermoderated, 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 civil reactors, such as the Magnox, UNGG, and AGCR type reactors, keep their cores blanketed with unreactive carbon dioxidegas, which cannot support a fire. Modern gas-cooled civil 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.

http://en.wikipedia.org/wiki/Nuclear_meltdown

How would a nuclear plant meltdown unfold?

• Control rods are driven back down into the core upon emergency (if rods don’t make it all the way… trouble)
• The coolant (water) could cease if backup systems fail (electricity, pumps, generators, batteries)
• Reactor continues to produce heat
• Numerous venting valve systems would release pressure above ~1,000 psi into containment vessel
• Eventually the uranium fuel encasement metal will melt (2,200 deg F)
• Radioactive contamination then released into the reactor vessel
• Radiation escapes into an outer, concrete containment building
• Radiation escapes into the environment as radioactive Fallout.
• …

Things to know about Cesium-137, “IF” there is a complete meltdown and radioactive Fallout released into the environment

(also spelled, Caesium)

Where does cesium-137 come from?

Radioactive cesium-137 is produced when uranium and plutonium absorb neutrons and undergo fission. Examples of the uses of this process are nuclear reactors and nuclear weapons.

What is the half life of cesium-137 ?

The half-life of cesium-137 is 30 years. Because of the chemical nature of cesium, it moves easily through the environment. This makes the cleanup of cesium-137 difficult.

How do people come in contact with cesium-137?

Walking on contaminated soil could result in external exposure to gamma radiation. People may ingest cesium-137 with food and water, or may inhale it as dust. It is distributed fairly uniformly throughout the body’s soft tissues. Exposure may also be external (that is, exposure to its gamma radiation from outside the body).

How can cesium-137 affect people’s health?

Exposure to radiation from cesium-137 results in increased risk of cancer. If exposures are very high, serious burns, and even death, can result. The U.S. Environmental Protection Agency says everyone is exposed to minute amounts of cesium-137. The average annual dose in the Northern Hemisphere is less than 1 millirem annually. That falls below the 100 millirem exposure limit the Nuclear Regulatory Commission recommends.

(information sourced from the U.S. EPA)

Fukushima II Nuclear Power Plant (Daini), has 4 nuclear reactors.

Reports point towards 3 reactors in trouble (or were in trouble) there with cooling systems. Details sketchy on Fukushima II.

Update, 13-Mar-2011, 1130 UTC
(TOKYO) JapanToday.com, Chief Cabinet Secretary Yukio Edano warned that a hydrogen explosion similar to one that blew away part of a building housing of another reactor (No. 1 at Daiichi) at the same facility on Saturday could occur at the reactor (No. 3 at Daiichi).

Tokyo Electric Power Co (TEPCO), began injecting fresh water into the No. 3 reactor’s core vessel on Sunday to deal with the problem that the tops of MOX fuel rods were 3 meters above the water inside.

Why did the Fukushima nuclear power plant reactor fail in Japan?
Following the magnitude 8.9 earthquake, the ensuing tsunami washed over the area and knocked out the backup power diesel generators. All that was left was battery power, which was not sufficient to keep the nuclear rods cool enough.

What is the local health danger from the nuclear accident?
People who are outside the immediate area could inhale radioactive particles. A nuclear reactor accident could release radioactive iodine and radioactive cesium. Breathing in or eating food contaminated with radioactive iodine can cause thyroid cancer. Potassium Iodide (or Iodate) tablets can help prevent this.

Contamination of food and water can result from radioactive dust that settles on water supplies, crops or grass. Cows or other animals eat, and it works up the food chain. Any suspected foods should be washed.

Radioactive cesium with its long half-life, can cause more long-term damage, including cancer.

How far might the radioactivity spread?
This depends of course upon how much radioactivity is released into the environment. Weather conditions, wind and rain, will mostly affect the spread.

Is there any danger to those outside of Japan at this time?
Currently there is no known danger, no. There is no evidence of a reactor core breach of containment vessel.

http://modernsurvivalblog.com/nuclear/west-coast-usa-danger-if-japan-nuclear-reactor-meltdown/