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Fukushima: Anatomy of a nuclear disaster [UPDATED]

After the earthquake of March 11, Japan is facing the worst nuclear disaster decades.
might be interesting to know details of the operation of the reactors at the Fukushima nuclear plant to understand how this accident occurred. The impact of this catastrophe, inevitably, will to reopen the debate on nuclear .

plant

The Fukushima Daiichi nuclear power plant has six reactors and was built in the seventies. The details of each reactor can vary, but the concept remains the same: the core is a pill-shaped glass, filled with several hundred fuel assemblies. Each fuel element turn contains hundreds of fuel rods. A fuel rod is a long narrow tube made from an alloy of zirconium , which contains within pellets of uranium enriched to 3-5% in the isotope U-235 (in the case of reactor # 3 For plutonium-239).
When there is enough fuel in the core, there is a chain reaction that generates heat that will ultimately be used to produce energy. The reactor core can be working for a year or more before replacement is needed fuel. The accident

accident The key is in the water. In addition to the fuel rods fuel elements are also channels through which circulates purified water. Water acts as a moderator of reactions and, in turn, serves as a coolant for the core, and of course, also to produce electricity: water, when converted into steam in the reactor, power turbines that generate electricity . After passing through the turbines, the water cools and re-introduced back into the core, repeating the whole process.
All goes well until the water stops flowing, which is exactly what happened during the 9.0 magnitude earthquake that struck the region last March 11. When the system failed electric, diesel generators, designed to ensure that water flow in the reactor # 1, stopped working for one hour after the earthquake. The next day, March 12, the water supply to the reactor # 3 was also interrupted. In both cases, the core was overheated.
crisis
Immediately after the earthquake, the reactors at the Fukushima, and many others, entered automatically in 'off'. Special bars neutron absorbing material, known as control rods, were introduced between the fuel elements to stop the chain reaction. But the chain reactions are not solely responsible for the heat produced in the core: as fuel is consumed, new elements are created that also produce heat in their own process of radioactive decay. This is a small amount of heat, yes, but there is no way off. Therefore, by not operating the emergency cooling, the temperature begins to rise. The water inside the reactor also began to boil and become vapor, increasing the pressure inside the reactor.
When the temperature reaches a thousand degrees centigrade, the zirconium alloy that holds the uranium pellets began to melt or crack. When this happens, it reacts with water vapor producing zirconium oxide and hydrogen gas, which is highly volatile.
Operators may or may not know what was happening when they decided to release some pressure in the reactor # 1 on Saturday. Hydrogen was apparently the cause of the explosion that blew the cover of the reactor, although the vessel appears to have suffered any damage (see diagram the Nuclear Energy Institute, NEI ).
If, as seems, the zirconium was broken, some of the pellets of uranium and plutonium fuel rods would have melted and deposited on the bottom of the glass. In that case, the cores # 1 and # 3 would be a huge moment in test tubes containing radioactive fuel, mixed with zirconium and water. That spilled fuel could initiate uncontrolled reactions. If that occurred, would lead to an unprecedented nuclear disaster. Emergency Actions

To prevent such a catastrophe, the central technical decide to flood the two reactors with seawater. The decision was not easy: the impurities from seawater definitely spoil the nuclei, leaving them absolutely useless. However, at least, get lower the temperature and prevent further breakage zirconium rods. In addition, boric acid is being injected, which is an excellent absorber of neutrons and should avoid reactions, even with the fuel deposited on the bottom of the core. Is also releasing excess steam, reducing the internal pressure.
What happens next?
is very difficult venture. In the best case scenario, the fuel may be cooled enough to stabilize the situation. What there is to know is that there is no way to 'turn off' the residual heat will stay inside these reactors. Unless all the fuel can be removed, something that seems impossible at present, the cores need refrigeration for weeks to avoid further crisis. Y after the danger has passed, the dismantling of the reactors may take decades.
Way: The Great Beyond more information do not hesitate to contact the author or the publisher (Need, Alfredo Landman 605 978 575). Prospectus. Need

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