The plants at Fukushima are so called Boiling Water Reactors or BWR for short. Boiling Water Reactors are similar to a pressure cooker. The nuclear fuel heats water, the water boils and creates steam, the steam then drives turbines that create the electricity, and the steam is then cooled and condensed back to water, and the water send back to be heated by the nuclear fuel. The pressure cooker operates at about 250 °C.
The nuclear fuel is uranium oxide. Uranium oxide is a ceramic with a very high melting point of about 3000 °C. The fuel is manufactured in pellets (think little cylinders the size of Lego bricks). Those pieces are then put into a long tube made of Zircaloy with a melting point of 2200 °C, and sealed tight. The assembly is called a fuel rod. These fuel rods are then put together to form larger packages and a number of these packages are then put into the reactor. All these packages together are referred to as “the core”.
The Zircaloy casing is the first containment. It separates the radioactive fuel from the rest of the world. The core is then placed in the “pressure vessels”. That is the pressure cooker we talked about before. The pressure vessel is the second containment. This is one sturdy piece of a pot, designed to safely contain the core for temperatures several hundred °C. That covers the scenarios where cooling can be restored at some point.
The entire “hardware” of the nuclear reactor – the pressure vessel and all pipes, pumps, coolant (water) reserves, is then encased in the third containment. The third containment is a hermetically (air tight) sealed, very thick bubble of the strongest steel. The third containment is designed, built and tested for one single purpose: To contain, indefinitely, a complete core meltdown. For that purpose, a large and thick concrete basin is cast under the pressure vessel (the second containment), which is filled with graphite, all inside the third containment. This is the so-called “core catcher”. If the core melts and the pressure vessel bursts (and eventually melts), it will catch the molten fuel and everything else. It is built in such a way that the nuclear fuel will be spread out, so it can cool down.
This third containment is then surrounded by the reactor building. The reactor building is an outer shell that is supposed to keep the weather out, but nothing in. (this is the part that was damaged in the explosion, but more to that later).
About Radiation
The radiation level getting higher around TOHOKU areas after Fukushima Nuclear Plant being damaged by tsunami. However Japan Government announced that the situation is still under control and no harm to human as for now. Most cities start to monitor the radiation level. We can see latest information about radiation level from MEXT (Ministry of Education, Culture, Sports, Science and Technology)
Chart below shows about radiation levels and its effect to human body. Yonezawa City's radiation is very low at about 0.1 microseiverts. However at Fukushima Nuclear Plant it is about 1000 milisieverts (near reactor 2). 1000 milisieverts is maximum range for measurement, it means they really don’t have the real value of radiation level there…
There is massive leakage that already pollutes cooling water near the reactor. As a result of finding plutonium and finding a high density of radiated water, it’s clear that the fuel has melted. Radiation levels are soaring in seawater near the crippled Fukushima plant core. Two weeks after the nuclear power plant was hit by a massive earthquake and tsunami, tests on 25 March showed radioactive iodine had spiked 1,250 times higher than normal in the seawater just offshore the plant. Radioactive water has been found in all four of the reactors at the plant, which workers are continuing to pump out in an attempt to restore the plant’s cooling system. Nuclear safety official Hidehiko Nishiyama said that the cooling the reactors is the priority at this point. The removal of the contaminated water is the most urgent task now, and hopefully we can adjust the amount of cooling water going in,” he told the Associated Press.
What Types of Radiation Are There?
The radiation one typically encounters is one of four types: alpha radiation, beta radiation, gamma radiation, and x radiation. Neutron radiation is also encountered in nuclear power plants and high-altitude flight and emitted from some industrial radioactive sources.
- Alpha Radiation
Alpha radiation is a heavy, very short-range particle and is actually an ejected helium nucleus. Some characteristics of alpha radiation are:
Most alpha radiation is not able to penetrate human skin. Alpha-emitting materials can be harmful to humans if the materials are inhaled, swallowed, or absorbed through open wounds. A variety of instruments has been designed to measure alpha radiation. Special training in the use of these instruments is essential for making accurate measurements. A thin-window Geiger-Mueller (GM) probe can detect the presence of alpha radiation. Instruments cannot detect alpha radiation through even a thin layer of water, dust, paper, or other material, because alpha radiation is not penetrating. Alpha radiation travels only a short distance (a few inches) in air, but is not an external hazard. Alpha radiation is not able to penetrate clothing. Examples of some alpha emitters: radium, radon, uranium, thorium.
- Beta Radiation
Beta radiation is a light, short-range particle and is actually an ejected electron. Some characteristics of beta radiation are:
Beta radiation may travel several feet in air and is moderately penetrating. Beta radiation can penetrate human skin to the "germinal layer," where new skin cells are produced. If high levels of beta-emitting contaminants are allowed to remain on the skin for a prolonged period of time, they may cause skin injury.
Beta-emitting contaminants may be harmful if deposited internally. Most beta emitters can be detected with a survey instrument and a thin-window GM probe (e.g., "pancake" type). Some beta emitters, however, produce very low-energy, poorly penetrating radiation that may be difficult or impossible to detect. Examples of these difficult-to-detect beta emitters are hydrogen-3 (tritium), carbon-14, and sulfur-35.
Clothing provides some protection against beta radiation. Examples of some pure beta emitters: strontium-90, carbon-14, tritium, and sulfur-35.
- Gamma and X Radiation
Gamma radiation or x rays are able to travel many feet in air and many inches in human tissue. They readily penetrate most materials and are sometimes called "penetrating" radiation. X rays are like gamma rays. X rays, too, are penetrating radiation. Sealed radioactive sources and machines that emit gamma radiation and x rays respectively constitute mainly an external hazard to humans. Gamma radiation and x rays are electromagnetic radiation like visible light, radio waves, and ultraviolet light. These electromagnetic radiations differ only in the amount of energy they have. Gamma rays and x rays are the most energetic of these. Dense materials are needed for shielding from gamma radiation. Clothing provides little shielding from penetrating radiation, but will prevent contamination of the skin by gamma-emitting radioactive materials. Gamma radiation is easily detected by survey meters with a sodium iodide detector probe. Gamma radiation and/or characteristic x rays frequently accompany the emission of alpha and beta radiation during radioactive decay.
Examples of some gamma emitters: iodine-131, cesium-137, cobalt-60, radium-226, and technetium-99m.
[Reference: Health Physics Society]
http://www.hps.org/publicinformation/ate/faqs/radiationtypes.html
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