Fuel pellet
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I mold nuclear fuel to use in a nuclear reactor like porcelain, and a fuel pellet (the Ryo pellet which does not lie down, fuel heartwood, English: fuel pellet) is an element of the first turn of five folds of walls of the nuclear reactor with fried ceramic which I hardened.
Table of contents
Summary
The most common shape is 1cm in height, a black column-shaped thing of a little less than 1cm in diameter, and, depending on a nuclear reactor, there is the pellet that the center becomes hollow. As for used uranium, low-grade uranium is used in one of the natural uranium or the low-grade uranium in Japan.
Not only because a melting point is 1,132 degrees Celsius, I melt easily in the nuclear reactor with the high temperature, but also the uranium metal has a fault such as crystal structure changing at approximately 670 degrees Celsius, and expanding. Therefore I mold it after having done oxide of uranium into powder form and raise a melting point to around 2,700-2,800 degree Celsius by baking it like porcelain, and hardening it (sintering).
The fuel pellet has ability to hold a radioisotope (fission products) formed by nuclear fission inside. The volumes of the fuel pellet increase by the accumulation of fission products. This volume increase is called a swelling. The swelling by gas fission products such as xenon in particular or krypton is called a gas swelling. The gas fission products scatter the crystal grain and become the air bubbles in a crystal grain boundary. These air bubbles are the causes of the gas swelling. It takes time so that a crystal grain is big before gas fission products arrive at the crystal grain boundary, and, as a result, the gas swelling is reduced. For a method to make a crystal grain big, addition of alumina and the silica is considered. In addition, it is performed by a swelling that I take (dish) and the corner touching a hollow in the base of the column-shaped fuel at the time of molding (Chan F) that a coating pipe contacts a pellet and may harm the soundness of the coating pipe. I inspect the appearance, dimensions, an uranium degree of concentration, a chemistry ingredient, impurities, density before the fuel pellet being able to be in the fuel coating pipe, and making a fuel rod, and only the thing which passed inspection is used.
Kind
The used uranium compound (ceramic) has three kinds of the next.
Carbonization uranium
Because carbonization uranium (UC) includes a lot of uranium which is nuclear fuel, and the thermal conductivity is good, it is excellent for a fuel pellet. However, a chemical reaction is easy to be caused because it is activity. In addition, I am not used in the light water reactor from consideration in the security because a chemical reaction is caused even if I touch it with water. In addition, there is a fault to wake up (cementation phenomenon) that carbon shifts to to stainless steel used in a fast reactor as a coating pipe.
Nitrided uranium
Nitrided uranium (UN) includes a lot of uranium like carbonization uranium, too and the thermal conductivity is good, too, but on the other hand active is easy to wake up a chemical reaction and is not used with the light water reactor to wake up a chemical reaction even if I touch it with water. In addition, absorption cross section of N14 is big (n,β), and there is a fault to generate C14 by a reaction.
Uranic dioxide
There is little content of uranium, and the uranic dioxide (UO2) is inferior to carbonization uranium in the thermal conductivity, too, but because it is inert, a chemical reaction is hard to be caused and is used in many nuclear reactors including the light water reactor. But a thermal conductivity is very lower than metal uranium mentioned above, carbonization uranium and is 4W/mK degree at 10W/mK, 1,000 degrees Celsius at normal temperature. This is because the heat conduction of the uranium oxide assumes phonon (lattice vibration) mediation. In addition, a thermal conductivity is 300 degrees Celsius and is very big at 44W/mK, 1,000 degrees Celsius with 28W/mK because metal uranium is heat conduction to assume an electron mediation.
Allied item
Outside link
This article is taken from the Japanese Wikipedia Fuel pellet
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