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Fission

In fission, a heavy nucleus absorbs a neutron and breaks apart to give back lighter nuclei plus 2 or more neutrons and some energy. Room temperature or slow thermal neutrons have enough kinetic energy to approach a nucleus of U-235. Lise Meitner and Otte Frisch were able to show that one of the rare isotopes, U-235, was able to split into roughly two equal parts. This they termed nuclear fission.

²³⁵₉₂U + ¹₀n -----> ²³⁶₉₂U -----> X + Y + b n^o

X + Y can be anything, but are usually in the midrange of mass numbers. The factor 'b' in this equation has a value of 2.47 which is the average of many fissions. When the U-235 is struck by the slow thermal neutron it becomes the unstable U-236. This U-236 fissions into Kr and Ba (most often) isotopes with the same n^o/p⁺ ratio as the U-236. This ratio is to high for the Kr and Ba and therefore they must eject n^o of a higher energy then the initial slow thermal neutrons.

An isotope capable of undergoing fission is called a fissile isotope. Only U-235 is a natural fissile isotope. U-233 and Pu-239 are synthetic fissile isotopes. Since each fission results in 2 or 3 n^o and, if these can be slowed down by collisions with their surroundings, then the potential for a nuclear chain reaction exists. A chain reaction is a self-sustaining process whereby products from one event cause one or more new events. If the mass of U-235 is small enough, the loss of n^o to the surroundings is to rapid to initiate chaining.

However, at the critical mass the loss is small. Most of the neutrons get trapped and strike other fissile nuclei. The result is a virtually instantaneous fission of the entire sample. An atomic bomb filled with weapons grade U-235 is essentially only 2 subcritical masses with an explosive trigger which drives them together. The explosion forces the sub-critical masses together and not only creates a critical mass but continues to compress the mass into an even more dense state. Any neutrons released spontaneously will not escape and start the chain reaction.

A reactor cannot be made to fission like a bomb. The bomb requires pure U-235 or Pu-239. The concentration of fissile isotopes in a reactor is only 2-4% with much of the remainder being U-238 (non-fissile). Even if all the fuel melted into a pool at the bottom of the reactor chamber (The China Syndrome) there would not be enough U-235 to form a critical mass. To have controlled fission take place in a reactor, the leakage of neutrons away from the core must be controlled, and the fast neutrons produced by fission must be slowed. Leakage occurs at the surface of the core, whereas n^o are generated throughout the volume of the core. By making the fuel core large enough, the ratio of surface area to volume can be reduced to where enough neutrons are retained to cause further fission events. To slow neutrons down, the fuel core is provided with a moderator. Water itself is a good moderator and D₂O is used in the CANDU reactors. Graphite is also a good moderator and it was in use in the Chernobyl reactor. The safe operation of a reactor requires that the multiplication of slow neutrons be controlled throughout the fission cycle. This cycle begins with the fission of a U-235 nuclei and the production of fast neutrons, their moderation and the use of the survivors of the moderation to launch additional fission events. The ratio of neutrons at the end of the cycle to those that started it is called the multiplication factor, k. For smooth safe, continuous operation, the value of k in the reactor should be exactly 1. When it is, the reactor is critical. When k>1 fission is accelerating and the reactor is supercritical. When k<1, fission is slowing down and the reactor is subcritical. The reactor must have a built-in ability to go supercritical and then be controlled. The system uses cadmium control rods to capture neutrons to keep the reactor at the desired level of criticality. The nuclear fuel is in the form of sintered pellets of uranium or plutonium oxide. These pellets are packed into cladding tubes made of zirconium alloy. These zirconium alloy tubes must be gas proof to prevent the escape of any gaseous radioactive wastes. As fission continues the wastes build-up in the cladding tubes and poison the U-235 still left. This slows the rate of fission. Only 3% of the U-235 in a tube actually fissions before it must be replaced. They still contain 97% of the original U-235 and newly synthesized Pu-239 made from U-238.

Alıntı: http://www.ucdsb.on.ca/tiss/stretton/chem2/nuc10.htm

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