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Atomic Pile





It was soon realized that a nuclear chain reaction could not be set up in a limited number of uranium. Even uranium-235 atoms will not necessarily always absorb a neutron that comes toward the uranium atom. The neutron may merely bounce off, unabsorbed.

If in the process of bouncing from atom to atom the neutron manages to make its way out of the uranium and, into the open air, it is lost. If enough neutrons do so, the nuclear chain reaction will be quenched. To prevent thus, one must see to it that the chances of loss of neutrons to the surrounding environment, before absorption and consequent fission have a chance to take place, are minimized. The simplest way to do this is to increase the size of the uranium core. The larger its size the more bounces a neutron must undergo before reaching the edge of the core and the greater the chance of its absorption.

If the core is just large enough to lose so few neutrons that the nuclear chain reaction may just keep going, it is said to be at critical size. A smaller core, one of "subcritical size" cannot maintain a "self-sustaining nuclear reaction".

Toward the end of 1942, the first attempt was made to set up a self-sustaining nuclear reaction. This took place under the guidance of Enrico Fermi.

At the time, some pure uranium was available in both metallic form and in the form of the oxide. It was not enriched and so the critical size was extraordinarily high. A very large "atomic pile" had to be built, (it was called a "pile" because it was a pile of bricks of uranium, uranium oxide, and graphite.)

When this first nuclear reactor was completed, it was 30 feet wide, 32 feet long and 21 1/2 feet high. It weighed 1400 tons, of which 52 tons were uranium. The uranium, uranium oxide, and graphite were arranged in alternate layers with, here and there, holes into which long rods of cadmium could be fitted.

 

Date: 2015-10-19; view: 290; Нарушение авторских прав; Помощь в написании работы --> СЮДА...



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