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The Basics
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Radiation
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Neutrons
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<h2>Fission</h2> <p>The word fission simply means 'split'. When certain heavy atoms such as uranium-235, are hit by a neutron, they can split into fragments. Those fragments will of course be atoms themselves, with weights about half the size of the original uranium atom. Uranium-235 might split into an atom of barium-141 and another of krypton-92. As that fission occurs, energy is released as well as two or three more neutrons.</p> <p>It is simple to see that one of those new neutrons could be used to split another atom of uranium, and the process could continue. That is exactly what a nuclear reactor does. It maintains a constant rate of uranium atoms splitting, creating neutrons which then split other uranium atoms. There is one important fact about the neutrons produced from fission which needs to be understood before you can build a reactor. When fission occurs, the neutrons are ejected from the uranium nucleus at great speed.</p> <p>In fact they are moving too fast to easily interact with neighbouring uranium atoms. Those neutrons need to be slowed down before they can cause new fissions. That process of slowing neutrons down is called moderation. A moderator is a substance inside which the neutrons can bounce around and lose a lot of energy. Once a neutron has traveled a certain distance through the moderator, it is moving a lot slower and can hit an atom of uranium-235 causing it to split and produce more fast neutrons.</p> <p> <a href="basics5.html" target="_top">next ></a></p> <p> <img src="px/fission1.jpg"></p> <p> <img src="px/fission2.jpg"></p> <p> <img src="px/fission3.jpg"></p>
