HKNIC - More about Nuclear Fission
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More about Nuclear Fission

The biologist James Arnold, who was working with physicist Niels Bohr in Copenhagen when the splitting of a uranium atom was discovered in 1938, is believed to have been the first person to use the term nuclear fission. He apparently noted the striking similarity of the picture of bacterial multiplication through the division of cells with the breaking up of a uranium nucleus, through the gradual deformation of the original nucleus, its elongation, forming a waist and finally breaking into two almost equal halves.

Uranium is a silvery gray metal that is slightly softer than steel. It has 92 protons, and between 141 and 146 neutrons, with three naturally occurring nuclides, U-238 (99.275%), U-235 (0.711%) and U-234 (0.006%). It has the highest atomic mass among naturally occurring elements. It is about 19 times heavier than water and slightly lighter than gold. It is weakly radioactive and has a low radiation hazard. Its main health risk is chemical poisoning, being about as poisonous as lead. It occurs naturally in low concentrations and is more plentiful than tin, cadmium or silver, and can be found in very minute quantities in rock, soil and sea water, at several parts per million. Before the discovery of radiation, uranium was primarily used to provide a yellow colour to glassware and pottery . In practice, it is very difficult for a quantity of uranium to be in the right condition to start a nuclear reaction. To start a nuclear reaction, not only is enough uranium needed, but either at a very high concentration of U-235, or if the concentration of U-235 is low, the uranium needs to contain a medium such as water which is capable of putting any free neutrons into the right condition to begin a nuclear reaction. Nuclear reaction is possible through rare combinations found in nature, as in some uranium ore bodies that occurred billions years ago, or in the sun or other stars which involve a different nuclear reaction called fusion. It is otherwise possible given a properly reactor engineering design.

The energy released in the fission of a U-235 atom is typically 3.2 x 10-11 joule, which is 50 million times more when compared to the chemical reaction of a carbon atom with oxygen, or burning carbon, which releases 6.5 x 10-19 joule of energy. Nuclear fission involves releasing some of the energy that holds the atomic nucleus together. In comparison, burning carbon releases energy by rearranging the electrons in the outermost orbits that hold the electrons of the carbon and oxygen atoms.