Nuclear+Fission

__**Nuclear Fission**__


 * **Nuclear fission**—also known as atomic fission—is a process in nuclear physics in which the nucleus of an atom splits into two or more smaller nuclei as fission products, and usually some by-product particles.


 * The by-products include free neutrons, photons usually in the form gamma rays, and other nuclear fragments such as beta particles and alpha particles.


 * Fission of heavy elements is an exothermic reaction and can release substantial amounts of useful energy both as gamma rays and as kinetic energy of the fragments (heating the bulk material where fission takes place).


 * Nuclear fission differs from other forms of radioactive decay in that it can be harnessed and controlled via a chain reaction: free neutrons released by each fission event can trigger yet more events, which in turn release more neutrons and cause more fissions. Chemical isotopes that can sustain a fission chain reaction are called nuclear fuels, and are said to be fissile. The most common nuclear fuels are 235U (the isotope of uranium with an atomic mass of 235) and 239Pu (the isotope of plutonium with an atomic mass of 239). These fuels break apart into a range of chemical elements with atomic masses near 100 (fission products). Most nuclear fuels undergo spontaneous fission only very slowly, decaying mainly via an alpha/beta decay chain over periods of millennia to eons. In a nuclear reactor or nuclear weapon, most fission events are induced by bombardment with another particle such as a neutron.

To see what happens to Uranium 235 visit this useful site (below): http://library.thinkquest.org/17940/texts/fission/fission.html · An uncontrolled chain reaction within a sufficiently large amount of fission fuel (critical mass) can lead to an explosive energy release and is the concept behind nuclear weapons. The chain reaction could also be adequately controlled and used as an energy source (nuclear reactor).
 * A **nuclear chain reaction** occurs when on average more than one nuclear reaction is caused by another nuclear reaction, thus leading to an exponential increase in the number of nuclear reactions.

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 * When a heavy atom undergoes nuclear fission it breaks into two or more fission fragments. The fission fragments consist of atoms more lightweight than the original heavy atom. The sum of their masses do not precisely equal that of the heavy atom, even while accounting for the incident neutron. The difference (mass difference) consists of ejected neutrons and the release of binding energy. The neutrons leave the reaction at high speed, and may collide with other heavy atoms in a phenomenon known as "fission capture". This could result in nuclear fission, forming the basis of a chain reaction.