Nuclear Fission

Nuclear fission Key words chain reaction fission

1 Reaction with uranium

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Nuclear fission nuclear fission, a heavy atomic nucleus divides to make two smaller ones. Some of the mass of the nuclei is converted into energy during the process.

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235 92 U

148 57 La

uranium-235

lanthanum-148

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85 35 Br bromine-85

+ 3 01 n three neutrons

1 Reaction with uranium ● In

a nuclear reaction with uranium and slow-moving neutrons, the nucleus of the uranium-235 atom undergoes fission and forms two smaller nuclei (lanthanum-148 and bromine-85) plus three neutrons. A small amount of mass is converted to energy.

2 Chain reaction

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2 Chain reaction nuclear chain reaction is a series of self-sustaining reactions in which the particles released by one nucleus trigger the fission of at least as many other nuclei. ● Under normal circumstances, only a very small proportion of fission neutrons act in this way. However, if there is a sufficient amount of a radioactive isotope, a chain reaction can start. ● In an atomic bomb, an increasing uncontrolled chain reaction occurs in a very short time when two pieces of uranium-235 (or plutonium-239) are rapidly brought together. ● In a nuclear power station, the chain reaction is steady and controlled, so only a limited number of fission neutrons bring about further fission reactions.

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N 1 neutron

3 neutrons

9 neutrons