Level 3: Nuclear Fission
The binding energy of a nucleus is the energy needed to separate a
nucleus into its respective individual nucleons and is proportional to the
stability of the nucleus. For atoms heavier than Nickel-62, binding energy and
also the stability of the nucleus decreases as atomic mass increases. Energy
given off by atomic bombs arise from nuclear fission in which heavier
isotopes such as uranium or plutonium split into more tightly bound stable
elements. In the example of Atomic
bombs using Uranium-235 as the fissile material, free neutrons may hit
the uranium, resulting in the formation of the highly unstable Uranium-236. It
then splits into the highly radioactive but more stable fission products of
Barium-144, Krypton-89 and 3 neutrons which collide with more uranium to cause
a chain reaction. The energy released during nuclear
fission is the difference between the binding energies of the isotope used in
the reaction and the fission products.
Chain reaction refers to
self-sustaining nuclear fission. As seen from the example of atomic
bombs using Uranium-235, neutrons produce in a fission reaction go on to
trigger more fission reactions. The critical
mass is the amount of fissile material needed to sustain an exponentially
growing chain reaction. What this means is illustrated in the diagram below
in which the number of nuclei undergoing fission reactions keeps increasing.
However, do note that not all neutrons produced in fission reactions will trigger
additional fission reactions.
© BBC Bitesize
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