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Nuclear Fission
This is where we enter the murky world of
isotopes; this is a study subject all by itself and if know what the term
means it may be best to study it a little further before proceeding;
otherwise read on!
The uranium we are left with consists of two isotopes, 238 which has 92
protons and 146 neutrons, and a much smaller quantity (less than 1%) of
235 which has 92 protons again, but only 143 neutrons. Both are
radioactive and will eventually decay down to form lead in 100,000 years
or so but but the major difference is that whilst the nucleus of an atom
of 238 has a full complement of neutrons it will deflect away any neutron
it is bombarded with whilst a 235 nucleus will accept it instead and
promptly split into different elements such as barium or krypton whilst
giving off a large quantity of energy and gamma radiation. More
interestingly it will also give off the spare neutrons which can then
collide with other neclei causing these to split and give off more energy
and more neutrons which can ....... etc etc etc. In practice the majority
of these protons either miss the nuclei completely, partly because there
are insufficient atoms of plutonium present or they are mainly of U238
which ignores them. However, if the purity or quantity of U235 increases
either sustainable energy generation can be realised (as in nuclear power)
or ultimately a chain reaction could occur which would cause a
near-instantaneous release of huge quantities of energy - in other word a
nuclear bomb. As little as 110 lbs of pure U235 is big enough to cause an
uncontollable explosion; this is known as the critical mass. However,
since about 99.3% of naturally ocurring uranium is the un-fissionable 238
isotope, how do we extract and purify the the 235?
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