The following is some very basic notes on U238 in Uranium (92 electrons, 92 protons and 146 neutrons per atom) — the fairly safe stuff
and U235 (92 electrons, 92 protons and 143 neutrons per atom) — the very dangerous stuff.
Click here for a recent article in the Weekend Australian dated 31st August 2019 on Australia's Nuclear Ambitions in the days of Robert Menzies, Harold Holt, John Gorton, William McMahon and Gough Whitlam (1950-1973).
The normal percentage of U238 in a chunk of mined uranium is 99.2%, with U235 at about 0.7%. For most kinds of nuclear power, the percentage of U235 needs to be increased (enriched), typically to between 3½% and 5%.
Apparently it takes a long time to raise the percentage of U235 all the way to 20%, but then a lot more quickly to get it up to 90% to build a nuclear weapon.
Chain Reaction principle with U235 and U238
When a free moving neutron gets added to a U238 nucleus, unless the neutron is moving very fast, which in fact makes the possibility of its being added to the nucleus far less likely, then very little happens. Over a few weeks, two of the neutrons will decay into protons, as that atom changes into Plutonium Pu239 (94 electrons, 94 protons and 145 neutrons).
When a free moving neutron gets added to a U235 nucleus, whether it's moving slow or fast, nuclear fission occurs, the nucleus breaks apart, there is an enormous release of energy, release of more neutrons, and ongoing chain reaction.
Now, to slow the speed of neutrons, so the likelihood of them being added to a U235 nucleus increases, a moderator is used in a nuclear reactor, such as water.
The reason water is useful is because it has a large number of single hydrogen protons in the nucleus of its vast number of hydrogen atoms, of roughly the same mass as that neutron. Regular water (light water) atoms are not so good as the nucleus of each of these atoms tends to absorb the neutron, creating atoms of "deuterium" or heavy water. Heavy water atoms are better, as each nucleus won't absorb the neutron, just slow its speed down.
Although two to three neutrons are produced for every fission, not all of these neutrons are available for continuing the fission reaction. If the conditions are such that the neutrons are lost at a faster rate than they are formed by fission, the chain reaction will not be self-sustaining. At the point where the chain reaction can become self-sustaining, this is referred to as critical mass.
In an atomic bomb, a mass of fissile material greater than the critical mass must be assembled instantaneously and held together for about a millionth of a second to permit the chain reaction to propagate before the bomb explodes. The amount depends on several factors, the shape of the material, its composition and density, and the level of purity. A sphere, for example, has the minimum possible surface area for a given mass, and hence minimizes the leakage of neutrons.
Secondly, by surrounding the fissionable material with a suitable neutron "reflector", the critical mass can be reduced further. At about 15 kilograms weapon's grade U235 i.e. 90% pure, or about 5 kilograms Plutonium 239, it will achieve critical mass.
With "Little Boy" – the first nuclear weapon that was dropped on Hiroshima in Japan – it worked by shooting a hollow U235 cylinder at a target "plug" of the same material. With "Fat Man" – the second nuclear weapon that was dropped on Nagasaki three days later – its plutonium core achieved critical mass via implosion (or compression).
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