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.
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.
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