system
Given the following post, respond as if you're commenting on Reddit:
human
What exactly is "weapons grade uranium/any other nuclear element"? What makes it different from other types of the same element?
gpt
"Weapons grade" refers to concentrations of _isotopes_ in _fissile material_ (fission/atomic bomb fuel). Isotopes mean they have the same number of protons (which defines what kind of element it is — uranium is element 92 because it has 92 protons, plutonium is element 94 because it has 94 protons), but different numbers of neutrons. 

There are two common isotopes of uranium. So the most common type of uranium (99% of it) found in the world is uranium-238 (92 protons, 146 neutrons). Less than 1% of it is uranium-235 (92 protons, 143 neutrons).  Only uranium-235 can work in an atomic bomb. It is just a little bit less stable than uranium-238 and so fissions (splits) more easily, with lower-energy neutrons. So to make weapons-grade uranium, you have to create a mass of uranium that is made up of 80-90% uranium-235. This is very difficult and requires huge factories and centrifuges and lots of energy to do. So "weapons-grade uranium" means "enriched uranium" which means "uranium where the uranium-235 isotope makes up around 80-90% of the uranium." U-238 will also inhibit a reaction, so the less of it, the better.

Plutonium is formed in nuclear reactors. It is created by letting uranium-238 absorb neutrons and then decay over a few days. The isotope of plutonium that is useful in weapons is plutonium-239. The problem here is that the longer nuclear fuel is in a reactor, the more likely that plutonium-239 you have made will also absorb another neutron and turn into plutonium-240. Plutonium-240 is very radioactive and doesn't fission very easily, so it both makes it hard to use plutonium-239 in a bomb and inhibits the reaction. So when you operate a reactor to make plutonium for a bomb, you take the fuel out relatively quickly, so not too much plutonium-240 is produced. If you are running a reactor for power purposes, you leave the fuel in as long as it is possible. Plutonium extracted from fuel that has been used in this fashion will have a large concentration of plutonium-240 in it, and thus not be as easy to use in a weapon. So "weapons-grade plutonium" means "plutonium that has no more than 7% plutonium-240 in it." ("Reactor grade" plutonium can have as much as 30% plutonium-240 in it.)