Eli5: What exactly are the conditions that have to be met for a nuclear reaction to occur? Why does it have to involve Uranium 235 and similar isotopes? The world is made out of atoms. If we were to split the nuclei of the atoms of like a T shirt would it cause a nuclear reaction?
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There are two types of nuclear reactions. The kind where you break an atom apart (fission) and the kind where you smash two atoms together (fusion).
The middle of atoms are made up of neutrons and protons. Protons are positively charged. You know how when you put two magnets together one way, they stick together, but turn one around and it’s extremely hard to get them to stick together? Protons are trying to get away from each other all the time just like those magnets that won’t stick together. But there’s something else keeping the protons and neutrons together: the nuclear force. It’s really strong, but only at really close range. With just a few protons and neutrons, the nuclear force is stronger and keeps them together just fine. But with a lot of protons and neutrons, the “magnetic” force that tries to force them apart starts to have a bigger effect. Uranium 235 and similar isotopes have a lot of protons and neutrons.
Sometimes a Uranium atom will spontaneously split on its own. This breaks the atom into multiple smaller atoms, along with some free neutrons. These neutrons might hit other Uranium atoms that are nearby, providing them with the energy to split also. Each time an atom splits, it releases some energy. If a lot of atoms do that really fast, it creates an explosion. If some of the atoms do it at a controlled rate, it can be used to heat water, turn it into steam, and turn a turbine for electrical energy. In order to do it to a lot of atoms really fast, there have to be a certain amount of the atoms in a small enough space so the atoms are nice and close together. Bombs will attempt to squish two or more pieces of the material together to create that situation. Nuclear reactors will have more spread out material that will have the nuclear reaction, but won’t explode.
The big factor here is that, due to the warring forces trying to push the protons and neutrons together (the nuclear force) and push the protons apart from each other (the magnetic force), since the nuclear force is stronger but at a shorter range, it takes a LOT of protons with the magnetic force to cause the instability required for easy atom splitting. Atoms with a high number of protons are atoms like Uranium and Plutonium. The atoms in your clothes have relatively small number of protons, and are much more likely to stay together in normal circumstances, or even circumstances that would cause Uranium to undergo fission. It is probably possible to split the atoms in your t-shirt (for instance, using a particle accelerator), but there wouldn’t be any kind of meaningful explosion from that. The key to creating an explosion or using the reaction for power is that each nuclei split starts or continues a chain reaction in nearby atoms.
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