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?


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?

In: 6

A T-shirt is mostly light atoms like carbon and hydrogen, which take more energy to split than they release. They can however be fused to release energy, but it’s stupidly difficult to make it happen, normally only happening in the cores of stars.

U-235 releases energy when split, but crucially, also releases neutrons that can hit other atoms causing those to split too.

Splitting a nuclei of an atom is a nuclear reaction. The problem is how to sustain it at a level that might prove useful. If an object is continuously bombarded by fast neutrons and collisions with nuclei cause the nuclei in that object to split, then yes, there will be a “nuclear reaction”. But it won’t be sustained or it will sustain itself about as long as a source of those neutrons are present. (let’s not discuss more complicated stuff for the ELI5)

The nature of radioactive material like U235 is that when it absorbs a neutron, the nuclei fairly quickly splits and releases two or three more fast neutrons. This is not true for most other nuclei – some simply absorb the neutron and remain fairly stable, others might split but not release 2-3 neutrons or have the release be of sufficient “speed”.

So, ELI5, 1 becomes 2 becomes 4 becomes 8. etc So this reaction sustains and builds on itself. If sufficient U-235 is present and in a dense enough configuration, then the reaction feeds back on itself and goes boom. But in a nuclear reactor, the number of reactions are controlled by design (use water or graphite to absorb excess neutrons) so that it doesn’t go out of control and remains at a steady state.

Bottom line the problem is not splitting the nuclei but sustaining that process.

A nuclear reaction happens in uranium 235 because when a neutron hits it, it splits into 2 and releases an average of 2.38 neutrons. Other molecules simply do not release neutrons when split. These neutrons hit more uranium molecules, which release more neutrons, which hit even more uranium, and so on. There is a condition which needs to be met, which is a large enough mass of uranium, known as a critical mass, or otherwise the neutrons would just fly out of the uranium before hitting other uranium molecules. You can decrease this critical mass by putting pressure on the uranium, which is usually done by putting explosives in a perfect sphere around a uranium ball and detonating them all at once. You can also decrease the critical mass by putting materials that reflect neutrons around the uranium. One more condition is that there needs to be an original source of electrons.

A nuclear reaction can mean any change in the nucleus of an atom such as fission, fusion, natural decay….etc. I’m going to assume based on your question that you mean a self-sustaining fission reaction like what occurs in a nuclear reactor.

First you some amount of a fissile isotope. Fissile means when split, the atom emits the right amount of neutrons and those neutrons have the right amount of energy. There aren’t that many isotopes that fit that criteria, and uranium 235 is one of them. Then you need to arrange the fissile material in such a way that neutrons from the fission of the atoms actually hit (and thus split apart) more atoms of that material.

Technically you can split any atom with 2 or more protons in the nucleus (so anything other than hydrogen) but small, light atoms are very stable. They don’t want to split apart, it takes a ton of energy to get them to do so, and it’s not self-sustaining. Only heavy, unstable atoms (like uranium) actually want to split apart.

T-shirts are mostly made out of cotton. Cotton is organic material that’s mostly made up of carbon, hydrogen, and oxygen. Those are all light atoms that are very stable and do not want to split apart, so you *could,* given the right conditions, split those atoms, but it would require a ton of energy and it would never be a self-sustaining reaction.

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.