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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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 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.
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.
Nuclear reactions happen all the time. The problem is sustaining it (and not letting it get out of control).
In fission an atom splits up and throws out some particles. If those particles hit another unsteady atom in just the right way the other particle will undergo fission and split too, causing a sustained reaction.
The problem is most of the time the initial reaction doesn’t hit another unstable atom in just the right way so it isn’t sustained. You need to get enough unstable atoms.together in a high concentration, then usually add something else to try and “catch” the particles flying about and direct them to another unstable atom.
You can cause a nuclear reaction in any element. The carbon in your T shirt can be split apart and fused together just like any other atoms.
However, for generating power, the vast majority of these reactions are not useful. What you want is something called a nuclear *chain* reaction, where causing one nuclear reaction will cause another one, and so on, so it can sustain itself, and you can get out more energy than you put in.
Uranium 235 is one of the few isotopes that has this capability. If you fire a neutron at it, it will split apart and release an average of about 2.5 neutrons. If you have enough U-235 in one place, those neutrons can split more U-235 atoms. By varying the setup, you can control the reaction – if 1 neutron out of that 2.5 on average splits a U-235, you have a stable nuclear reactor, and the rest of the energy goes into boiling water to generate power. If 2.2 out of 2.5 go on to split another atom, you have a bomb.
The kinds of isotopes you need for a nuclear chain reaction are called _fissile_ isotopes. They are isotopes that have just the right nuclear properties so that they will split when they absorb a neutron of basically any energy level, including the energy level of neutrons that are released by fission. Which is always a little tricky to explain, but what you’re basically asking for is an atom that can be split by the same neutrons that are produced by splitting atoms. If you have that, then you can have the neutrons from one atom split another atom, and so on and so on.
There are isotopes that are _fissionable_ but not _fissile_. U-238, for example, can fission, but only from very high-energy neutrons, higher-energy than the ones typically released by a fission reaction. So the odds are that if they absorb a neutron created by another fissioning atom, they won’t split, they just absorb it. Even if they do split, their neutron has a very low chance of splitting another U-238 atom. So the presence of U-238 — or any other non-fissile isotopes that absorb neutrons — will kill a chain reaction.
There are some atoms, typically very light ones, that will essentially bounce neutrons off of them, which lowers the energy of the neutrons. These are used in nuclear reactors, because decreasing the energy of a neutron increases the chance (for quantum mechanical reasons) of it splitting a fissile atom.
The exact conditions for a nuclear chain reaction are complicated and can be adjusted by a lot of variables. The long and short of it is that you need the atoms that can be split by neutrons to be near other atoms that can be split by neutrons, and you need to overcome the many ways in which a neutron creating by that splitting won’t split another splittable atom (e.g., captured by a non-splitting atom, lost to the environment, etc.). There are a few ways to do this for the purposes of nuclear weapons, and many ways to do this for the purposes of nuclear reactors.
At the center of atoms is the nucleus that contains one or more protons and(with the exception of hydrogen) one or more neutrons. The protons have a positive charge which is import for determining the overall charge of the atom which affects its chemical properties and is why elements are determined by the number of protons, this charge also causes protons to repel eachother, the neutrons have no charge and, to massively oversimplify hold the nucleus together. With a relatively low number of protons the nucleus can remain stable with roughly a 1:1 proton/neutron ratio, as you move up the periodic table the nucleus gets bigger and the proton:neutron ratio for stable atoms starts to approach 1:1.5. Eventually you reach a point(lead) where there there are no more stable nuclei and eventually they all undergo some form form of decay. Normally this decay involves minor changes like the nucleus spontaneously ejecting a couple protons/neutrons or a neutron converting to a proton and releasing energy etc, but with some really large nuclei under the right conditions like uranium 235 they can be split roughly in half and this process is called fission.
Splitting smaller nuclei like the carbon in your t-shirt is technically possible, but would require some exotic scenario like using it as a target for a partical accelerator or something. While this would produce a nuclear reaction it would be very very small and wouldn’t be very useful unless you wanted an extremely expensive radioactive t-shirt for some reason.
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