My knowledge about school level physics is admittedly not that fleshed out, but we were told that it is possible to convert mass to energy. My google-fu has sadly left me for my question here 🙁
So why can’t we just take e.g. nuclear waste and convert it to energy? After that so is my understanding it wouldn’t simply exist as matter anymore and wouldn’t require to store dangerous trash if you can convert it all to energy.
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In a nuclear fission reaction, we take a bunch of one or more unstable elements (e.g. uranium) and turn it into slightly less of one or more other more stable elements (e.g. barium and krypton) + a bunch of energy. The barium and krypton can’t be nuclear fissioned (at least, not easily), so we’re stuck with them.
Additionally, the nuclear waste still has a lot of the original uranium, just not in high enough concentrations to do more fission. That’s why you sometimes hear about [nuclear reprocessing](https://en.wikipedia.org/wiki/Nuclear_reprocessing) where the waste is chemically/physically processed to re-concentrate the uranium for use as fuel again.
It is possible to convert mass to energy, but it isn’t possible to just convert *any* mass to energy. The main methods we have of doing this are nuclear fission and nuclear fusion. Nuclear fission is the thing that causes nuclear waste in the first place. It involves bombarding uranium atoms with little particles called *neutrons*. This causes the uranium atoms to split apart into a couple different pieces, and it turns out the pieces together have less mass than the original uranium atoms did. The rest of the mass has turned into energy.
The thing is, though, that this mass was actually energy all along. It was the energy that held the uranium atoms together (the so-called *binding energy*), and it got released when they broke into pieces. And that’s kind of the point. Objects always gain mass if they have more energy. A spring has more mass when it is under compression or tension, because it has stored up potential energy. When you release the spring, that energy is released and the spring’s mass decreases again. Warmer objects also have more mass than colder ones, and spinning balls have more mass than stationary ones. So in a way, there’s not anything special about nuclear fission. It’s just a matter of how much energy you can harvest. And with nuclear fission, that happens to be a *lot* of energy from every single gram of nuclear fuel.
Anyway, the point is that we don’t have some kind of machine that you can put mass into and get energy out of, with all the mass turned into energy. You have to be able to release the energy somehow, and that’s not always possible.
A useful comparison is burning fuel. Let’s say you burn methane (natural gas). If you burn it hot enough all you’re left with is CO2 gas and water vapor. These products will have (very very slightly) less mass than the original methane did. The (tiny) difference in mass is the chemical bond energy (now released as heat) that held some of the atoms together inside the methane molecules (really quite similar to nuclear fusion, only there is a lot less harvestable chemical energy in a single gram of methane than there is usable binding energy in a gram of uranium). Okay, so now we have CO2 and water. They still have mass – can’t we convert more of that mass to energy? Well, how would you do that? You can’t keep burning them – they’re not combustible (water is rather famously used to put out fires, not fuel them). And that’s the problem: you have to have a way to harvest more energy from them – there is no magical mass-to-energy machine.
Nuclear waste is a broad catch all category for things used in a nuclear power station, so gloves and protective suits can become contaminated while protecting the wearer. The old suits are considered low level nuclear waste.
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You don’t want to just release energy stored in mass, you want to do so in a slow, sustained manner.
We’ve found ways to harness energy from certain nuclear isotopes in a way that is slow and sustained. Even then, there are many steps and safety measures involved to make sure that it doesn’t go from ‘slow and sustained’ to ‘fast and uncontrolled.’ Fast and uncontrolled effectively means a bomb.
Other isotopes that fall out of that desirable range either don’t release enough energy to make it worth our time and effort, or is way too eager to release energy all at once thus making them much harder to keep under control and very dangerous. (Again, bomb.) Some of the materials in ‘spent’ nuclear fuel fall in those two categories.
All that said, some nations do recycle ‘spent’ nuclear fuel by combining an amount of them with fresh fuel, since there is still much (harnessable) energy potential left in the ‘spent’ fuel rods, as long as you keep the ratio of undesirable isotopes (decays too slow or too fast) below a certain threshold. France does this; the United States does not.
As an aside, the aforementioned ‘fast and uncontrolled’ isotopes are a desired byproduct of uranium-based reactors for many nations, as they are well suited for—you guessed it—nuclear bombs. That’s why many nations opted for nuclear reactors that use uranium (and breed plutonium) instead of using other isotopes such as thorium, which doesn’t breed weapons-grade fissile materials.
But now that the superpowers have more than enough nuclear weapons to end the world several times over, there’s not much use for the bomb-grade nuclear byproducts, except for replacing the degraded material in our existing arsenals with fresh material.
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When the nuclear fuel undergoes fission, we actually do convert some mass to energy. That’s where the energy from the fission reaction comes from. The thing is, it’s only a very tiny amount of mass being converted to energy. In order to convert the rest to energy 100% efficiently, we would need antimatter to annihilate the nuclear waste. The problem with that is that in order to create antimatter, we would need a massive amount of energy, and we would also create an equal amount of matter, so we still have a byproduct in the end. We have only created a few thousand atoms of antihydrogen, nowhere near enough to annihilate all of the nuclear waste.
Only a tiny bit of the mass is converted to energy.
Most of it can’t be converted to mass.
The left over mass is still dangerous.
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You can’t arbitrarily just turn things into energy. You can only do that under very specific circumstances-like, say, within the nuclear reactor in the first place.
The reason the waste is waste is exactly because it’s in a form that’s difficult to turn into energy-so we can’t delete mass like that without first inputting absolutely colossal amounts of energy doing something first-like say, making antimatter to interact with it to destroy it.
Due to the amount of energy and effort that would be involved with doing so exceeding the amount of energy you’d get out of this process by several orders of magnitude, this is infeasible as a power source-and in terms of trash removal ‘load a rocket with it and shoot it into the sun or deep space’ would be cheaper option than attempting it.
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