How comes there is nuclear fuel waste when uranium stays radioactive for hundreds of years shouldn’t we be able to use the same fuel rod until 2122+?
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The fission process creates lighter elements from the heavier uranium.
Uranium is used up to some degree. The daughter products cannot undergo fission and basically poison the process when there is too much of them.
There are a variety of daughter products, with different types of radioactive decay.
We *could* use the fuel for much longer, by using more efficient reactor designs such as [breeder reactors](https://en.wikipedia.org/wiki/Breeder_reactor).
We don’t because it’s just cheaper to dig more Uranium out of the ground. Uranium is dirt cheap compared to how much energy you can get from a kilogram of it.
– Uranium is very weakly radioactive, so it stays radioactive for hundreds of _millions_ of years (or more, depending on the kind of uranium), and isn’t that dangerous by itself.
– When you put it in a reactor, you transform the uranium into new elements (lighter ones) by splitting it (nuclear fission), which releases energy. To be clear about it, it is not the low-level of radioactivity in the uranium that makes it useful as a power source. It is the fact that you can make it split that makes it a power source. (I just note this because that seems to be a fundamental confusion not just for you, but for many discussions about nuclear power on ELI5. The key term here is that uranium-235 is “fissile” — capable of sustaining a fission chain reaction — and that is what makes it special and useful to us.)
– These new elements are more radioactive than the original uranium (they have shorter half-lives), so you have to treat them carefully. They are called “fission products.”
– Eventually you will use up enough of the U-235 in the fuel that it either becomes uneconomic to keep it in the reactor, or it makes the reactor more difficult to control. So they take the fuel out. This is now your “nuclear waste”: some U-238, a little leftover U-235, some new heavy elements (like Pu-239, which the U-238 turns into in the reactor), and the fission products.
– You could, if you wanted to, extract the U-235, U-238, and Pu-239 from the fuel, and turn it into new fuel. This is called “reprocessing.” It is done by some countries. It is very expensive — usually more expensive than just making new fuel.
– If you did that, you would still have to dispose of the fission products as nuclear waste. But the total volume of waste would be much smaller.
– There are many different types of reactors out there, and the above just describes how it usually works. But there are other ways to set it up. But the core issues are the same.
We split U-235 to get heat.
There is less than 5% U-235 in the fuel.
When we split enough of the fuel, it can’t maintain full power. This is when we refuel the reactor. When we pull the fuel out, it has about 0.7% U-235, about 0.7% Pu-239 (which is made in the reactor) about 93% U-238 which is not fissile, and the rest is waste.
The U and Pu are essentially harmless from a radiation perspective. The waste products cannot be used to make energy and that is the stuff that’s super hazardous.
“Radioactive” is not the same thing as “can sustain a nuclear chain reaction”. Very very few radioactive materials can do that, and those are the ones we use in a reactor.
Spent nuclear fuel is what’s left after you’ve burned off a lot of the material that *can* sustain a chain reaction. It’s still decaying, but it’s not decaying at a rate you can run a power plant off of. Unfortunately, there’s a very wide gap between “safe for life” and “can power a city”, and spent fuel spends a long time in between the two.
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