Why do we have e.g. nuclear waste, if mass can be converted to energy?

Question

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.

In: 77

Answer 1

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.