This might be a stupid suggestion/question, but maybe it'll become cheap enough to just toss the spent fuel rods into space?
To be more precise: achieving a velocity high enough to e.g. detonate on the surface of the sun (in particular carrying tons of nuclear waste) won't be cheap for a long time. [1]
Stuff we "toss out there" would probably just keep floating in an uncontrolled (in the long run) earth bound orbit and might come back at some point.
What you'd want to do is to shoot it out into an orbit around the Sun that's slightly lower or higher than Earth's. The chance of it somehow returning to Earth is so low that the Sun will go out before it happens.
Still, the whole concept is wasteful and dangerous anyway. For one, as others mentioned, a launch failure would mean large-scale contamination on the planet. Two, you can safely store this waste on Earth for many orders of magnitude less money and work.
> People still seem to make the same mistake our ancestors did regarding tossing stuff into the ocean.
“Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.” Throwing something out into orbit around the Sun isn't going to be a problem, like, ever.
It's still a pretty impressive way to show how strong they are...
Approximately 1/10,000 as much energy would be involved, and it would be 100% safe.
I've heard, for example, that geothermal energy generates nuclear waste (because digging so low digs up radioactive material anyway)
The challenge is that fuel rods are quite heavy and the $$$ costs to launch a kilogram on a path beyond earths orbit are incredibly high, let alone the carbon costs. Launching into space is just not a net positive.
I should add also that generally it's not the super high activity waste like the fuel rods or the low activity waste million years half-life stuff that makes things problematic but rather the heaps of intermediate activity waste that's active enough to require special equipment and procedures, too irregular to easily streamline, too long lived to just wait out and too voluminous to just stick it in a very secure container container somewhere. For that, a deep geological repository is mihc safer, simpler and cheaper.
Far better to just have it in a hole in the ground somewhere geologically inactive.
Edit: I'd far rather it be buried than whirling around above my head!
With reusable spacecraft, the cost could conceivably approach that of the propellants required, making such disposal permanent and cost effective.
That said, we're still not sure we want to get rid of high level nuclear waste; it's potentially a valuable source of fuel if some reactor technologies are developed.
https://www.theatlantic.com/science/archive/2018/08/parker-s...
My understanding is the ideal target would depend on the time you wished to launch, and the orbital phase of planets useful for gravity assists.
[NB Not that I would be in favour of that, but seems easier than the other options].