[QUOTE=chorpler]
Both, essentially – it’s too expensive and risky. On the risk side, putting spent radioactive fuel on top of a giant tube full of explosives and then lighting them off is clearly just not politically feasible – even if you put all the waste in practically indestructible containers, if the rocket blows up it scatters those containers who knows where, potentially making the material available to terrorists. Or even to idiots or tinkerers who just want to take a peek.
Plus, those indestructible containers are probably pretty heavy, significantly reducing the payload of waste you can lift on a single rocket, making it more expensive.
And heck, there’s always the chance that we could develop some new technology that turns today’s waste into valuable fuel.
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Well summed up. It’s difficult, expensive and risky to launch heavy nuclear fuel in heavy safety containers into space. It’s complicated by the fact that there are only so many launch sites, and depending on where in outer space you want to send the rocket to, there are only so many places you can reasonably build a launch facility. So now you need infrastructure to transport radioactive fuel from all over the place to the launch site, with all the risks that entails.
The one other issue is just where in space you want to put it? Of course the answer isn’t to let it burn up in the Earth’s atmosphere, so it has to either be a higher Earth orbit (which leaves a bunch of hazardous junk in potentially useful orbits where it could pose a collision or debris hazard), or on an Earth-escape trajectory. This means, again, extra expense from a bigger rocket, and then comes the next challenge.
Sure, space is big, but if you just chuck something into space near the Earth, there’s a good likelihood that it’s orbit will bring it back around to our neighbourhood again. An orbit out of the Earth’s orbital plane (the ecliptic) is less likely to intersect with the Earth again, but is hard to get to. We don’t want to crash it into any of the planets, probably, for fear of messing up future research/exploration/habitation/economic activities or affecting any local biosphere (the question still isn’t settled for Mars, yet). And dropping it into the sun seems like a nice idea, but it’s actually really fuel-intensive to get there, too.
So the best possibility is perhaps a solar orbit that steers well clear of the planets, and is relatively easy to get to. And then you run the risk of losing track of it, and, in the long run, having its orbit altered by the perturbations that the big planets produce. So perhaps just a solar escape trajectory, which is extra fuel-intensive.
So anyway, the upshot is, even if the rocket doesn’t blow up on the way to space, it’s not clear where you could sent it to cheaply.
Edited to add: Ah, I see OtakuLoki already covered this. Ah well.