[gambiting]

Nuclear waste is waste because it doesn't produce enough heat anymore to be useful in production of electricity. So I doubt that spent nuclear fuel would be able to produce 1000C+ temperatures for 30 years to bury itself deep enough.

[tarre]

Nuclear waste is waste because it doesn't contain enough fissile nuclides (i.e. nuclides, which could produce heat in fission). It has almost nothing to do with radioactivity or decay power. Radioactivity just makes it harder to manage.

But you have it right: The decay power density of nuclear waste is quite soon not enough for such a probe.

[DCKing]

IANAG, but it probably doesn't need to get 100km under the ground. It only needs to go to the depth at which the nuclear waste stops being a problem.

With a good amount kilometers of rock between the surface and the waste, the danger is already gone. Furthermore, melting the waste and have it move thorugh the mantle will lower its concentration and will over time have the heavy materials fall into the core.

My guess is that the point at which the nuclear waste becomes less problematic than current methods will probably occur quite quickly, just a few kilometers if that. The point at which the tungsten corodes and the waste can be swept away doesn't seem to be that deep either.