Seems like a silly idea, but thinking about it more deeply I'm not so sure. Uranium is almost 7 times as dense as even the densest magma, so it _should_ sink in theory. The only issue I can think of is that "maybe" isn't good enough when you're talking about risking a large amount of radioactive waste getting spread out over a large area in a volcanic eruption if it turns out you're wrong. But if we could figure out a way to do it reliably... maybe?

Another potential issue: is getting rid of nuclear waste _permanently_ really such a good idea? What if we discover some other use for it in the future?