I was reading a question about why not to drop nuclear waste into volcanoes; the short answer is it would come back out and not be rendered safe.
Ignoring the cost and energy requirements, why can't waste be assimilated into artificially melted rocks?
If conditions could be controlled, it as if one could allow rocks to cool very hard subsequent to an appropriate melting process; the gradual weathering of these rocks could allow the integrated nuclear waste to disperse harmlessly over huge periods of time or be contained long enough to realize the biologically meaningful portion of their half-lives.
The rocks could perhaps be located to a geologically stable environment (for example polar ice)
What I'm asking is, what would prevent a society with access to massive amounts of energy (such as fusion or advanced solar power) from simply melting their old nuclear waste into rocks under controlled conditions?