re: Launch it all into the Sun
Assuming the existence of a launch system and container reliable enough for this, just launch it into deep space at escape velocity. Or take it to a facility in some distant orbit, and save it until you can do something useful with it. But we don't need to go to these lengths.
I think we need to stop treating this problem as though we're dealing with hundreds of tons of antimatter. The idea that we can't use any solution that doesn't keep the material utterly safe for tens of thousands of years is inane.
This debate is being driven by those who have a fundamentalist opposition to nuclear power in any form, and will continue to find obstacles no matter what technological answers we come up with.