Although I suppose by melting it all together, it'd make it easier to store it, what with the smaller surface area to coat with radiation-containing materials?
They do that and it is called vitrification. The melt the waste into a glass-like substance. I guess their reasoning behind it is that it puts the waste in something that won't get easily eroded and it makes it near impossible for anyone to use it for anything nefarious.
But wouldnt you need a helluva lot to float a cannon, and then the recoil from the fired weapon would produce problems, and lastly, what if there wasnt enough oxygen to burn the propellent?
ITT: cannons mounted on balloons launching crap into space.
This is like rule 34 on engineering.