Seems silly to me to just leave it sitting there for decades if its internal heat is being generated by how the isotopes are in close proximity to each other. Or I might be misunderstanding why it's generating its own heat. Could they contain and cool down the waste by slicing off chunks of the mass and containing each chunk in lead-lined boxes? Essentially re-processing the lump into crude 'control rod' sized chunks…
It's a misunderstanding I had as well before I read detailed discussions of the Fukushima incident: stopping the chain reaction does not make a nuclear reactor "safe". It just prevents new highly radioactive material from being generated. But the material you already have is still generating massive amounts of radiation and heat, and if you're not able to dissipate that heat (which you want to do without also dissipating any of the material!) then the heat will build up, cause it to melt, and that will usually restart the chain reaction!