I consider these passively-stable waste disposal solutions that are supposed to last 10,000-1,000,000 years without maintenance over-engineered and founded on dubious assumptions. I think that something like surface dry cask storage in a dry, geologically stable area is a lot better. The facility should have ongoing monitoring and the capability to re-cask units every few decades or centuries. If there's some weird failure mode that can't be discovered in artificial accelerated aging tests, future caretakers will be much better poised to cope with the problem if they actually have access to the waste. You don't need to try to invent a language that will survive into deep time, you don't have to use gargantuan safety margins to in an attempt to offset the unknowns of deep time, and so on.
What if the maintainer-civilization collapses? And what if nobody a thousand years from now knows how to build steel and concrete vessels or detect ionizing radiation? The somewhat cold but IMO correct answer is then those future low-tech humans will not suffer much risk from our spent fuel, relatively speaking. Making steel and concrete, making radiosensitive photographic emulsions -- these are 19th century technologies. If humans of the distant future don't have even 19th-century-equivalent technology, they probably have such high ambient morbidity and mortality from infectious disease that leaking nuclear waste will not much diminish their quality nor quantity of life. Not to mention that the most dangerous components of spent fuel -- short- and medium-lived fission products -- will be diminished a billion-fold or more after 1,000 years pass.
Spent light water reactor fuel fresh from the reactor is orders of magnitude more hazardous per unit mass than any stable element. But after 1,000 years or so the hazard is greatly diminished. At that point it's more akin to a mundanely-hazardous waste like cadmium. Nobody plans how to communicate the hazards of cadmium exposure into geological time even though it endures forever.
What if the maintainer-civilization collapses? And what if nobody a thousand years from now knows how to build steel and concrete vessels or detect ionizing radiation? The somewhat cold but IMO correct answer is then those future low-tech humans will not suffer much risk from our spent fuel, relatively speaking. Making steel and concrete, making radiosensitive photographic emulsions -- these are 19th century technologies. If humans of the distant future don't have even 19th-century-equivalent technology, they probably have such high ambient morbidity and mortality from infectious disease that leaking nuclear waste will not much diminish their quality nor quantity of life. Not to mention that the most dangerous components of spent fuel -- short- and medium-lived fission products -- will be diminished a billion-fold or more after 1,000 years pass.
Spent light water reactor fuel fresh from the reactor is orders of magnitude more hazardous per unit mass than any stable element. But after 1,000 years or so the hazard is greatly diminished. At that point it's more akin to a mundanely-hazardous waste like cadmium. Nobody plans how to communicate the hazards of cadmium exposure into geological time even though it endures forever.