I'm going to say something where people's immediate reaction is going to be negative. Ocean. We have stored waste in the ocean in the past and thus have data on it, from a few different sites. In most cases they were put in sealed containers, but have since leaked, which has given us the chance to see the environmental impact on those sites. Virtually none, even very close to the leaks itself. Turns out we could just start doing that again. We can still do a better job. My proposal is sealed containers put into the bottom of the marianas trench. If a leak does occur (all nuclear waste strategies will have leaks so we should assume them), then difference in density and height of the narrow column will make diffusion into the rest of the ocean basically zero. All nuclear waste is going to the ocean anyway because all containers leak and all water soluble substances flow into the ocean eventually if we are talking very large scales. The question is if it contaminates large amounts of land on its way there. You can prevent that. There is one case of nuclear waste having a negative impact on the ocean that I should mention. Off the coast of Somolia someone unknown dumped nuclear waste with no effort at containing it. That has had some negative impact. So open dumping isn't a good idea.