For example it could get dispersed an irradiate people, maybe to an immediately fatal degree or contribute to cancer etc. That's the main problem. Seeping into the ground water, worst case getting blown up in an explosion. The latter may also happen intentionally, as a dirty bomb. Because betting big on SMRs means they are going to be everywhere...
You normally don't hear about these kind of problems because of course the people responsible don't let the simple and stupid stuff happen. But this has to be prevented forever for all practical purposes, for all waste ever generated.
I think ground water contamination is not an issue for a properly chosen site. I think the terrorist issue is an overblown boogeyman. More reactors would give bad actors more access to nuclear material though but I don't think that's an issue with the end of life storage part of the chain.
Erm, "I think the terrorist issue is an overblown boogeyman" is cold comfort. ISIS already comtemplated building dirty bombs in Mossul.
And so far most of the nuclear waste has been stored right were it was used. SMRs are supposed to be everywhere. So nuclear waste will be everywhere, ready to get stolen, or shot at or blown up.
They didn't know that they had Cobalt-60 on the college campus, which would have made a superb dirty bomb. They did get their hands on some Uranium, though.
So it is a relatively close call. To close to hand-wave away the possibility of terrorists or failed states abusing nuclear material.
By the numbers, any kind of terrorism is an overblown boogeyman. Nuclear terrorism particularly so, and doubly overblown is nuclear terrorism that isn't state sponsored.
You'd hope so. But in the case of ISIS you could argue that it was a near-state actor. And we just can't predict the trajectory of state actors. Pakistan is troublesome. So is Iran, even with its "civilian" nuclear program. North-Korea. Those are just examples from the present. There are lots of other countries that would need nuclear power but whom I wouldn't count on surviving the next few decades. And many are too corrupt to regulate a nuclear industry well enough.
Just hand-waving away future risks like that does sound intellectually dishonest and careless to me.
You normally don't hear about these kind of problems because of course the people responsible don't let the simple and stupid stuff happen. But this has to be prevented forever for all practical purposes, for all waste ever generated.