Uranium (even-235) on its own honestly isn't that dangerous, I was oversimplifying to say that the enrichment is what made it dangerous.
You're incorrect here on the nature of moderators - they moderate neutron flux, but they don't moderate the reactivity, they increase it. Fast neutrons are less likely to cause a uranium atom to fission so you have to slow emitted neutrons down to generate a sustained chain reaction.
In the absence of a whole lot of work a (fresh) uranium fuel rod is fairly inert. It doesn't produce much heat or very dangerous levels of radiation. Once you put the work in to generate a sustained chain reaction the U-235 starts turning into vastly more radioactive isotopes. For comparison U-235 has a half life of 700-million years, while Caesium-137 has a half life of 90 days.
The corium underneath Fukushima is fissioning because it's full of shorter half life fission products from its time as an active reactor, not because of the remaining Uranium. Now some of that Uranium is presumably getting fissioned by neutrons flying off of all of the other stuff decaying in there, but in 20,000 years time when the core is fairly inert nearly all of the U-235 left in the core as of today will still be there.
Yes, spent fuel rods are exceedingly dangerous, far more so than unused ones. We should really use the term "spent fuel rods" instead of the nonspecific nuclear waste.
The fact that every nuclear power plant produces these and we store them in swimming pools is really not great.
Not to push coal, but by comparison, nothing that can ever happen in a coal plant can create the unfixable mess at Fukushima, or the one at Three Mile Island, or the one at Hanford, or many other nuke accident sites. These are simply not "cleanup-able" situations and they will continue to fission for thousands of years.
You're incorrect here on the nature of moderators - they moderate neutron flux, but they don't moderate the reactivity, they increase it. Fast neutrons are less likely to cause a uranium atom to fission so you have to slow emitted neutrons down to generate a sustained chain reaction.
In the absence of a whole lot of work a (fresh) uranium fuel rod is fairly inert. It doesn't produce much heat or very dangerous levels of radiation. Once you put the work in to generate a sustained chain reaction the U-235 starts turning into vastly more radioactive isotopes. For comparison U-235 has a half life of 700-million years, while Caesium-137 has a half life of 90 days.
The corium underneath Fukushima is fissioning because it's full of shorter half life fission products from its time as an active reactor, not because of the remaining Uranium. Now some of that Uranium is presumably getting fissioned by neutrons flying off of all of the other stuff decaying in there, but in 20,000 years time when the core is fairly inert nearly all of the U-235 left in the core as of today will still be there.