Holding 400 tonnes of coal in one hand

Nuclear fuel

I assume it is uranium oxide?

Yes. UO2 is uranium oxide. It's a processed uranium product used for mixed oxide (MOX) fuel for nuclear reactors. It is relatively safe to handle and store, but the mining and processing of raw uranium as well as the wage byproducts of spent UO2 fuel are very hazzardous.

Don't fret. They'll always find a way. Maybe an "environmental remediation tax" for reactor byproducts?

Uranium is limited too, they'll be fine