The US did a half-assed clear up. They dumped tons of radioactive waste into a crater on Runit Island and capped it in concrete, which is now deteriorating.
The material outside the dome is hotter than the material inside the dome. I agree it was a half-ass cleanup, but OTOH dilution and time are wonderful things. The damage has already been done to the local marine life.
https://en.wikipedia.org/wiki/Runit_Island