> severe contamination of ocean water

No it didn’t

Like I said at the time, you could melt all of the cores down at the Fukushima Daiitchi site and dissolve them all in to the oceans and it would be undetectable in sea water.

The oceans weigh around 10^21 kilograms, and the six reactor cores at Fukushima Daiichi would weigh, what, several hundred tons and contain, what, several tens of tonnes of radioactive products.

We’re talking beyond parts per trillion.