Its the case a nuclear material was dunped into the pacific, and you can measure that and right after there was a lot of fear mongering about it.

However the emperical evidence does not seem to show that it is as bad as one might think. From "the environmental impact of the fukushima nuclear power plant disaster" paper: despite the significant increase in ceasium isotope levels in the water, their risk is below thode generally considered harmfull to Marine animals and human consumers.

The same goes for the japanese population, there has not been found an increase in the cancer rate after the incident. (At least last i checked) Thise who died mostly died from being moved and not from the actual incident.