Very little. The ocean is pretty good at diluting this stuff. In fact, one solution that would actually work very well would be to dump the whole thing into the center of the pacific. Trouble is, reactors are enormous.300 tons of waste water are still dumping into the Pacific as well. What is that doing to the Ocean and marine life?
Revelations 8: 8-10
…8Then the second angel sounded his trumpet, and something like a great mountain burning with fire was thrown into the sea. A third of the sea turned to blood, 9a third of the living creatures in the sea died, and a third of the ships were destroyed. 10Then the third angel sounded his trumpet, and a great star burning like a torch fell from heaven and landed on a third of the rivers and on the springs of water.…
Compared to our own nuclear testing, the amount of radiation added to the environment would be paltry.