>The volume of the Atlantic with its adjacent seas is 354,700,000 cubic kilometers
1 cubic kilometer is 1 000 000 000 000 liters, so 354,700,000 cubic kilometers is 354,700,000,000,000,000,000 l.
1 ton of water is 907 l water, and Fukushima leaks 450 tons a day, that's 408,150 l.
Divided and multiplied by 100 that's 1.3*10^(-13)%.
Keep in mind, as I said above, that this water itself is a dilution of radioactivity.