i think the primary issue is cesium - though not 100% sure.
"Something like 326,000,000,000,000,000,000 gallons (
326 million trillion gallons) of the stuff (roughly
1,260,000,000,000,000,000,000 liters) can be found on our planet. This water is in a constant cycle -- it evaporates from the ocean, travels through the air, rains down on the land and then flows back to the ocean."
that is 1.26 * 10^21 liters of water on earth (mostly oceans)
from
https://www.ncbi.nlm.nih.gov/pubmed/17836351 , there is 0.37*10^-6 grams of cesium per liter
that comes out to about 0.4*10^15 grams of natural cesium in the ocean.
that is 0.4*10^12 kilograms
so that is about 0.4 billion tons of cesium
i guarantee the fuel rods in those reactors are much much much less than that.
the rods are usually about 4 meters long and 1 cm diameter
you may have ~30 of them.
lets say that is 1 ton of fuel rods.
i make a conservative assumption that it is ALL cesium (it is not).
so that is 1 radioactive atom of cesium to 0.4 billion atoms of nonradioactive cesium.
the plutonium/uranium issue - have not looked into it.