Just to put things in perspective, the last time I worked the numbers, it turns out that a cubic kilometer of seawater contains about a metric ton of uranium in solution. We don't give it any thought at all. Dilution of cesium-137 is massive. The best shielding against serious (gamma) radiation is U-238.

Just to put things in perspective, the last time I worked the numbers, it turns out that a cubic kilometer of seawater contains about a metric ton of uranium in solution. We don't give it any thought at all. Dilution of cesium-137 is massive, and the isotope dwindles to nothing in a matter of weeks (my cat had it to treat a hyperthyroid condition; had to store the litter for 90 days before disposal). The best shielding against serious (gamma) radiation is U-238.