Saw this not long ago.I question how much radiation the animals tested were actually exposed to. Radioactive material and sediment was probably moved around and spread out fairly quickly. Water is also a good insulator of ionizing radiation. You could swim in the spent fuel pool at a nuke plant and be fine; it would probably be nice and warm. Unless they were in direct contact with radioactive material, they wouldn't get much dose.
Been reading xkcd for years.