Scientists say plutonium may be the worst of all the fission byproducts that could enter the environment as a result of the Fukushima nuclear disaster. That's why MOX fuel rods that are piled up in spent fuel pools near the Unit 3 reactor, which consist of a mix of plutonium and uranium isotopes, have become the number one concern of workers at the plant.

Plutonium-239, the isotope found in the spent MOX fuel, is much more radioactive than the depleted Uranium-238 in the fuel.

Plutonium emits alpha radiation, a highly ionizing form of radiation, rather than beta or gamma radiation. External exposure to alpha particles isn't much of a health risk, because they have a low penetration depth and are usually stopped by skin. When alpha-emitters get inside cells, on the other hand, they are extremely hazardous. Alpha rays sent out from within cells cause somewhere between 10 and 1,000 times more chromosomal damage than beta or gamma rays.

According to the Environmental Protection Agency, plutonium enters the bloodstream via the lungs, then moves throughout the body and into the bones, liver, and other organs. It generally stays in those places for decades, subjecting surrounding organs and tissues to a continual bombardment of alpha radiation and greatly increasing the risk of cancer, especially lung cancer, liver cancer and bone sarcoma.

There are documented cases of workers at nuclear weapons facilities dying within days of experiencing brief accidental exposure to plutonium, according to the Hazardous Substances Data Bank.

Furthermore, among all the bad things coming out of Fukushima, plutonium will stay in the environment the longest. One isotope of plutonium, Pu-239, has a half-life of 24,100 years; that's the time it will take for half of the stuff to radioactively decay. Radioactive contaminants are dangerous for 10 to 20 times the length of their half-lives, meaning that dangerous plutonium released to the environment today will stick around for the next half a million years.