I have been following the news and reading about the events in Japan, and I have noticed some things which I think I should comment on.
The fires in the spent fuel store are serious problems, while less of the shortlived but very toxic iodine-131 will be present in this older fuel this spent fuel could still pose a threat.
Plutonium has been discussed, it so happens that unit 3 was fueled with MOX. I am well aware that the public and nuclear workers are aware that plutonium can be a threat to health. On yahoo news it was said that plutonium once absorbed into the blood will linger for years in the bone marrow and the liver. This is true but might not be relevent.
Plutonium dioxide which is the chemical form of plutonium in nuclear fuel is very very insoluble in water, in fact it is very hard to dissolve it in acid. As a result it is unlikely to get into the blood of a worker or member of the public in a water soluble form. But in a paradoxical way the insolubility of plutonium dioxide makes it more dangerous, if plutonium dioxide is inhaled it can stick in the lungs and stay there for a very long time. This could lead to lung cancer. Overall I think that airborne insoluble plutonium is a greater threat to health than a water soluble form of plutonium.
Some of the plutonium dioxide will be cleared from the lungs by the action of the little hairs which sweep the dirt out of your lungs, this plutonium together with your phlegm will then be swallowed. The insoluble plutonium dioxide then will pass through the digestive system unchanged and come out at the other end. One of the classic ways to check a nuclear worker for exposure to airborne plutonium dioxide is to get a poo sample and then test it for plutonium.
Because plutonium dioxide is very insoluble it is unlikely to get into the public drinking water assuming that the Japanese water company filters the water before selling it.
On the other hand because plutonium dioxide is very insoluble and has a very high boiling point, it is unlikely to escape from the reactor if the reactor is just over heated. A mechanically violent event which is able to scatter fuel would be required for the plutonium to be ejected into the air from the reactor. So as long as no violent event occurs inside the reactor we are safe from the plutonium, currently I hold a view that the iodine, noble gases (krypton and xenon) and cesium from the reactor will be a greater threat right now.
If you do get plutonium dioxide into a cut then it would be able to deliver an alpha dose to the tissues where the powder ends up, if however you inhale, eat or inject a soluble form of plutonium into your body. Then things are rather different, what will happen is 10 % of the soluble plutonium will be lost in your urine rapidly while the rest of it is split equally between your liver and bones. In the liver it has a biological half life of about 40 years while in the bone the biological half life is about 100 years. This is much longer than the biological half lives of lead (circa 10 years) and cadmium (circa 30 years) in bone.
It is possible to prevent 90% of soluble plutonium entering the liver and bones but this requires rapid treatment with special drugs.
So the good news is that the plutonium is not likely to escape from a simply overheated reactor at an early stage. Also plutonium dioxide is not likely to invade ground water and then get into the drinking water supply.
While the bad news is if your inhale plutonium dust then it is very bad for your lungs.