Ben considers four elements (or groups of elements). He concentrates on tritium, iodine, cesium and the actinides. I will say what I think of each of these in turn. These elements range from a raging lion which is locked in a cage (the actinides), through a wolf in the garden (cesium), through to a cobra (iodine) which is able to slide through an open window into your bedroom through to a pest of a fly (tritium).
Tritium this is an isotope of hydrogen which forms inside nuclear reactors, it is formed by neutron activation of boron, lithium and water. The boron is present in control rods, while the lithium is present because it is used as the hydroxide to adjust the water chemistry in the reactor. By using lithium-7 the tritium production can be greatly reduced, this is because lithium-6 is converted by neutrons into alpha particles and tritium while lithium-7 can not do this reaction. The production of tritium from water in a light water reactor like the Japanese plant will be much less than what I would expect in a heavy water plant such as a CANDU.
I think that as soon as the reactors started to leak that any tritium in the cooling water would have started to escape. I think that this isotope is a minor isotope in this accident.
While tritium in the form of water is very mobile, I hold the view that is one of the least dangerous isotopes. Tritium only emits a very low energy beta particle, it is so low in energy that it can not be detected with a Geiger counter and it is so unable to pass through human tissue that tritium (outside the body) is not able to reach a fetus growing inside a woman. As a result some radiochemical workers are able to carry on working while carrying their unborn baby inside them. Internal contamination with tritium (getting it inside your body) is not good, but when compared with most isotopes it is not that bad a deal. The tritium is rapidly washed out of your body; it has a short biological half life. So it will not concentrate in the human body, as long as you drink non tritium contaminated water it will be soon washed out of your body. I am aware that in cases of humans who have been very strongly contaminated with tritium at work that one treatment was to give the person about 5 litres of water (or beer) to drink. This method of tritium removal was used many years ago at Harwell (nuclear research centre near Reading in England). I do not think that it is likely that any member of the general public will get contaminated with tritium to the point at which they need the 5 L of beer treatment, I will also say that for mothers to be (and the operators of cars & bicycles) that 5 L of beer is a very bad idea.
Also the beer trick only works for people who have been contaminated with tritium; it does not work for any other isotopes. Also if the normal drinking water is contaminated with tritium then it will not work, all the beer drinker will do is to swap one group of tritium atoms for some others, waste a lot of money, get very drunk and get a nasty hangover the next day.
If you want to read about tritium and reactor water then see http://www.nap.edu/openbook.php?record_id=9263&page=113
Cesium is an important element, in a reactor accident the cesium isotopes are often the isotopes which will have the greatest medium term effect on the public’s health and farming, the cesium isotopes are made by nuclear fission (Cs-137) and by neutron activation of fission products (Cs-134 and Cs-136). I have to agree with Ben that the cesium is very important.
The cesium has a moderate ability to escape from fuel and form fallout, it has a greater ability to escape from fuel than strontium but a much smaller ability than iodine.
It is interesting that a Cs-134 measurement is an easy way to distinguish between fallout from a recently detonated atom bomb and the radioactivity from a nuclear power plant accident.
Iodine is more important in the short term than cesium, this is because the radioactive iodine from a nuclear reactor accident is more mobile than cesium, and it also concentrates into a single small organ (thyroid) in the human body while cesium is spread over the whole body. This concentration into a single small organ makes exposure to iodine-131 a greater health hazard than exposure to the same amount of cesium-137. Ben is right the iodine is very important, after Chernobyl the iodine-131 exposure of the general public was responsible for the vast majority of the health effects which have been seen so far.
For pregnant women iodine is of special concern, radioactive iodine is more toxic to neonates and the foetus than it is to an adult. As a result women who are radiation workers are often strongly advised not to work with radioactive iodine if they become pregnant. Also the breastfeeding woman is often advised not to work with radioactive iodine, if a lactating woman has to be treated with radioactive iodine it is wise precaution for her to stop breast feeding her baby. If this ever happens to you, I suggest that you discuss it with your doctor as some actions will be needed to enable a woman to return to breastfeeding.
Ben also concentrates on the actinides; these are elements such as neptunium, plutonium and americium. While these elements are much more toxic to humans, thankfully they are far less mobile than the tritium, iodine and the cesium. While I think Ben was right to talk about them, I think under the conditions of the reactor accident they are much less important. However in the case of a fire in the used fuel storage pond, the actinides are very important.