A very serious situation situation may now be emerging in Japan. Despite repeated tabloid inflation of the dangers, it is`t until recently that the dangers really have started to increase. According to news-sources the radiation in the air around the reactor is 100-400 milli sieverts. Which could be deadly with only a few hours of exposure. This is serious enough, but are most likely from gases that will decay into harmless elements quite quickly. This has at least been the situation the last few days.
So far it is only moderately radioactive gases from inside the concrete container that have been vented into the air. If radioactive material from the core leaks out into the air directly, the disaster is pretty much a fact. There has been a lot of comparisons with Chernobyl in the aftermaths of the Japan Earthquake, but there are several important differences between the two.
First of all, the reactor in Chernobyl used slightly enriched Uranium-235 rods. While the Japanese plant uses a mixture of different fuels (MOX) from weapons grade plutonium and re-processed nuclear waste, partly put into civilian use to prevent proliferation of radioactive materials to terrorists. This fuel is much more radioactive and toxic than the U-235 rods used in Chernobyl. It releases far higher doses of gamma and neutron radiation than Uranium,and according to a paper released by the Japanese Atomic Energy Institute the release of radioactive gases and the deformation of fuel rods can be expected to be much higher than it was in the Chernobyl-accident.
Secondly, the Chernobyl-plant was designed to produce power in a much more aggressive and yielding way than the Japanese plant is. This gives the Japanese more time to control the situation, but given a complete cooling failure, the accident may be of a bigger magnitude than Chernobyl, due to the higher energies that are stored in the fuel used at the Fukushima plant. Possibly much bigger.
Contrary to popular belief, Plutonium is not very dangerous in its solid form. You could very well hold a piece of it in your hand without it causing you any damage. Since the radiation that comes from it can`t penetrate your skin. If it gets into your lungs on the other hand, you are in deep trouble. The Plutonium will stick in your lungs, and spread to your lymph-nodes and bone marrow. And stay there, bombarding your cells with alpha particles that ionize your tissue and DNA. And depending on the dose, it may cause radiation sickness and possibly a very painful death. If plutonium is spread into the air and sea it will cause widespread death of wildlife and plants and any humans exposed to the particles
However the Plutonium oxide that may leak from the core, is not very reactive, and will not burn or mix into air or water easily. So these particles are most dangerous in the vicinity of the plant. It is noteworthy on the other hand, that these radioactive particles, some with a half-time of 24.000 years or more. Can be re-activated into the air or water during forest-fires or later tsunamis, making them dangerous for future generations.
The most likely and dangerous culprit is Cesium-137. This was the compund that increased the radiation levels across the world after the Chernobyl accident, and subjected millions of Europeans to abnormally high doses of radiation. It it is very reactive and can create a wide variety of chemical derivatives that are radioactive and transportable in the food-chain. Cesium emits beta particles and strong gamma radiation, the latter being very penetrative, going through skin, clothes and walls. Making it more dangerous to the general population even with a limited degree of protection. If C-137 gets into the air from a reactor core breach, it could reach Tokyo in a matter of hours, and possibly USA within 36 hours. C-137 has a half-time of 30 years, so it is not so bad as certain Plutonium or Uranium isotopes, but still a silent killer if there is enough of it. Strontium-90 is an isotope that carries similar properties to C-137 in case of a meltdown, the main difference may be that the strontium mimics calcium in your body, and is therefor deposited in bone-structures.