OK, in a nutshell...
"Radiation" is a catch-all term. There are many different types of radiation and untold different levels of radiation. The best definition I have is
that radiation is energetic release during transmutation (decay). Each radioactive isotope has its own intensity, half-life, and type(s) of radiation
associated with it.
Nuclear plants use uranium (U-238) as a fuel, which decays into other elements, some radioactive, by a nuclear chain reaction. As one atom decays, it
emits energetic neutrons, which then can intercept other atoms and cause them to spontaneously decay. At some point, the fuel is packed tightly enough
and in enough quantity to produce a critical mass, which simply means atoms are producing so many neutrons that the chain reaction will continue
indefinitely until all the fuel has been exhausted.
How does this affect the US? Well, here are the different types of radiation:
- Alpha radiation is the emission of a helium nucleus, 2 protons
and two electrons. This is possibly the most carcinogenic of the different types of radiation, because the helium nuclei are electrically charged and
massive on an atomic scale. Plutonium decays via alpha emission.
- Beta radiation is the emission of electrons or positrons (anti-electrons). This also has a carcinogenic effect since the emissions are ionizing
like alpha emission, but they do not have the atomic mass of alpha radiation. Beta emission is used in food sterilization, as it quickly kills
bacteria. Cesium-135, Cesium-137, and Iodine-131 all decay by beta emission.
- Neutron emission occurs when energetic neutrons are emitted. Energetic neutrons then release energy (as do the alpha and beta particles) into the
surrounding environment. The problem with neutron emission is that neutrons are easily absorbed by certain materials, and this absorption does not
decrease their energy levels by itself. One notable material which is very good at absorbing neutrons is water. U-238 decays by neutron emission.
- Gamma radiation is the release of energy rather than particles. It can be compared to being exposed to an unshielded microwave oven. Most elements
exhibit some gamma radiation, but significant amounts come from iodine-131 and U-238.
The only type that poses any significant threat to the US would be neutron radiation, with some beta and gamma radiation possible for a short period
should something like iodine-131 manage to survive the trip across the Pacific. Any element that crosses the Pacific must have three characteristics
to be significant:
- It must have a long enough half-life to still be significant when it gets here.
- It must have a significant intensity of radiation.
- It must have a transport mechanism to carry it.
The last requirement is the critical one. Plutonium, for instance, is heavy and insoluble, so any plutonium that attempted to reach us would drop out
of the atmosphere well before it got close to the US. Iodine-131 could reach us (that was probably the cloud of radiation encountered by a US
warship), but by the time it did, the short half-life of 8 days would mean it would not have much radioactivity left. Cesium-135 or -137 could reach
us easily enough, and has a very long half-life, but cesium has very little intensity... it is mostly used for experimentation where a low-level,
mostly safe radiation source is needed.
Neutrons have all three characteristics. They can remain energetic for extended periods of time. They carry a good deal of energy, and by their very
nature, they can cause other quasi-stable atoms to split and produce more radiation of different types. And they are easily absorbed by water, which
provides them a free ride on any trade winds or currents available.
Neutrons in the atmosphere would fall as radioactive rain, accumulate in radioactive groundwater, be absorbed into radioactive plants, and then be
eaten by radioactive animals. There are plenty of areas in nature that will tend to concentrate energetic neutrons.
There is also really no defense against neutron contamination. The concrete walls inside a reactor building are the barricade used to protect the rest
of the plant. There are elements, like boron, that tend to absorb neutrons, but if they are already absorbed, this does not help.
The thing to watch for now is a reactor breach that releases neutron-rich steam into the atmosphere, in a huge white plume. That has not happened, and
I have no reason to believe it will at this point. It remains a possibility, but not a probability.
Not time to panic over here. Japan... yeah, panic time.
P.S.: Iodine pills only provide protection against I-131.