I have seen many alarming headlines and news-stories lately, saying things like "radiation clouds heading for the US from Japan" along with calls to protect your family now from this looming danger. Yesterday, I addressed the issue of using potassium iodide pills to block the radiation. Please read it if you haven't yet, but the short version is: those pills will not help much and they certainly won't block all the radiation as many people believe.
Radiation comes in many different forms. We are surrounded by and exposed to radiation all the time, since sunlight is radiation, as are radio waves, TV waves and microwaves. This type of radiation is called "non-ionizing" because when it is absorbed by matter (like your body) it does not produce charged particles known as ions.
Ionizing radiation, on the other hand, will strip electrons from the atoms in your body (or any other type of matter) when it is absorbed. This can wreak havoc in your tissues, since the result can be free radicals or other highly-reactive species that cause a cascade of worrisome events.
We are exposed to ionizing radiation all the time, and our bodies have developed means to defend against a certain level of radiation in the environment. Free radical scavengers exist in our bodies and clean up the byproducts of ionizing radiation--as long as there's not too much of it.
Examples of ionizing radiation include X-rays, gamma rays, alpha rays and beta rays. The latter three are all produced in nuclear reactors and emitted by the substances which may be released when a nuclear reactor is damaged or explodes. An excellent, and very detailed, article about radiation can be found here.
A lot of attention has been focused in media reports on whether or not the explosions seen at the troubled reactors in Japan were "nuclear explosions" or not. (Apparently, they were not.) To me, this is irrelevant. An explosion that spreads radioactive material into the environment is a problem whether or not the source of the explosion was a thermonuclear event or a build-up of combustible gas. Nuclear explosions are much more powerful, of course, but the main reason they are so feared is because of the radioactive materials left behind afterward.
Last night, the special defense forces in Japan began an operation to flood the number 3 nuclear reactor at the Fukushima Daiishi power plant with water, and many wondered why they chose that particular reactor for their initial assault, when three others (numbers 1, 2 and 4) appear, visually, to be in much worse shape. I certainly don't know what led them to this decision, but perhaps the nature of the fuel rods inside reactor 3 had something to do with it.
Apparently (and I only know this from media reports, but I assume it is correct) the number 3 reactor uses a type of fuel rod known as MOX fuel, which stands for "mixed oxide." This name does not sound too alarming, until you read further to find that the MOX fuel rods are composed of a mixture of Uranium-238 (the usual fuel in a nuclear plant) and Plutonium-239, or Pu-230.
Pu-239 is not naturally occurring. It is produced in so-called breeder reactors by bombarding Uranium-238 with neutrons. It is used in nuclear weapons, which is why it's use in nuclear power plants has been controversial.
Pu-239 is chemically toxic, but it is also radioactive with a half life of 24,000 years. This means that half of a sample of Pu-239 will still be radioactive after 24,000 years have passed; the other half will have decayed by emitting what's known as alpha radiation.
Alpha radiation is actually a beam of helium atoms that have been stripped of their electrons, giving them a positive electrical charge. They are heavy and the energy they carry is, thus, low. Alpha rays can penetrate only 2-3 inches of air and can be stopped by a single sheet of paper or a thin layer of human skin.
The problem with alpha rays occurs when substances that emit them are breathed into the body or ingested through food and drink. Pu-239 is an alpha emitter and if it is present in particulates in the smoke coming out of a damaged reactor, anybody nearby needs to be shielded immediately. It is not clear to me how far Plutonium will travel in a "cloud" since that seems to depend on the size and nature of the particles in the smoke.
Another type of ionizing radiation produced in radioactive decay is beta radiation. These rays are actually electrons that travel with high speed. They can penetrate about an inch of water and about the same thickness of human flesh, but can be blocked by an eighth of an inch of aluminum, so special protective clothing is needed to block beta rays.
Gamma radiation is, by far, the most dangerous of all radiation produced by radioactive substances. Gamma radiation is pure energy (no particles are associated with it) and is very similar to X-rays. To block gamma radiation, 3 - 4 feet of concrete is required or, better yet, a few inches of lead. It is likely that the high radiation levels being measured near the Fukushima reactors is gamma radiation. Shielding workers from this type of radiation will be very difficult, if not impossible.
It is important to realize that gamma radiation will not travel indefinitely through air as it will be absorbed by molecules of oxygen, nitrogen and water as it moves along, so there is no danger to anybody who stays far enough away. The bigger danger comes from the radioactive nuclei that may be escaping from the reactors.
I would like to see more detailed information from the Japanese government and its electric utility TEPCO about the precise nature of the radiation they are measuring. A few bits of information are coming through about monitoring of Cesium and Iodine, but a fuller picture would help experts around the world do a better assessment of any dangers we all may be facing.