This damage has released some radioactive materials, and there are widespread fears about the health effects of current and possible future releases. Travel time: The longer radioactive material is in the air, the more of it will decay. Rain and snow: Precipitation washes some particles out of the air. A press report of March 22 stated that equipment in Charlottesville, VA, detected radiation from the reactor incident, but that "health experts said that the plume's radiation had been diluted enormously in its journey of thousands of miles and thatâat least for now, with concentrations so lowâits presence will have no health consequences in the United States." What Is Radiation?9

Many atoms are stable: they will remain in their current form indefinitely. Some atoms are unstable, or radioactive. They have no electrical charge and may be highly penetrating, depending on their speed. For example, when uranium-235 decays, it emits gamma rays, most of which are of 186 keV (a low energy) or less, and alpha particles; cesium-137 emits gamma rays, virtually all of which are of 662 keV, a medium energy, and beta particles. Uranium-235 and plutonium-239 can support a nuclear chain reaction: to oversimplify, one neutron fissions one atom, which releases two neutrons that fission two atoms, releasing four neutrons that fission four atoms, and so on. A nuclear reactor cannot explode like an atomic bomb because the fuels and configurations differ. In addition, a bomb must be configured in one of two ways to create a large enough mass to support a runaway chain reaction; reactors are arranged in an entirely different configuration. A nuclear reactor uses pellets of LEU or mixed oxides (MOX, i.e., uranium oxide and plutonium oxide) for fuel. If the crusts are thick enough, they can block water from circulating between the fuel rods. As the rods heat up, their zirconium cladding can ignite, which may cause the uranium inside to melt and release radioactive material. When the ratio of fission products to fissile material rises to the point at which a fuel rod can no longer efficiently maintain a chain reaction, it is referred to as spent fuel. When fuel rods are first removed from a nuclear reactor, they have a high level of short-lived radionuclides, unlike new fuel rods, so they are intensely radioactive. This radioactivity generates intense heat, so spent fuel rods are placed in pools of water to cool them, typically for several years, until most of the short-lived radionuclides decay. This possibility led to concern about the spent fuel rods at Fukushima Daiichi NPP reactor 4:
The spent fuel pools can be even more dangerous than the active fuel rods, as they are not contained in thick steel containers like the reactor core. This explains the concern about that reactor, as it is the only one that uses MOX fuel, although irradiation of uranium fuel also creates plutonium. Water is being pumped into the spent fuel pools at the Fukushima Daiichi NPP reactors as well to cool the fuel rods and prevent additional radiation release. The biological impact of ionizing radiation, however, depends not just on the absorbed dose (i.e., the amount of energy absorbed) but on the type of radiation. Iodine-131 posed the most important health risk following the incident at the Chernobyl nuclear power plant in 1986. As a result, potassium iodide tablets, taken shortly before exposure to iodine-131, offer protection from thyroid cancer. As the next section of this report discusses, the amount of radioactive material that has reached the United States from the Japanese nuclear reactor incident is minuscule. http://www3.nhk.or.jp/ daily/ english/ 01_33.html 
Nuclear and Industrial Safety Agency
"Countermeasures for TohokuâPacific Ocean Earthquake": includes updates of information on radiation levels http://www.nisa.meti.go.jp/ english/ 
Prime Minister of Japan and His Cabinet
"The Prime Minister in Action": includes messages, statements, and a blog by the Prime Minister http://www.kantei.go.jp/ foreign/ index-e.html 
Tokyo Electric Power Company
Home page of the company that owns the Fukushima Daiichi nuclear power plant: includes press releases and monitoring data http://www.tepco.co.jp/ en/ index-e.html 
Other
American Nuclear Society
"Fukushima": Links to many news sources, updates on reactor status, an archive of updates and news http://ansnuclearcafe.org/ fukushima/ 
Areva
"The Fukushima Daiichi Incident," by Matthias Braun: provides a detailed description, with photos and diagrams, of plant design, accident progression, radiological releases, spent fuel pools, and sources of information http://www.seyth.com/ ressources/ quake/ AREVA-Document.pdf 
Health Physics Society
"Fukushima Nuclear Plant Update": health aspects of radiation exposure in general and from the Fukushima Daiichi incident http://www.hps.org/ fukushima/ 
International Atomic Energy Agency
"Fukushima Nuclear Accident Update Log": daily detailed updates of the situation http://www.iaea.org/ newscenter/ news/ tsunamiupdate01.html 
New York Times
"Status of the Nuclear Reactors at the Fukushima Daiichi Power Plant": for each reactor, daily updates of the situation along with recent developments http://www.nytimes.com/ interactive/ 2011/ 03/ 16/ world/ asia/ reactors-status.html?
Japan's nuclear incident has engendered much public and congressional concern about the possible impact of radiation on the Japanese public, as well as possible fallout on U.S. citizens. This report provides information on technical aspects of the nuclear incident, with reference to human health.
While some radioactive material from the Japanese incident may reach the United States, it appears most unlikely that this material will result in harmful levels of radiation. In traveling thousands of miles between the two countries, some radioactive material will decay, rain will wash some out of the air, and its concentration will diminish as it disperses.
Many atoms are stable; they remain in their current form indefinitely. Other atoms are unstable, or radioactive. They "decay" or "disintegrate," emitting energy through various forms of radiation. Each form has its own characteristics and potential for human health effects.
Nuclear reactors use uranium or mixed oxides (uranium oxide and plutonium oxide, or MOX) for fuel. Uranium and plutonium atoms fission, or split, releasing neutrons that cause additional fissions in a chain reaction, and also releasing energy. A nuclear reactor's core consists of fuel rods made of uranium or MOX encased in zirconium, and neutron-absorbing control rods that are removed or inserted to start or stop the chain reaction. This assembly is placed underwater to carry off excess heat. The incident at the Fukushima Daiichi Nuclear Power Plant prevented water from circulating in the core of several reactors, causing water to evaporate and temperature to rise. High heat could melt the fuel rods and lead to a release of radioactive material into the air.
When uranium and plutonium fission, they split into smaller atoms that are highly radioactive and generate much heat; indeed, fuel rods that have just been removed from a reactor are much more radioactive, and hotter, than fuel rods before they have been inserted into a reactor. After fuel rods can no longer efficiently produce energy, they are considered "spent" and are placed in cooling pools of water for several years to keep them from overheating while the most radioactive materials decay. A concern about the spent fuel pool at reactor 4 is that it may have lost most or all of its water, yet it has more fuel rods than pools at the other five reactors, as it contains all the active fuel rods that were temporarily removed from the reactor core in November 2010 to permit plant maintenance in addition to spent fuel rods.
A nuclear reactor cannot explode like an atomic bomb because the concentration of the type of uranium or plutonium that fissions easily is too low to support a runaway chain reaction, and a nuclear weapon requires one of two configurations, neither of which is present in a reactor.
Some types of radiation have enough energy to knock electrons off atoms, creating "ions" that are electrically charged and highly reactive. Ionizing radiation is thus harmful to living cells. It strikes people constantly, but in doses low enough to have negligible effect. A concern about the reactor incident is that it will release radioactive materials that pose a danger to human health. For example, cesium-137 emits gamma rays powerful enough to penetrate the body and damage cells. Ingesting iodine-131 increases the risk of thyroid cancer. Potassium iodide tablets protect the thyroid, but there is no need to take them absent an expectation of ingesting iodine-131.
This update adds a section on useful links.