How does a nuclear reactor work?
Most nuclear reactors, including those at Japan's Fukushima Daiichi generating station, are essentially high-tech kettles that efficiently boil water to produce electricity. They rely on harnessing nuclear fission—the splitting of an atom into two smaller atoms, which also yields heat and sends neutrons flying. If another atom absorbs one of those neutrons, the atom becomes unstable and undergoes fission itself, releasing more heat and more neutrons. The chain reaction becomes self-sustaining, producing a steady supply of heat to boil water, drive steam turbines and thereby generate electricity.

How much electricity does nuclear power provide in Japan and elsewhere?
With 54 nuclear reactors generating 280 billion kilowatt-hours annually, Japan is the world's third-largest producer of nuclear power, after the U.S. and France, according to data from the International Atomic Energy Agency. The Fukushima Daiichi station, which has been hit hard by the March 11 earthquake, houses six of those reactors, all of which came online in the 1970s.

Worldwide, nuclear energy accounts for about 15 percent of electricity generation; Japan gets nearly 30 percent of its electricity from its nuclear plants. The U.S. produces more nuclear power overall, but nuclear constitutes a smaller share of its energy portfolio. About 20 percent of U.S. electricity comes from nuclear power plants, making it the third-largest source of electricity in the country after coal (45 percent) and natural gas (23 percent).

What fuels a nuclear reactor?
Most nuclear reactors use uranium fuel that has been "enriched" in uranium 235, an isotope of uranium that fissions readily. (Isotopes are variants of elements with different atomic masses.) Uranium 238 is much more common in nature than uranium 235 but does not fission well, so fuel manufacturers boost the uranium 235 content to a few percent, which is enough to maintain a continuous fission reaction and generate electricity. Enriched uranium is manufactured into fuel rods that are encased in metal cladding made of alloys such as zirconium.

Reactor No. 3 at the Fukushima Daiichi station runs on so-called mixed oxide (MOX) fuel, in which uranium is mixed with other fissile materials such as plutonium from spent reactor fuel or from decommissioned nuclear weapons.

How do you turn off a nuclear reaction?
Sustained nuclear fission reactions rely on the passing of neutrons from one atom to another—the neutrons released in one atom's fissioning trigger the fissioning of the next atom. The way to cut off a fission chain reaction, then, is to intercept the neutrons. Nuclear reactors utilize control rods made from elements such as cadmium, boron or hafnium, all of which are efficient neutron absorbers. When the reactor malfunctions or when operators need to shut off the reactor for any other reason technicians can remotely plunge control rods into the reactor core to soak up neutrons and shut down the nuclear reaction.

Can a reactor melt down once the nuclear reaction is stopped?
Even after the control rods have done their job and arrested the fission reaction the fuel rods retain a great deal of heat. What is more, the uranium atoms that have already split in two produce radioactive by-products that themselves give off a great deal of heat. So the reactor core continues to produce heat in the absence of fissioning.

If the rest of the reactor is operating normally, pumps will continue to circulate coolant (usually water) to carry away the reactor core's heat. In Japan the March 11 earthquake and tsunami caused blackouts that cut off the externally sourced AC power for the reactors' cooling system. According to published reports, backup diesel generators at the power plant failed shortly thereafter, leaving the reactors uncooled and in serious danger of overheating.

Without a steady coolant supply, a hot reactor core will continuously boil off the water surrounding it until the fuel is no longer immersed. If fuel rods remain uncovered, they may begin to melt, and hot, radioactive fuel can pool at the bottom of the vessel containing the reactor. In a worst-case meltdown scenario the puddle of hot fuel could melt through the steel containment vessel and through subsequent barriers meant to contain the nuclear material, exposing massive quantities of radioactivity to the outside world.

How can a meltdown be averted?
The Japanese plant's operators have made a number of attempts to cool the reactors, including pumping seawater into the reactor core to replenish the dwindling cooling fluid. The Tokyo Electric Power Company has also injected boric acid, an absorber of neutrons, into the reactors.

How does this incident compare with Chernobyl or Three Mile Island?
At present, three of the reactors at Fukushima Daiichi station are seriously crippled. Units 1 and 3 have experienced explosions that destroyed exterior walls, apparently from buildups of hydrogen gas produced by the zirconium in the fuel rods reacting with coolant water at extremely high temperatures—but the interior containment vessels there thus far seem to be intact. A third explosion was reported March 15 at reactor No. 2, and the situation there appears direr. Pressure in the suppression pool—a doughnut-shaped water vessel below the reactor—dropped after the explosion, indicating that the containment vessel had been compromised.

In reactor Nos. 1, 2 and 3 water levels dropped enough to leave the fuel assemblies temporarily uncovered; those fuel rods are presumed to have suffered damage. And a fire at a pool storing spent fuel rods at dormant reactor No. 4 is posing additional hazards to the few workers remaining at the site.

Japanese officials initially rated the incident a level 4, an "accident with local consequences," on the seven-tier International Nuclear and Radiological Event Scale (INES), but Princeton University physicist Frank von Hippel told The New York Times that the Fukushima Daiichi situation is "way past Three Mile Island already." Three Mile Island, the highest-profile U.S. nuclear accident, was classified level 5—an "accident with wider consequences".

At that Pennsylvania nuclear station in 1979 a cooling malfunction combined with worker error led to a partial meltdown—about half of the reactor core melted and formed a radioactive puddle at the bottom of the steel pressure vessel. The vessel remained intact, but some radiation did escape from the plant into the surrounding environment.

The 1986 Chernobyl accident was far more devastating; it rates as a 7, or a "major accident," on the INES scale. In Ukraine, then part of the Soviet Union, a power surge caused an explosion in one of the plant's reactors, releasing huge doses of radioactive fallout into the air. Two plant workers died within hours, according to the U.S. Nuclear Regulatory Commission; 28 more died in the following months from radiation poisoning. The fallout from Chernobyl was widespread, and the health effects of the disaster are difficult to quantify. A report from the United Nations Scientific Committee on the Effects of Atomic Radiation found that 6,000 individuals who were under the age of 18 in Ukraine, Belarus or Russia at the time of the disaster had by 2006 contracted thyroid cancer, "a substantial fraction" of whom likely contracted the disease due to radiation exposure.