In the aftermath of Friday's earthquake, Japan continues to fight a nuclear catastrophe at its damaged Fukushima Daiichi power plant, about 150 miles northeast of Tokyo. New explosions raised the amount of radiation in the area to levels that "without a doubt would affect a person's health," says Cabinet Secretary Yukio Edano, and the government has ordered all residents within a 20-mile radius of the plants to stay indoors. What exactly is going on at Fukushima, and will the fallout be "serious or catastrophic"?

How do the Fukushima fission reactors work?
The nuclear plants are Boiling Water Reactors, in which fuel rods heat water in a thick concrete or steel container to create steam, which in turn generates power by spinning a turbine. The fuel rods — groups of metal poles filled with nuclear fuel pellets — have to be covered in circulating cool water or else they overheat and melt.

What went wrong?
"Everything that could go wrong did," says William Saletan at Slate. The earthquake triggered an automatic shutdown of the nuclear reactors, as designed, but electricity was still needed to pump cold water through the slowly cooling core. With the main power down and the backup diesel generators incapacitated by the tsunami, the last-ditch battery power lasted only eight hours. "If that sounds like a design flaw, you're right," says Maggie Koerth-Baker at BoingBoing. Newer reactors don't need pumps at all to cool the core; water circulates via gravity.

What's Japan doing to stop a total meltdown?
In Fukushima Daiichi's reactors No. 1, 2, and 3, workers have used outside power sources or fire hoses to pump in seawater mixed with boric acid to cool the nuclear core. This is a last-ditch maneuver, as seawater will render the nuclear reactors inoperable. The water has caused dangerous steam buildups inside the core, and to prevent a bigger blowup, engineers have vented some of the mildly radioactive steam into the air.

Is it working?
Not entirely. Japanese officials say the fuel rods in all three reactors have partially melted. If the rods completely melt, the worst-case scenario is that molten rods eat through the floor and contaminate the environment. In reactor No. 2, officials say an explosion may have already breached the inner containment capsule, which would increase the amount of radiation leaked.

What's causing the explosions?
Flammable hydrogen gas vented from the containment capsule, which ignited and blew the roofs off the buildings protecting the nuclear cores. The core capsules have held intact in reactors No. 1 and No 3. However, a suspected hydrogen explosion at reactor No. 4 sparked a now-extinguished fire that could be releasing dangerous levels of radioactivity directly into the air.

So is this another Chernobyl?
Not yet, but "it's at least as bad as Three Mile Island," the 1979 partial-core meltdown in Pennsylvania, says nuclear physicist Frank von Hippel. Things could get as bad as the 1986 Chernobyl disaster in Ukraine — the worst nuclear accident to date — if the nuclear core melts through the reactor capsule. Nonsense, says William Tucker at The Wall Street Journal. Nothing about the Japanese problem "amounts to 'another Chernobyl.'" The Ukrainian plant had two "crucial design flaws" that have been fixed on the later Fukushima reactors. Even if there was a Chernobyl-sized meltdown, says Roddy Campbell at The Telegraph, the damage from that disaster is "far less bad than people thought and indeed still think."

So what's the best we can hope for?
That minimum amounts of radiation leak out and are blown over the Pacific Ocean, while the Japanese crews manage to keep the cores submerged until either power is restored or the fuel rods cool off enough to not be a threat. In that case, Japan would only have to deal with a years-long, costly cleanup effort, not a spike in cancer and a miles-wide toxic wasteland.

Sources: LA Times, BBC News, MSNBC, BoingBoing, Wall Street Journal, AOL News, Slate, Economist, Washington Post, Telegraph