Can’t find anything on the wires yet, but news is breaking all over Twitter and at Kyodo News. The headline:

BREAKING NEWS: Fukushima nuke plant might be experiencing nuclear meltdown

And:

BREAKING NEWS: Radioactive Cesium detected near Fukushima plant: nuke safety commission

Cesium would be the signature of a true meltdown. There’s no danger to civilians so long as the containment dome holds. If it doesn’t hold, oh boy.

Another ominous note via Twitter:

Four other Fukushima nuke reactors are struggling with similar problem. If multiple meltdown begins, it will be uncontrollable.

Looking for further details at news sites. Stand by for updates.

Update: Here’s a bare-bones report at Kyodo News repeating the bits quoted above.

Update: More from Al Jazeera:

“The events that occurred at these plants, which is the loss of both offsite power and onsite power, is one of the rarest events to happen in a nuclear power plant, and all indications are that the Japanese do not have the situation under control,” “Edwin Lyman, a nuclear expert at the Union of Concerned Scientists, a US-based nonprofit organisation, said…

However, Naoto Sekimura, a professor at the University of Tokyo, said a major radioactive disaster was unlikely.

“No Chernobyl is possible at a light water reactor. Loss of coolant means a temperature rise, but it also will stop the reaction,” he said.

“Even in the worst-case scenario, that would mean some radioactive leakage and equipment damage, but not an explosion. If venting is done carefully, there will be little leakage. Certainly not beyond the 3 km radius.”

Update: They’ve reached the point where they’re pouring water on the reactor out of a fire truck to try to reduce the temperature.

Parts of the reactor’s nuclear fuel rods were briefly exposed to the air after cooling water levels dropped through evaporation, and a fire engine was pumping water into the reactor, Jiji Press reported. The water levels are recovering, said operator Tokyo Electric Power, according to Jiji.

A TEPCO spokesman told AFP that ‘we believe the reactor is not melting down or cracking. We are trying to raise the water level.’ Kyodo News agency moments later said radioactive caesium had been detected near Fukushima plant, citing the nuclear safety commission.

Update: Another Japanese official tells the AP that even if a meltdown occurs, it shouldn’t affect anyone beyond a six-mile radius — and most of those people either have been evacuated or, no doubt, are in the process. All eyes now are on the containment dome.

Update: The WSJ confirms that people within a six-mile (i.e. 10-kilometer) radius were already being shipped out hours ago. Also, an update on the control room, where radiation levels reached 1,000 times the normal rate earlier:

Radiation levels aren’t supposed to rise in a control room, which is designed to allow operators to continue working during emergencies and is equipped with filtration systems and other design features to protect workers from radiation exposure. Nevertheless, experts said that a level that is 1,000 times normal probably isn’t immediately harmful.

The technicians on the scene battling this thing must be signing years of their lives away, if not decades.

Update: Finally, some good news from Kyodo News:

BREAKING NEWS: Pressure successfully released from Fukushima No. 1 reactor: agency (15:31)

That won’t stop a meltdown but it might prevent a fracture in the containment dome, which is now the last line of defense.

Update: Here’s some nice news to wake up to:

A huge explosion has rocked a Japanese nuclear power plant damaged by Friday’s devastating earthquake.

A pall of smoke was seen coming from the plant at Fukushima. Four workers were injured…

The Japanese government’s chief spokesman, Yukio Edano, said the concrete building housing the plant’s number one reactor had collapsed but the metal reactor container inside was not damaged.

He said radiation levels around the plant had fallen after the explosion.

Why would radiation levels around the plant fall because of an explosion?

Update: People within a 12-mile radius have now been evacuated. The core hasn’t completely melted down, Japanese officials say, and new cooling efforts are underway:

Tokyo Electric Power, which operates the plant, which is located 160 miles north of Tokyo, now plans to fill the reactor with sea water to cool it down and reduce pressure. The process would take five to 10 hours, Mr. Edano said, expressing confidence that the operation could “prevent criticality.”…

Japanese nuclear safety officials and international experts said that because of crucial design differences the release of radiation at the Fukushima plant would likely be much smaller than at Chernobyl even if the Fukushima plant has a complete core meltdown, which they said it had not.

As for why the concrete roof on the building would blow when they’d already released some gas to lower the pressure inside:

Those releases apparently did not prevent the buildup of hydrogen inside the reactor [Update: NYT error, see update below], which ignited and exploded Saturday afternoon, government officials said. They said the explosion itself probably did not result in dramatic increases in the amount of radioactive material being released into the atmosphere, but they expanded the evacuation area around the Daiichi plant from a six-mile radius to a 12-mile radius.

They’ve moved from battery power to the cooling system back to generators, which is a hopeful sign, but they still need to vent some gas in the containment dome to keep the pressure stable.

Update: Amazing video of the explosion via rdbrewer. Skip ahead to 50 seconds in for the blast. Even more amazing, given the force, is that the containment dome is still intact.

Update: A commenter at Ace’s site translates an article at Japan’s Asahi news site: “Hydrogen built up in a storage vessel and exploded. It was not the reactor.” I haven’t seen anyone else report that, and it contradicts the excerpt from the NYT quoted above. It’s also unclear why pressure would be building at the plant outside the reactor. Presumably it’s a byproduct of quake damage. If you come across an English-language news site confirming that it was a storage vessel, please tip us.

Update: Here’s a reassuring Reuters round-up of thoughts from several nuclear experts. Uniformly, they’re skeptical that this will turn into a major disaster. A quote from an official at the Chernobyl Nuclear Safety Center:

“The explosion at No. 1 generating set of the Fukushima nuclear plant in Japan, which took place today, will not be a repetition of the Chernobyl nuclear disaster,” Interfax quoted the Ukrainian expert as saying.

He said that the Japanese nuclear power plants use reactors of a totally different design to Chernobyl’s.

“Japan has modern-type reactors. All fission products should be isolated by the confinement (the reactor’s protection shell). Only gas emission is possible.”

Another expert speculates that the explosion might have been caused by the coolant overheating and turning to steam more rapidly than expected.

Update: Rod Adams, an activist who supports nuclear energy and a former operator at a light-water nuclear plant, also argues that the fears here are way overblown:

At [Three Mile Island], the widely predicted and discussed “China Syndrome” did not happen, even though 20-30% of the core melted and slumped to the bottom of the pressure vessel. That melted corium froze again once it contacted the thick metal walls – the maximum measured penetration was just 5/8ths of an inch. Anyone who has ever watched as welder employs a torch to cut through a thick steel wall will understand just how much concentrated power it takes to melt several inches of steel. Avoiding the China Syndrome was not a matter of luck – the scenario is imaginary and only works in fiction. Physics and material science make it impossible….

Radiation levels inside the containment will be many times higher than usual, but that is okay because no one needs routine access inside containment buildings and no humans will be over exposed. The containment walls, reactor coolant piping, and other equipment inside the containment building will condense and capture much of the radioactive materials that are entrained in the water. Other than those vented noble gases mentioned above, essentially nothing will be released to the environment.

His point about the strength of the containment dome is well taken, but the X factor here is the mega-quake. What happens to the structural integrity of a dome when it gets hit with a 9.1 tremor and dozens of giant aftershocks?

Update: Speaking of which, the aftershocks are still coming. The latest to hit the plant — after the explosion — was a 6.4.

Update: Contrary to the “calm down” message being pushed by nuclear experts this morning, Stratfor’s analysis of the situation at the plant is frightening:

However, the earthquake in Japan, in addition to damaging the ability of the control rods to regulate the fuel — and the reactor’s coolant system — appears to have damaged the containment facility, and the explosion almost certainly did. There have been reports of “white smoke,” perhaps burning concrete, coming from the scene of the explosion, indicating a containment breach and the almost certain escape of significant amounts of radiation…

And so now the question is simple: Did the floor of the containment vessel crack? If not, the situation can still be salvaged by somehow re-containing the nuclear core. But if the floor has cracked, it is highly likely that the melting fuel will burn through the floor of the containment system and enter the ground. This has never happened before but has always been the nightmare scenario for a nuclear power event — in this scenario, containment goes from being merely dangerous, time consuming and expensive to nearly impossible.

Follow the link for harrowing data on radiation exposure. That was written a few hours ago, but it seems to be behind the news curve. Japanese officials insist that the containment dome around the reactor core hasn’t been breached; in fact, they claim, radiation levels around the plant are dropping, not increasing. That’s not to say things couldn’t change — see my point above about aftershocks — but for now it looks like the nightmare scenario has been averted.

Update: NPR says that flooding the reactor core with sea water will effectively destroy the plant, but will (hopefully) prevent a meltdown. And if it does melt down? “[T]he only thing left to do will be to ‘seal it up with concrete. You sort of entomb it.'”

Update: Japanese officials keep insisting that the radiation leak thus far has been small, but three people in the area — randomly selected from a group of 90 — have already tested positive for radiation poisoning.

Update: A reader e-mails to point out that the Times has now changed the passage I quoted above about the explosion being caused by hydrogen building up inside “the reactor.” It now reads, “Those releases apparently did not prevent the buildup of hydrogen inside the plant, which ignited and exploded Saturday afternoon, government officials said.” Hence the confusion between the NYT report and the Asahi report on what caused the explosion.

Speaking of which, more details from the Times on what happened:

David Lochbaum, who worked at three reactors in the United States similar to the Fukushima design, and who was later hired by the Nuclear Regulatory Commission to teach its personnel about that technology, said that from pictures he had seen of the stricken plant, the explosion appeared to have occurred in the turbine hall, and not the reactor vessel or the containment that surrounds the vessel.

The technology used at Fukushima is called a boiling-water reactor, in which the reactor, inside a containment, sends its steam out of containment to a turbine. The turbine converts the steam’s energy into rotary motion, which turns a generator and makes electricity.

But as the water goes through the reactor, some water molecules break up into hydrogen and oxygen. A system in the turbine hall usually scrubs out those gases. Hydrogen is also used in the turbine hall to cool the electric generator. Hydrogen from both sources has sometimes escaped and exploded, he said, but in this case, there is an additional source of hydrogen: interaction of steam with the metal of the fuel rods. Operators may have vented that hydrogen into the turbine hall.

Update: The Guardian thinks the hour of crisis has passed but notes that “the Japanese nuclear industry has a bad reputation for owning up to accidents and many observers remain cautious about accepting these claims [that things are under control] too quickly.”

Update: I’m now completely confused. As noted two updates ago, the going theory is that it was a turbine in the plant, not anything happening inside the reactor, that had exploded and blasted the walls off of the building housing the containment dome. But now comes this, which is being headlined by Drudge:

The Nuclear and Industrial Safety Agency (NISA) said Saturday afternoon the explosion at the Fukushima No. 1 nuclear plant could only have been caused by a meltdown of the reactor core.

The Bellum blog at the Stanford Review notes that no one else is reporting this and that NISA’s latest statement on the situation at the plant says nothing about a meltdown. What we’re seeing here, I think, is a perfect storm of journalistic confusion: The facts on the ground are changing rapidly, so some information is constantly outdated; the details of nuclear engineering are obscure to laypeople, so reporters aren’t sure what to make of new developments; and Japanese officials may themselves be lying about how bad the situations, either to keep people calm or to cover their own asses while they can. According to the Times, they’re now saying that “a major meltdown is no longer imminent.” True? False? Somewhere in between? Exercise caution going forward. In fact, here’s a worthy read from Sci Am as we veer from calm to hysteria and back again: “Beware the fear of nuclear … fear.” Quote:

We know from studying the survivors of [the Hiroshima and Nagasaki] bombings, who were bathed in horrific doses of high level radiation – far worse than anything that could come from the Daiichi plant (or that came out of Chernobyl) – that ionizing radiation from nuclear energy is a carcinogen, but a relatively weak one. The roughly 100,000 survivors of the two atomic bomb blasts are known in Japan as hibakusha, and they are honored, and given special rights…

Based on studies of atomic bomb survivors, the World Health organization estimates the maximum lifetime death toll from cancer due to radiation exposure from Chernobyl, of roughly 800,000 people, will be about 4,000.

And what about environmental damage? A huge area around Chernobyl is off limits to humans for hundreds of years. But that’s to limit human exposure to ionizing radiation which, while dangerous, is less so than many of us presume. With people removed, wildlife in those areas is thriving.

Update: The number one reactor at Fukushima isn’t the only reactor at the plant that’s in trouble. Reuters reports that they’ve now lost their emergency cooling system at reactor number three.

Update: At 5:48 p.m. on Saturday, CNN reports that one of the reactors might be melting down:

A meltdown may be under way at one of Fukushima Daiichi’s nuclear power reactors, an official with Japan’s nuclear and industrial safety agency told CNN Sunday.

A meltdown is a catastrophic failure of the reactor core, with a potential for widespread radiation release. However, Toshiro Bannai, director of the agency’s international affairs office, expressed confidence that efforts to control the crisis would prove successful.

A spokesman for NISA told reporters that as long as they can continue pumping sea water in to the containment vessel (which one nuclear expert describes as a “Hail Mary pass”), the situation shouldn’t get any worse. And yet … it sounds like it’s getting worse. Maybe the meltdown is happening in reactor number three, which is also in a state of emergency right now?

Update: And with this, I’m officially ready to give up on this story. CNN now quotes Japan’s ambassador to the U.S. as saying there’s no evidence of a meltdown in progress:

“We are now trying to cope with the situation by putting salt water into the reactor,” he said. “There are some other issues with other reactors as well, which need also injection of water or taking out vapor because of increasing pressure into the container and we are now working on it.”…

Engineers have been unable to get close enough to the core to know what’s going on, an official with Japan’s nuclear and industrial safety agency told CNN Sunday. He based his conclusion on the fact that they measured radioactive cesium and radioactive iodine in the air Saturday night.

So after hours and hours of blogging this thing, there’s the verdict: No one knows what’s going on. Terrific.

Update: A reader passes along the link to this live feed — in English — of Japan’s national public broadcasting network, NHK. That’s probably your best bet for news on the reactor and other quake fallout in Japan.