This was at reactor number two, where the fuel rods were completely above the water line inside the containment vessel earlier today for more than two hours. How come? Partly it was due to a malfunctioning valve, but according to WaPo, the explosion at reactor number three earlier today actually damaged four of the five water pumps that were being used to cool number two. It’s only the fifth that’s still being used to try to contain the heating.

According to the Times, this blast might have been “different.”

This explosion, reported to have occurred at 6:14 a.m., happened in the “pressure suppression room” in the cooling area of the reactor and inflicted some degree of damage on the pool of water used to cool the reactor, officials of Tokyo Electric Power said. But they did not say whether or not the incident had impacted the integrity of the steel containment structure that shields the nuclear fuel.

Radiation levels around plant spiked after the explosion to 8,217 microsieverts an hour from 1,941 about 40 minutes earlier, the company said. Some emergency workers there were evacuated, though the levels would have to rise far higher to pose an immediate threat to health, officials said…

In the predawn hours of Tuesday Tokyo Electric announced that workers had finally succeeded in opening a malfunctioning valve controlling the vents, reducing pressure in the container vessel. It then resumed flooding the reactor with water.

But the company said water levels were not immediately rising to the desired level, possibly because of a leak in the containment vessel.

The LA Times claims there’s no evidence of damage thus far to the containment vessel in number two, and Kyodo News says radiation reached 8,000 micro Sievert only “temporarily” — although, for reference purposes, that’s eight times the amount you’d normally be exposed to in a year. As for what a “suppression pool” is, you can see it in the second frame of WaPo’s illustration of a boiler-water reactor. It’s a donut-shaped structure that sits below the containment vessel and uses water to relieve pressure inside the reactor by condensing the steam produced. Any nuclear technicians out there willing and able to offer input on what a damaged suppression pool might mean? Would it reduce Japan’s ability to vent steam from the reactor, or are they already doing that through other means by now?

Updates may be coming. Please stand by.

Update: If one of these reactors is destined to melt down, better that it be this one than reactor number three.

But the situation a reactor No. 3 was being closely watched for another reason. That reactor uses a special mix of nuclear fuel known as MOX fuel. MOX is considered contentious because it is made with reprocessed plutonium and uranium oxides. Any radioactive plume from that fuel would be more dangerous than ordinary nuclear fuel, experts say, because inhaling plutonium even in very small quantities is considered lethal.

Update: Abandon ship.

They initially suggested that the damage was limited and that emergency operations aimed at cooling the nuclear fuel at three stricken reactors with seawater would continue. But industry executives said that in fact the situation had spiraled out of control and that all plant workers needed to leave the plant to avoid excessive exposure to radioactive leaks.

If all workers do in fact leave the plant, the nuclear fuel in all three reactors is likely to melt down, which would lead to wholesale releases of radioactive material — by far the largest accident of its kind since the Chernobyl disaster 25 years ago…

“It’s way past Three Mile Island already,” said Frank von Hippel, a physicist and professor at Princeton. “The biggest risk now is that the core really melts down and you have a steam explosion.”

A Japanese cabinet minister confirms that part of the containment vessel itself in reactor number two has been damaged. Meanwhile, if you can believe it, a fourth reactor is now on fire and radiation levels have risen to the point where, in the vicinity of the plant, they’re actually a threat to human health. Which, of course, is another major problem since there are only so many workers with the know-how to perform the necessary tasks. Even if they’re willing to be rotated in or out, there may not be enough of them to safely staff the rotations.

And guess what? Even if a meltdown is averted — which now seems unlikely — the worst may be yet to come:

Even as workers race to prevent the radioactive cores of the damaged nuclear reactors in Japan from melting down, concerns are growing that nearby pools holding spent fuel rods could pose an even greater danger…

The threat is that the hot fuel will boil away the cooling water and catch fire, spreading radioactive materials far and wide in dangerous clouds

The pools are a worry at the stricken reactors at the Fukushima Daiichi plant because at least two of the three have lost their roofs in explosions, exposing the spent fuel pools to the atmosphere. By contrast, reactors have strong containment vessels that stand a better chance of bottling up radiation from a meltdown of the fuel in the reactor core.

It would take days or weeks for the spent rods to boil off so there’s time to deal with this problem if workers can get back into the plant. But if they can’t due to radioactivity — what then?

A nuclear engineer tells the Times that spent rods catching fire would actually be worse than a meltdown; a 1997 study says 138,000 “eventual deaths” could result within a 500-mile radius. If I’m understanding it correctly, it would be the equivalent of a giant dirty bomb going off.

Update: Looks like the fire has taken its toll on reactor number four. Kyodo News:

BREAKING NEWS: Hydrogen explosion occurs at Fukushima No. 4 reactor (11:53)

Update: A shred of good news: The fire at reactor number four is out. Why it’s out is a mystery, though. Are there still workers on the scene fighting it, or did it burn itself out? Or did the explosion somehow extinguish it?