In the 24 hours since my last post, we've learned a few things:
1. The explosion that happened on Unit #1 may happen again on Unit #3. (There were 3 reactors at Daiichi operating when the quake hit: Units 1, 2, and 3.) Below is a (now famous) video of the explosion ... fast forward to the 45-second mark and you'll see it go.
Why did this happen? How could such an enormous explosion not be catastrophic to the plant? And should we panic if this can happen to another unit?
First, the why -- and an admission that my initial guess / explanation wasn't 100% accurate: When metals get really hot -- like around 1800F, far hotter than normal operation -- metals can interact with steam to create hydrogen. In the case of the 439 megawatt (electric) Unit #1 reactor, it's probably 200 to 300 kilograms of H2. In the case of the 760 megawatt (electric) Unit #3 reactor, it's probably 300 to 400 kilograms of hydrogen. The release of hydrogen causes pressure to build up very fast, and the operators had no choice but to burp the reactor, like I described before.
(My initial explanation neglected to explain the steam / metal interaction. Sorry.)
Okay, so you burp the reactor ... where does it go? Into the containment structure. See picture below.
The lower structure (called the primary containment) is a very, very robust (6 foot thick, steel reinforced walls) containment structure. When they burp the reactor, the gases initially vent to that area. But pressure can rise too high there, too, over time (above 840 kPa, as I mentioned before), and the primary containment needs to be burped ... into the secondary containment. The upper structure, perhaps the top 1/4 of the picture above, is the secondary containment, and it's just steel framing and siding. It holds the refueling crane and ... unfortunately ... the spent fuel pool.
When the pressure from the primary containment was burped into the secondary containment, the pressure and humidity dropped suddenly. With less steam in its environment, the hydrogen was free to autoignite -- which is does, happily, at about 900 degrees Fahrenheit. What we see is the top 1/4 of the containment building -- the part with steel framing -- blowing off.
Here's a picture from the NYT of what's left.
The warnings that TEPCO is giving out now are indications that the pressure has risen in Unit #3's primary containment enough that they're going to have to burp to the secondary containment ... and the hydrogen could accumulate there (without steam) to cause a second explosion. Since Unit 3 is about 50% bigger than Unit 1, the resulting explosion could be bigger. The primary containment has been designed to withstand this kind of shock.
But note one thing that I've learned in this process -- do you see those spent fuel pools in the above picture? That's where they put spent nuclear fuel after it has been in the reactor ... and they're outside the primary containment. (In hindsight, that's a very silly place to put spent fuel.) I DO NOT know if there was any spent fuel in there at the time the quake hit. I HAVE NOT seen any evidence or news reports mentioning the presence of spent fuel. But the fact that there could have been spent fuel there makes me nervous. If there was, the pools were probably emptied as part of the explosion, and without water to cool / shield the spent fuel, the risk of spreading contamination increases.
Again, I haven't seen any mention of whether there WAS or WAS NOT spent fuel in there.
2. News agencies are really trumpeting the fact that radiation was detected and that people had been tested for contamination and -- gasp! -- some levels were as high as 100,000 counts per minute. These numbers sound terrifying, but it's important to keep some perspective: the level at which you can detect radiation is far, far less than the level at which it becomes a hazard to health.
For example, here is a link to where you can purchase your very own radiation sources, and no NRC license is required. The strength of these sources? 1 microCurie, or about 2.2 million counts per second. Literally, I could buy one of these sources, cut it up and distribute it evenly to 20 of my best friends, and the level detected would still be higher than what they're finding on the general public.
The level at which you can detect radiation is far, far less than the level at which you need to be concerned for health reasons.
3. Some core damage appears to have occurred. I got a mailing from the American Nuclear Society yesterday, and it mentioned that some of the core fuel may have been damaged as part of the increased temperature and reduced water supply. The Nuclear Energy Institute is also saying that there may have been some core damage. This is unfortunate. But would somebody please tell the Japanese chief cabinet secretary to quit saying things like, "We're assuming there was a meltdown"??? Good grief; this is really feeding the trolls.
The word "meltdown" has very big implications, and instills fear and panic in the general public. With the decision to throw seawater on the core -- a basically limitless supply of coolant -- the chance of a meltdown is ridiculously small. As long as backup power and / or fuel for the generators can keep putting water in, the core will stay covered and cool.
I'm still following these sites for good info: