Over the past 48 hours or so, it seems like there is a lot of misinformation and disinformation being floated regarding what's happening with the troubled nuclear plants in Japan. I'll try to clarify things a bit, in as non-technical terms as possible. By way of credentialing myself, I'm an old school, Institute for Nuclear Power Operations-certified systems engineer in GE Boiling Water Reactors (BWRs), and worked for over 20 years in the industry with a large architect engineering (A/E) firm and an electric company with a nuclear portfolio. I've worked directly at four BWR plants, including one similar to Fukushima. Ok, with that out of the way...
Disclaimer - much of what follows is simply a compilation of comments that I made earlier today. I've been tied up most of the day, and haven't been able to keep a close eye on developments.
The bottom line: take everything you read or hear about the events at the Fukushima plants with a grain of salt. There is a lot of anecdotal information that leads to some obvious conclusions, but there is very little hard data that allows even someone with my experience to fully understand what's going on. If someone on TV says the reactor core is melting out of the bottom of the reactor vessel, don't believe them. If someone on TV says there are flowers blooming and everything's peachy in the nuclear world, don't believe them, either. More after the jump.:
Q: Why isn't there much hard data?
A: There's most likely plenty of hard data - but it's not being released. TEPCO and GE have the hard data. They've had the hard data since moments after this thing started.
Q: Ok, smart guy - how bad is it?
A: Did you see the explosion of the secondary containment building (which I'll refer to from here on out as the reactor building, because that's where the reactor vessel sits)? That was not smoke from a gradually increasing fire. That was a prompt explosion - as Fishgrease might say, ka-fucking-boom. Those buildings look small in far away photos. They're not. Think of it like the pictures of shit blowing up in Baghdad when Cheney and Junior started lobbing 1000lb cruise missile warheads at buildings - but worse. The secondary containment building in a nuclear plant of Fukishima's vintage is feet thick, reinforced concrete meant to withstand an 747 crashing into the building without breaching containment. This is not hyperbole. They really are.
It's bad. Everything that's used to control the reactor and cool the reactor and shut it down in an emergency is housed within that building, as well as pools where spent fuel and new fuel are stored. And the systems that keep them cool. All that shit is gone. Ka-fucking-boom. Did you see that building fly apart?
Q: Why did the reactor building fly apart?
A: Basically, a lot of hydrogen gas collected in the building as the reactor core started overheating. For the moment, just take my word for this one - it happens. Hydrogen is explosive in the extreme (think "Hindenburg"). Under normal conditions, the reactor building is kept at a negative pressure (less than atmospheric) to keep the bad shit in the building. But with just a spark in an environment with elevated H2, ka-fucking boom. So, everything that I write past this point is predicated on the fact that there ain't anything left inside of the reactor building that's working. Not a toilet, water fountain, or High Pressure Coolant Injection pump. It's all gone.
See the left hand half of that plant, above? The walls and roof are gone or compromised. Ka-fucking-boom. See the swimming pool looking thingies right below the roof, on either side of the penis-shaped looking do-hicky? Those are the fuel pools. They keep the old and new fuel cool. Except. It's very probable that there is no water circulating through those pools anymore to keep the old (or new) fuel rods cool. And, those aren't even the operating fuel rods inside of the penis-shaped looking do-hicky (the reactor vessel, or "primary containment" as you'll hear it called by CNN, Fox, MS-NBC, etc. - think of the reactor vessel as a large pressure cooker).
Here's what I want you to get out of this - see all of the equipment around the penis looking do-hicky? It ain't there anymore, or at least it isn't working. This is not good, because this is all of the equipment that keeps the reactor core (which is inside of the reactor vessel) cool.
Q:So, I keep hearing about a meltdown? What's that all about? Why is that bad?
A: Part of what keeps a nuclear plant contained and safe is the way the fuel rods (the individual housing units for the uranium inside of the reactor vessel) are made. Normally, the rods are covered by water in the reactor vessel, and when the fuel rods heat up, the water is boiled in a controlled manner, and steam is sent to the plant turbine to make electricity. When that normal flow of water stops, usually, the nuclear reaction is automatically stopped by insertion of "control rods" into the middle of four bundled fuel rods. However, heat generation remains. And even if the plant is shut down, cooling water must continue to flow to the reactor vessel to keep the fuel rods covered. If not, fuel rods start to overheat, and start to literally melt apart. It gets pretty hot.
At least one of the Fukashima plants has apparently at least partially melted down. I won't get into the whys and wherefores right now, but there are certain "markers" (the detection of the element Cesium, for example) that would indicate fuel rod failure and meltdown.
Joanneleon has a good backgrounder here. But to answer the basic question, yeah, I do believe there's no question that at least a partial meltdown has occurred.
A: So, if you were speculating, what's the worst thing going on there right now?
Q: First, the fuel pools. No one in the media is asking about the conditions of the fuel pools. Bad stuff, toxic, toxic stuff, major releases of radioactivity possible. Second, is the plant actually shut down? I'm hearing reports of boron injection being used. This is a last resort. If the boron injection system is being used, it means the plant has experienced an Anticipated Transient Without Scram (ATWS) event - in layman's terms, one or more of the control rods did not insert when the plant shutdown was triggered during / after the earthquake. Boron (which absorbs neutrons produced by the fuel rods) is the nuclear reaction moderator of last resort in a BWR. Basically, if the boron injection system is used, the plant will never operate again under any circumstances. But again, the primary question is: is this plant actually shut down?
Alrighty, then, it's late, I'm extremely tired, and going to bed. Unfortunately, I can't stick around to answer questions right now, but will do my best to respond to some in the morning, and perhaps update with some more info. Hopefully, there are some other nuclear folks here who can field questions in my absence.