This cutaway diagram shows the central reactor vessel and thick concrete containment in a typical boiling water reactor of the same era as Fukushima Daiichi 1 (image: www.world-nuclear-news.org)

First, thanks much to our great commentors — Masoninblue, earlofhuntingdon, Professor Foland and many others — who continue to provide updates and expert analysis on the Japanese nuclear crisis. And hat tip to commentor “lobster” for finding a better schematic that helps illustrate the conflicting stories about a “meltdown.” [Updated to reflect suggestions for clarity.]

You can find other articles at the these links:

Explosion At Quake Damaged Fukushima Nuclear Power Plant

Facts On What A Melt Down Is

Japan Nuclear Watch – Struggling To Prevent Meltdowns

Disaster is not too strong a word for what has happened to Japan in the last three days. The biggest earthquake recorded, the subsequent massive tsunami and now the follow-on of at least two partial meltdowns of nuclear reactors at Fukushima and the critical status of three more there, and others in the nation.

Let’s start with a little vocabulary so things are as clear as they can be in this confusing situation. When I talk about the “Reactor Vessel,” I am talking about the stainless steel bottle where the fuel and control rods are housed. This also includes the entire pressurized system for generation of electricity as it is a closed single loop system.

When I talk about the “Reactor Building,” that is a concrete structure primarily designed to keep the weather off of the reactor and generator equipment. It was not designed to be another sealed radiation containment building like AT some of the plants elsewhere the in the world.

That said, let’s talk about what we do know. Reports vary but between 200,000 and 450,000 people have been ordered evacuated from the area around the Daiichi and Daini power stations. The Daiichi complex is the older of the two with 6 reactors built in the 1970’s and Daini has 4 reactors built in the 1980’s

It is the Daiichi Unit Number 1 that has experienced the explosion (more on that shortly) and has been the focus of an attempt to cool the reactor core with seawater. Reports have come in that Unit Number 3 there has also had a complete failure of the emergency cooling system and rods are exposed to a level of three meters. (more after the jump)

This is a very dire situation as some percentage of the rods (it is unclear how many) have been loaded with Plutonium instead of Uranium235. Plutonium is not only radioactive for a much longer time (a half life of 24,100 years) than uranium but is directly toxic to humans in small doses. If this reactor completely melts down and ruptures its containment vessel exposure to the resulting fallout pollution would be devastating.

Right now the Japanese officials are doing all they can to prevent this. Remember that a partial meltdown is not the same as a full blown loss of containment. The 3 Mile Island plant suffered a partial meltdown, and while radioactive steam was released, they did not lose containment of the reactor core.

Going back to Daiichi One, there has been a lot of back and forth about the hydrogen explosion yesterday. It is important to know that this type of reactor always generates hydrogen in its normal operations. In fact the hydrogen generated is used to help cool the turbines that produce the electricity. When they were performing the emergency venting to reduce the pressure on the reactor vessel, hydrogen gas would be vented along with the steam. That it built up in the generation building is probably not as surprising as you might think given that the plant had been hit by a major earthquake prior to this accident event.

That there was an explosion does not mean that the reactor vessel ruptured. The fact that the radiation levels at the plant had been falling all day yesterday tends to indicate the containment of the core had been maintained. The detection of Cesium around the plant is an indicator that fuel rods had indeed been melting and exposing the inner core of the rods. But again, those elements would be released with hydrogen and radioactive steam in the venting.

In fact the venting may have contributed to the exposure of the rods themselves. Prior to the seawater injection the loop was a closed system. When the steam at the top of the system generated pressure on the whole system, it raised the boiling point of the water, which allows operations at higher temperatures.

Once venting begins it is a race to see if you can bring the pressure of the overheated system down to safe levels without allowing the drop in pressure to flash too much of the water into a gaseous state and reduce its cooling of the core. It seems that they lost this race before they could pump in seawater.

What the authorities are trying to do now is prevent a catastrophic loss of containment at both Unit 1 and Unit 3. There is fear of an impending hydrogen explosion at Unit 3 as the same conditions that existed at Unit 1 are present there. They have been venting steam, but as I said above it is a temporary solution, at best.

Right now the New York Times is reporting that the radiation levels are once again raising at Daiichi. The level in the reactor control room is 1000 times normal. This is very dangerous for the brave technicians who are desperately working to prevent a much more massive disaster than has already occurred.

The evacuations and distribution of iodine tablets are precautionary measures. This is a good and right thing to do. Of course this is all complicated by the fact that Japan has just experienced a major natural disaster and there are tens of millions of people without power and tens of thousands in need of aid and rescue.

It is a scary and desperate time in the area around Fukushima right now. Things continue to develop and what really happened and in what order will not be fully known for some time. Right now it is all about preventing these and other reactors from melting down and having a complete loss of containment. If that happens it will be a death sentence for anyone working at the plants, though I have no doubt those still there will stay as long as they can to mitigate that catastrophe. Please keep these brave and desperate people in your thoughts.

The floor is yours.

More on this story as it becomes available.