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)

Things continue to develop at the Daiichi and Daini nuclear power stations in Japan today. As you have already heard, a second reactor building exploded at Daiichi, it was the Unit 3 which had been experiencing the same problems as Unit 1, which also exploded.

What is believed to have happened is that hydrogen gas which was released along with the steam to reduce the pressure in the reactor vessel built up in the generation building and then a random spark ignited it. The building is seen to have the same kinds of damage that the Unit 1 building had.

Additionally Unit 2 has been reported to have exposed the rods and is in the process of at least a partial melt down at this time. What does this mean? It is likely that they will be experiencing the same pressure and hydrogen problems that Units 1 and 3 have had, with a similar danger of hydrogen explosion.

There has been a lot of talk that the reactor vessel (the actual structure of the reactor) might have been ruptured by these explosions because of the detection of Cesium (a fission by product). The main reason I find this unlikely at this time is that if the reactor vessel had failed two things would have happened.

The first is that the pressure within the vessel would have fallen to normal atmospheric pressure, this would have flashed most of the water in the vessel to steam by lowering the boiling point of that water. The second is that there would have been a massive increase in the amount of radiation detected.

While there have been reports of radiation detected at long distances (60 miles) we have to remember that venting of radioactive steam has been going on for three days now and that it could have traveled. We also are not being told how high this increase in radioactivity is. That is an important metric as very small increases can be detected.

We are now getting reports through the BBC that fuel rods in all three of these reactors are melting. The issue here is the way that they have been adding sea water to the reactors. It is being done through a system of fire hoses. They are using diesel powered pumps and the New York Times quotes and official as saying:

“The pump ran out of fuel,” Mr. Edano said, “and the process of inserting water took longer than expected, so the fuel rods were likely exposed from the water for a while.”


Note the uncertainty in Mr. Edano’s statement. This is another factor in the trouble with these reactors. The monitoring systems which they rely on are no longer as accurate as they were.

They can not just fill the reactor to the brim, because if they do, the steam generated from the still very hot rods would cause pressure to build up to the point where the reactor vessel ruptured. They have to add water, let it boil, then vent steam, then add more water.

When the steam vents the overall water level goes down, which risks exposing the core. It is also a problem when you can not vent steam. As more water becomes steam it lessens the cooling efficiency at the same time increasing the pressure.

The CBC is reporting that Unit 2’s problems are related to a stuck venting valve. If they can not release steam then they can not control the level of water or the pressure within the reactor.

Worse, and this has been a problem (we think) for all three reactors, every time part of the core is exposed there is additional heating of the whole reactor. While the control rods damp down the fission reaction, the water is there to help as well. When there is even a partial meltdown this changes the efficiency of the rod configuration. The water becomes even more important.

If it drops that moderating affect goes away, and small amounts of fission begin with the resulting heat that this reaction produces. So when they drop the water level too low, it sets back the overall cooling process. Under normal circumstances it can take days for a core to cool off and as we all know this is far from normal.

The nightmare scenario is that one of the reactor vessels ruptures and the core goes into a fully uncontrolled melt down. There is a containment vessel around the reactor, and the Japanese officials have been flooding this with water as well, but if the core fully melts leaves the reactor vessel there is almost nothing that can be done. The melted rods would be reacting without moderation and would likely rupture the containment vessel in short order.

At this point the whole plant would have to be abandoned. It would be lethal in short order for anyone there. What makes this triply as bad is that they could not continue to work to prevent the same thing at the other two reactors that are in trouble or the other three that are also at the site.

Worst case, all six of the Daiichi reactors melt down. The radiation levels from this kind of event would be off the chart. It would be as bad as Chernobyl but not as wide spread. Part of the reason that Chernobyl was so bad was that it burned, this lifted radioactive graphite and fission products up to the upper atmosphere. That is unlikely to happen here.

Things continue to develop and we will continue to provide as much detail as we can over the next hours and days. Keep your fingers crossed. As bad as things are they can get a lot worse, they have not thanks to the heroic efforts of the Japanese technicians. They are working with very little power and without their usual level of information to prevent a cataclysm.

The floor is yours