Physics graduate student
University of Illinois
In the aftermath of the devastating earthquake and tsunami in Japan, the world is watching an unfolding crisis in Japan’s Fukushima nuclear reactors. Below is an attempt to explain the science behind the crisis to the best of our knowledge. Not a nuclear expert myself, I asked my son, a Physics graduate student at the University of Illinois. Yesterday he attended a seminar on campus where professors explained what we currently know about the situation. Here’s his report.
In order to understand what's going on, you need a good image of what the reactor looks like. Here is a good picture: http://1.bp.blogspot.com/-YCQGP12Grn0/TXwmsKea0YI/AAAAAAAAAp8/Gc3-w9VHPcI/s1600/BoilingWaterReactorDesign_3.jpg.
This type of reactor is called a boiling water reactor. The reddish cylinder in the middle is the reactor core. Inside are fuel rods, which are zirconium clad tubes filled with uranium pellets. The fuel rods are surrounded with water being pumped in from outside. The water serves two purposes in this type of reactor: it is a coolant, and it slows down neutrons so that they are more easily absorbed by the uranium (it is called a moderator in this context). The problem at the Fukushima reactors occurred when power was lost. When power is lost, the reactor automatically shuts down (control rods are inserted, which stops the sustained reaction). Water has to be actively pumped into the reactor to cool it. Without power, the water circulation stops. The Fukushima reactors had multiple diesel generators on-hand, but the tsunami appears to have knocked all of them out. They had backup battery power, but it didn't last long. Once power was lost, the water in the reactor just sat there and began to boil off. The core remains hot long after the reaction has stopped because the fuel still contains a lot of radioactive material which will continue to decay. That is the main issue right now. If they could guarantee a constant safe water level in all of the reactor cores, the problem would be solved. They are currently pumping in seawater (which will ruin the reactors, but they aren't worried about that), but this is difficult because the core is under such high pressure. They need very strong pumps to get water in.
Now, the big news lately has been these explosions. To the best of anyone's knowledge, these were hydrogen explosions. It is not at all obvious where the hydrogen comes from, since it is not a decay product. It comes from the fact that steam can react with the fuel rods' zirconium cladding to create zirconium oxide (in effect, it rusts). Steam normally does not come in contact with the zirconium but it did in this case because the water level sank below the tops of the fuel rods (this is what the media means when they say the fuel rods were "exposed"). As oxygen is leached off of the steam molecules in the formation of zirconium oxide, hydrogen is produced. At some point, the pressure of the steam became dangerously high and had to be vented. It was vented out of the core and then out of the primary containment vessel (labeled in the diagram) into the secondary containment vessel. This is where the explosions occurred. Interestingly, the steel plates surrounding the secondary containment vessel are "meant" to be blown off in the case of an explosion. If they were on too tightly, an explosion might damage the primary containment vessel, and that would be bad. So that's exactly what happened in all of the explosions. You can see the steel skeleton in pictures of the exploded reactors.
Now, one major concern at this point is the spent fuel ponds. You can see one of these in the image; it is the pool of water just to the right of the core containing rectangular slabs. These slabs are spent fuel. The spent fuel is put in this pool to cool off after most of its uranium-235 has reacted. It stays hot for a long time, though, so it is kept in the pool until it is cool enough to move to a secure location. After the explosions, you can see that the spent fuel pond is exposed to the outside. That's fine as long as there is water in there to absorb/block radiation. The fear is that either a) the water will boil off, or b) the pools were damaged in the explosions (or the fire a couple days ago) and could be leaking. The more worrisome scenario is situation b. If the water level drops enough, the spent fuel will no longer be cooled effectively, and it may begin to melt. If it gets hot enough to ignite, the fire would send large amounts of radiation into the air. That is what happened at Chernobyl, though at Chernobyl the core itself caught fire, so it was much worse. It is unclear how secure the spent fuel ponds are at this point.