What is happening at Japan’s Fukushima Daiichi power plant?

This is a good question to ask — especially amid speculations about “possible Chernobyl-like nuclear meltdown” and pictures of explosions at the plant.  Knowing a little bit of physics (and reading press-releases from TEPCo — Tokyo Electric Power Company), one can make some initial analysis. Clearly, a complete picture will follow in the near future.

So, first of all, what is the problem? To understand this, let me note that nuclear reactors at the Fukushima Daiichi plant are of the Boiling Water Reactor (BWR) type — quite different in design from Three Mile Island’s PWR-type reactor and  Chernobyl’s RBMK reactor. In order to see the logic of what Japanese engineers are doing, it is useful to see how the BWR reactor works.

Here is the scheme of BWR-type reactor, taken from the Wikipedia page on BWR. The physics here is very simple. Fission reaction in the uranium fuel assemblies (2) heat water (blue stuff, 7), which turns into steam (red stuff, 6) in the reactor vessel (1). The steam exits the vessel and spins the turbine (8 and 9) that generates electricity. That steam is cooled down and returned into the reactor vessel (as water) and the process begins again.

Simply speaking, fission reaction happens when a slow (thermal) neutron is absorbed by a uranium (U-235) nucleus, which then splits into several (two) lighter daughter nuclei, and several fast neutrons (about 3), releasing energy that is converted into heat. In order to have sustained nuclear reaction one needs to slow down those produced neutrons so that they could be absorbed by other U235 nuclei to initiate chain fission reaction. Different reactor designs use different moderators to do that: water (BWR, PWR), graphite (RBMK), etc.

This simple excursion into nuclear physics tells us that the rate of power generation can regulated by controlling the flux of thermal neutrons. This is indeed what is done by the control rods (3) that are usually made of a material (boron) that absorbs neutrons.

What happens in case of an earthquake? Well, the automatic control systems first and foremost would kill the sustained fission reaction that is going in the fuel elements. This was done at the Fukushima plant immediately by inserting the control rods (notice that the control rods are inserted from below). So, what’s the problem then? Why is the water vapor’s pressure rising?

The problem is that during the fission reaction one also produces a lot of short-lived nuclear isotopes. Normally, if you would like to shut down a reactor (say, to refuel), you need to allow for some time (several days) for those isotopes to decay. During that time, water is still being circulated through the reactor core in order to take away the heat produced in the decays of those short-lived isotopes. This is done via pumps that are operated via electricity from (a) power grid or (b) diesel generators or (c) batteries. After the earthquake, the grid was  knocked out and the diesel generators got damaged. The pumps are now running on the batteries and the water vapor pressure inside the reactor vessel is rising — by the way, the normal operating pressure there is about 75 atmospheres!!! TEPCo reports that the pressure there rose twice that, so the plant operators decided to release steam from the vessel.  Now, to cool down the reactor (until those short-lived isotopes decay) they decided to flood the containment vessel with sea water.

So, as you see, the Chernobyl-type of explosion is highly unlikely at the Fukushima plant. I think the reactor will cool down in a couple of days. BTW, it appears that the reported explosion happened at the pumping system…

The only troubling news is that the monitoring stations appear to detect small amounts of iodine and cesium isotopes (to quote TEPCo’s press release “The value of radioactive material (iodine, etc) is increasing according to the monitoring car at the site (outside of the site). One of the monitoring posts is also indicating higher than normal level.”). Those isotopes are normally produced in the nuclear fuel rods. This might indicate that one or more rods are damaged.

Update (3/14/2011): It appears that water circulation systems in reactors 1 and 3 of the power station failed on 3/12-13. The reactor containment is now cooled by sea water (with added boric acid to further capture neutrons). Sea water is not an ideal coolant — purified fresh water is — sea water contains salt and other things that can become radioactive in the core of a reactor. Thus, the spent water will most likely be transferred to the spent fuel pools (place on the power station’s campus where spent fuel rods spend some time before being transferred to the permanent storage facility). It also appears that there were two hydrogen explosions in Units 1 and, recently, 3. Where did the hydrogen come from? It most likely came from the chemical reaction on the zircalloy’s casings of the fuel assemblies. Zircalloy, an alloy of zirconium, tin and sometimes other things, contains zirconium. That zirconium  reacts with oxygen in water and releases hydrogen. It is, however, believed that in both cases the containment vessels held up. Those containment vessels did not exist at the Chernobyl’s power station.

Update (3/15/2011): I decided to put updates in the separate post.