So… I was wondering how an earthquake could possibly cause a meltdown at the Fukushima Daiichi nuclear facility in Japan, given that the plant operators had been able to fully insert the control rods into the core. I had to search for a few minutes to find an explanation by someone who actually knew what they were talking about – in this case, Alexey Petrov, an Associate Professor at the Department of Physics and Astronomy at Wayne State University. Here is an informative excerpt from his post, which can be found here–
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 and the process begins again.
Simply speaking, fission reaction happens when a slow (thermal) neutron is absorbed by uranium (U-235) nucleus, which then splits into several (two) lighter daughter nuclei, 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 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 is 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 some time (several days) for those isotopes to decay. During this time, the 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 (1) power grid or (2) diesel generators or (3) batteries. After the earthquake the grid was knocked out and the diesel generators 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 [Tokyo Electric Power Company, the plant’s owner – ed.] 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.
Well, that’s a relief… But (UPDATE 3/15) that doesn’t mean we’re not in for a disaster, according to the New York Times–
“We are on the brink. We are now facing the worst-case scenario,” said Hiroaki Koide, a senior reactor engineering specialist at the Research Reactor Institute of Kyoto University. “We can assume that the containment vessel at Reactor No. 2 is already breached. If there is heavy melting inside the reactor, large amounts of radiation will most definitely be released.”
Another executive said the chain of events at Daiichi suggested that it would be difficult to maintain emergency seawater cooling operations for an extended period if the containment vessel at one reactor had been compromised because radiation levels could threaten the health of workers nearby. If all workers do in fact leave the plant, the nuclear fuel in all three reactors is likely to melt down, which would lead to wholesale releases of radioactive material — by far the largest accident of its kind since the Chernobyl.