As I wrote yesterday I have known about issues and concerns with the GE BWR Mark I (Boiling Water Reactors), the type installed in Fukushima, for decades. This type of reactor, present in about 1/3 of all US nuclear facilities was developed in the 1950's by GE and the Idaho National Laboratory.
Here's what I don't like an why I don't like it:
Inside the reactor core are columns of fuel rods - they are about 4 meters long (say 15 feet or so). Inside each fuel rod is a set of uranium pellets - stacked up to fill the rod. The rods are laid out in a grid-type arrangement. The rods sit inside a metal vessel. Water is pumped into the vessel - it passes between the rods moving upward and turns to steam. The steam drives the turbines to make electricity.
Individual pellets by themselves are not dangerous and do not react. For a nuclear reaction to occur a "critical mass" is required. So, for example, say two grams of uranium is a critical mass - meaning an uncontrollable nuclear reaction will occur if that much uranium is present in a single pellet. In the fuel rods the uranium is broken down into 1/4 gram pellets (just for illustration). These are stacked up in a rod 15 feet long and because there isn't 2 grams of uranium in one place the rod is basically inert.
A nuclear reaction occurs when neutrons from an atom of uranium hit other atoms of uranium causing them to emit more neutrons. So one atom emits two neutrons - which hit two other atoms which each emit two more neutrons which hit four atoms which emit eight neutrons until a critical reaction occurs. (This is like growing at the power of two: 1, 2, 4, 8, 16, 32, ... things get very large very fast.)
A reaction is controlled by inserting a substance (a "moderator") between uranium pellets that causes the number of neutrons to grow much less rapidly: water or cadmium. These substances offer a place where neutrons emitted by the uranium atoms are harmlessly absorbed.
When you have a lot of these rods located near each other without a moderator present the reaction will sustain until all of the uranium fuel is depleted.
Thus in a nuclear reactor the reaction proceeds when the reactor is fully loaded with fuel unless either 1) control rods have been inserted or 2) water is placed in the system.
So my first problem is this: Unless something is done to moderate (slow down the rate of reaction) the reactor will overheat and the rods holding the fuel will melt. Melting rods are bad because the pellets inside will fall down into the base of the reactor vessel and react uncontrollably.
In the GE BWR design this requires that control rods be present and/or water be present in the reactor vessel. The control rods enter this type of reactor from the bottom under hydraulic power (generated by electricity). If the rods have been withdrawn and there is an incident like an earthquake its possible that the rods will not return to the "fully inserted" position which would shut down the reaction. (Other reactor designs pull the control rods out from the top and use gravity in an emergency to pull the rods back into the reactor - to my mind a better idea.)
So the problem is that when there is a catastrophic failure of some sort, like an earthquake, that damages the power available to run the water pumps and/or the control rod assemblies, you have a situation like you do in Japan.
Now the reports also say that they are having trouble keeping the rods "covered". Normally this would mean water flowing through the system which would remove heat in the form of steam and moderate the reaction.
So if the control rods were all the way "in" then one presumes the reaction would stop completely. However its either the case that both water and fully inserted control rods are required for this or there is a problem with the water and/or control rods.
We know that there isn't enough power to drive the temporary water cooling pumps - which is why there was a problem after the earth quake in the first place...
So I think that the real problem is the basic BWR reactor design itself. To my mind a good design would be one where the default configuration of the reactor is to not react unless something is done. And further, in case of any problem, the reactor should automatically return to its default "do nothing" state.
What we are seeing in Japan is the case where the reactor cannot be "shut off" for some reason. There may be leaks in the cooling system leaving the core uncovered, the control rods may not have returned to the fully off position, or a combination.
This in turn is causing the reaction to continue and boil away any water. In addition, the nuclear reaction breaks down the water and converts it to hydrogen.
The recent explosions at the plants are said to be hydrogen based.
Pumping in sea water would slow the reaction - but since they are doing this and the reaction clearly has not stopped there must be other issues as well - like broken cooling pipes or other unreported problems. (And its unlikely we'll ever hear the full truth.)
BTW - "Officials in President Barack Obama's administration sought to reassure Americans that nuclear power is safe..." according the the San Jose Business Times.
Really?
What part of my scenario is unclear?
What part of a reactor design that melts down in its "default" configuration is "safe?
If this energy is so clean why is the US naval fleet off the coast of Japan withdrawing to a safe distance as the crisis proceeds?
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