Good report by an American expert.
The root of the problem is the lack of cooling to the fuel rods. The residual heat that cannot be carried away if the water pumps have no energy will lead to the water cooking and vapour collecting in the reactor vessel. Engineers have been venting the reactor vessels of various reactors for hours to prevent massive build up of steam that can blow up a complete reactor.
When the water is not replaced fast enough the water level obviously will fall and the top of the fuel rods will not be cooled at all. The danger is that the steel tube which contains the fuel (uranium/plutonium) will overheat and pieces of the radioactive fuel will start to contaminate the water and the vapour. That has obviously already happened because Caesium has been found in the area surrounding the plant. This can only happen when the barrier of the containment of the fuel rod and the reactor vessel have been overcome by radioactive matter from the fuel. Under normal circumstances no Caesium will be found.
The steam or hydrogen explosion which we heard about indicates that one of the reactor must have at least partially run away and that the reactor basically is lost. All that can be done now is to try and keep the damaged core as cool as possible as and when cooling fluid is available. Because the reactor is now open to the environment it means that the process will continually produce radioactive water and steam that is contaminated with fuel and waste debris.
In that situation the Japanese will try to avoid a Chernobyl type situation where the complete core melted and a steam explosion distributed a substantial percentage of the radioactive fuel and waste over Europe. That has obviously not happened at Fukushima. Let's hope it does not come this far. It probably makes sense to follow the American hint and kill the reactor with radiation absorbing boron coolant.
We can be happy that this was one of the conventional light water reactors that was hit by the problem. The new breading reactors that Japan and many other countries are developing and some are actually running are using liquid metal (sodium) instead of water. When the sodium gets in contact with air as in an overheating scenario that we see now, it starts to burn and creates a huge fire. I just can't get it through my head that people think this can ever be safe, particularly in an earth quake and tsunami zone.