I have been researching for information on the reactors.

They are of a type called BWR - Boiling Water Reactor. Pretty much just like it sounds. The core boils water to steam which directly drives the turbines. The steam is cooled and condensed by outside water (and pumps) back into liquid water and sent back into the reactor. It's the most common type of reactor and has MANY safeguards.

From what I've been able to gather, this is what seems to have happened in Japan:

When the quake struck, the reactors were automatically shut down. However, because the plant is no longer generating power, it needs an outside source to power the equipment. Most important are the cooling pumps.

Although the reaction has been shut down there is still a lot of residual heat that must be pumped away. Think of your car suddenly boiling over when you shut off the engine. Many new cars have electric fans that keep running for a time after you turn off the engine. This is also why race cars have to take a cooling lap before the end of the race.

Anyway, either due to the quake or the tsunami, the external power sources were knocked off line. The nuclear plant has backup generators, but those got taken out by the tsunami. The next backup are on site batteries, but those won't last long.

There may also be structural damage to the cooling systems.

The explosion this morning is supposedly part of the cooling system. I opine that it was something under high pressure, likely the primary water/steam loop through the reactor, because watching the video it appears to be steam released and you can see a detonation wave from the explosion indicating that it was under high pressure. Only the primary loop and reactor are under this kind of pressure, and there have been reports that the reactor was at about 1.5 times normal operating pressure. According to the wikipedia article, these reactors run around 75 atmospheres.

Also, according to news reports, cesium and iodine has been detected which indicates that the core rods have been exposed. They are supposed to stay submerged in the water.

This all brings in the question of what if any damage there is to the core and the containment vessel. Ideally, in the event of core exposure or even a meltdown, the containment vessel is supposed to do just that - contain it.

Nuclear power and weapons has been a minor interest of mine, likely from growing up in the Cold War.

http://en.wikipedia.org/wiki/Boiling_water_reactor
http://en.wikipedia.org/wiki/Boiling_Water_Reactor_Safety_Systems
http://en.wikipedia.org/wiki/Fukushima_I_Nuclear_Power_Plant

http://www.cnn.com/2011/WORLD/asiapcf/03/12/japan.nuclear/index.html?hpt=T1

Brian