Please use this Discussion Thread for the situation in Japan with respect to the earthquake and the local nuclear reactors. Here is a summary of the situation as I understand it:
1. There is no credible risk of a serious accident. All reactors responded by insertion of control rods to shut down their nuclear reactions. Thus, power levels in all cases dropped quickly to about 5% of maximum output, and the nuclear chain reaction ceased.
2. The concern is providing emergency cooling water to the reactor cores to remove decay heat from the fuel rods. This heat comes from the fission products, and will persist for weeks but diminish rapidly over time.
3. At one plant, the 40-year old Fukushima Daiichi, the backup diesel generators supply power to the core cooling system failed. This allowed pressure to build up in at least one of the reactors cores to about 50% higher than normal (unit 1), and required some venting of mildly radioactive steam.
4. New generators have been flown in to provide power to the pumps. The emergency core cooling systems have not yet needed to be invoked. We are still only a couple of levels into defense in depth.
5. The plant closest to the earthquake epicentre, Onagawa, stood up remarkably well, although there was a fire in a building on site but not associated with the reactor operations.
(I will edit the above section and provide further updates below, as more information comes to hand)
Some useful links for further information:
Battle to stabilise earthquake reactors (World Nuclear News)
ANS Nuclear Cafe updates (useful news feed)
How to Cool a Nuclear Reactor (Scientific American interview with Scott Burnell from the NRC)
This is a critical time for science, engineering and facts to trump hype, fear, uncertainty and doubt.