About as simple an explanation and as complete as I can find right now regarding Fukushima I:
http://en.wikipedia.org/wiki/Fukushima_I_Nuclear_Power_Plant#2011_earthquake_eventsAfter the March 11, 2011, earthquake, Nuclear Engineering International reported that units 1 to 3 were automatically shut down, and units 4 to 6 were already in maintenance outages.[1] However electrical problems prevented cooling pumps for Reactor 1 from functioning. Because cooling pumps are needed to remove residual reactor heat, a nuclear emergency was declared. It was the first time a nuclear emergency had been declared in Japan. The emergency cooling systems also were activated for Reactor 2. Batteries, which last about eight hours, are being used to power the reactor cooling during the electrical outage, and the government can use military helicopters to fly in replacement batteries.[2]
An evacuation order has been issued to people living within 3 kilometers of the plant, affecting approximately 5800 residents living near the power plant. People living less than 10 kilometers from the power plant have been advised to stay indoors.[3]
David Lochbaum, who worked at three American reactor complexes that use General Electric technology and now works for the Union of Concerned Scientists, said the plant was probably equipped to function for some hours without emergency diesel generators.[4] The United States Air Force assisted in delivering backup diesel generators for powering the cooling pumps to the plant site. Japanese ground forces are also trucking generators and batteries to the site.[5] The water level inside the reactor has dropped; however, the rods were not exposed.[6]
Past midnight local time, it was reported that The Tokyo Electric Power Company was considering venting super hot gas from the reactor vessel into the atmosphere, which could result in the release of radiation.[7] Saturday, the Tokyo Electric Company reported that radiation levels were rising in the turbine building for reactor 1.[8] At 20:30 GMT the pressure inside Reactor 1 was reported to be 2.1 times the "design capacity."[9] At 21:10 UTC, the IAEA reported that unit 2 is also experiencing cooling problems.[10]
To reduce mounting pressure potentially radioactive steam has been released from the primary circuit, into the secondary containment.[11] On March 11, 2011 at 21:40 (GMT), Chief Cabinet Secretary Yukio Edano stated that the amount of potential radiation would be small and that the prevailing winds are blowing out to sea.[12] Measured radiation levels inside the plant control room were reported to be 1000 times greater than normal.[13] Radiation levels measured at a monitoring post near the plant's main gate are more than eight times above normal, Japan's nuclear safety agency said.[14]
