Many of the news reports on the situation of the Japanese nuclear plants adversely effected by the earthquake and ensuing tsunami have been criticized for being shoddy, sensationalist, light on facts, etc... In other words 'par for the course' for the media.
Below are some good links that explain in greater detail what has happened, viz. the plants were automatically shut down after the earthquake i.e. there was a scram and the control rods were inserted, shutting down the nuclear reaction. However, even with the active heat generation being stopped there is a very significant residual heat that needs to be cooled... The cooling was initially being enabled by the generators, but when these were rendered inoperable by the tsunami, the cooling systems went to battery power. Unable to restore power there have been problems ever since... See the interactive graphic at: How a Reactor Shuts Down and What Happens in a Meltdown
What's the bottom line? If the containment vessels are not breached (e.g. by one of the explosions that have occurred), then even a partial meltdown in which the fuel melts and pools at the bottom of the containment vessel, should not result in an escape. In Three Mile Island, where half the core melted, it did not even get close to melting through the bottom of the containment vessel (though I've heard reports that the containment vessels here are less sturdy than those at TMI...). The operators have been venting steam to reduce pressure... This steam contains small amounts of radioactive material, so it is 'scrubbed' through filters to remove radioactive components such as iodine and cesium, before being released into the atmosphere. As a result relatively small amounts (thus far) of radioactive materials have been released...
Boiling Water Reactor (BWR)
Some Perspective On The Japan Earthquake
Why I am not worried about Japan’s nuclear reactors
Nuclear Experts Explain Worst-Case Scenario at Fukushima Power Plant
Discussion Thread – Japanese nuclear reactors and the 11 March 2011 earthquake
Information on the Japanese Earthquake and Reactors in That Region
Fukushima Nuclear Accident – a simple and accurate explanation
Drumbeat Special Edition: Fukushima Thread
IAEA on Fukushima
How Black Is the Japanese Nuclear Swan?
Some think that the far greater danger is posed by the spent fuel pools, that are situated "... several stories above ground adjacent to the top of the reactor...", see the links below:
All Bets Are Off: Meltdown at Fukushima
The Post-Tsunami Situation at the Fukushima Nuclear Plants
Safety and Security of Commercial Spent Nuclear Fuel Storage: Public Report
Below are some good links that explain in greater detail what has happened, viz. the plants were automatically shut down after the earthquake i.e. there was a scram and the control rods were inserted, shutting down the nuclear reaction. However, even with the active heat generation being stopped there is a very significant residual heat that needs to be cooled... The cooling was initially being enabled by the generators, but when these were rendered inoperable by the tsunami, the cooling systems went to battery power. Unable to restore power there have been problems ever since... See the interactive graphic at: How a Reactor Shuts Down and What Happens in a Meltdown
What's the bottom line? If the containment vessels are not breached (e.g. by one of the explosions that have occurred), then even a partial meltdown in which the fuel melts and pools at the bottom of the containment vessel, should not result in an escape. In Three Mile Island, where half the core melted, it did not even get close to melting through the bottom of the containment vessel (though I've heard reports that the containment vessels here are less sturdy than those at TMI...). The operators have been venting steam to reduce pressure... This steam contains small amounts of radioactive material, so it is 'scrubbed' through filters to remove radioactive components such as iodine and cesium, before being released into the atmosphere. As a result relatively small amounts (thus far) of radioactive materials have been released...
Boiling Water Reactor (BWR)
Some Perspective On The Japan Earthquake
Why I am not worried about Japan’s nuclear reactors
Nuclear Experts Explain Worst-Case Scenario at Fukushima Power Plant
Discussion Thread – Japanese nuclear reactors and the 11 March 2011 earthquake
Information on the Japanese Earthquake and Reactors in That Region
Fukushima Nuclear Accident – a simple and accurate explanation
Drumbeat Special Edition: Fukushima Thread
IAEA on Fukushima
How Black Is the Japanese Nuclear Swan?
Some think that the far greater danger is posed by the spent fuel pools, that are situated "... several stories above ground adjacent to the top of the reactor...", see the links below:
All Bets Are Off: Meltdown at Fukushima
The Post-Tsunami Situation at the Fukushima Nuclear Plants
Safety and Security of Commercial Spent Nuclear Fuel Storage: Public Report
Update 11:00 PM: Latest reports seem to indicate that one of the containment vessels was damaged in a blast, and that "... emergency workers were withdrawn from the plant, and much larger emissions of radioactive materials appeared imminent..."
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