UPDATE, 1:45 pm, Saturday, It is being reported that Japanese authorities are preparing to distribute potassium iodide to the public in the most affected zones. This helps protect against thyroid cancer caused by exposure to radioactive Iodine-131. The release of Iodine-131 to the environment indicates melting of reactor fuel.

UPDATE, 1 pm, Saturday, World Nuclear News is reporting that a worker who was apparently trapped in the exhaust stack of Unit 1 at Fukushima Daiichi has died. Tokyo Electric Power Co. (Tepco) has announced that it will begin venting the containments of three of the four reactors at the Fukushima Daini site in an effort to reduce pressure inside those containments. This will cause additional radiation releases to the air. Evacuation of a threekilometer zone around the Daini site has begun; people from 3 to 10 kilometers from the site are being told to take shelter indoors and have been notified that they may be asked to evacuate.

UPDATE 12:30 pm, Saturday, Japanese government officials state that radiation levels at the Fukushima Daiichi site have decreased from 1015 micro/Sieverts per hour to about 70 microSieverts/hour. There is no independent confirmation of these levels. Officials state that the primary containment remains intact.

UPDATE, 11:30 am, Saturday, Official reports are that radiation levels have decreased from those reported below; however radiation monitoring systems are either still down or not available to the public.

UPDATE, 9:30 am, Saturday, An explosion has occurred at Fukushima Daiichi Unit 1. Video of the aftermath of the explosion shows that the containment building has been destroyed. In a General Electric Mark I reactor, the containment building is fairly weak and is considered the secondary containment. The primary containment is a steel liner that surrounds the reactor core. So far, video and photos have not been clear enough for us to determine whether this steel liner is intact.

UPDATE, 8 pm, Friday, Venting of the containment began at Fukushima Daiichi Unit 1 around 9:00 am Tokyo time. This will release some radiation into the air. Because radiation monitors at the site are inoperable (see below), it will be impossible to know how much radiation is being released.

UPDATE, 7:45 pm, Friday, An evacuation of a three kilometer zone has been ordered at a second reactor site in the Fukushima area. Apparently there are also problems cooling three of the four reactors at the Fukushima Daini site. All of these reactors are later model General Electric Boiling Water Reactors.

UPDATE, 7:30 pm, Friday, Units 1, 2 and 3 at the Fukushima Daiichi site have lost offsite power. Pressure is building up inside Unit 1 (a General Electric Mark I reactor, see below). The normal procedure to release the pressure is to vent the containment (a procedure that releases radiation into the air), but the utility has been unable to do so because of the lack of power. Plant workers may need to enter the containment to do so manually.

UPDATE, 6: 45 pm, Friday Numerous sources are reporting radiation levels at Fukushima to be 1,000 times higher than normal inside Unit-2 (though some sources have also reported high radiation levels inside Unit 1) and at least eight times higher than normal at the plant gates outside. The evacuation zone has been expanded to 10 kilometers (about 6 miles) from the Fukushima reactor site.

UPDATE, 3:30 pm, Friday, A Japanese Cabinet Office report released at 12:30 am Saturday, March 12, 2011 (Japanese time) predicts that fuel damage will occur at the Fukushima Daiichi Unit-2 reactor late Saturday night (Japan time, early Saturday morning US time) if cooling is not restored before then.