UPDATE 7:30 pm, Monday, Tokyo Electric Power is holding a press conference at this hour. NHK TV reports that there has been an explosion at Unit 2 at Fukushima Daiichi. There is speculation that this explosion has damaged the primary containment (inside the concrete containment building, which is the secondary containment. Tepco is evacuating some nonessential personnel from the reactor site. 2.5 meters of the core are currently uncovered by water—which means it is almost certainly melting. Winds from the site are currently blowing toward the North.

UPDATE 12:30 pm, Monday, According to our colleagues in Japan, Tokyo Electric Power states that Fukushima Daiichi-2 “has again lost its coolant (sea water was pumped in but is dropping). They cannot ease the reactor pressure because the relief valve is stuck closed. Air dose rate on site (outside the reactor building) was 3,130 at around 9:30pm.” We believe the 3,130 figure means 3130 MicroSievert/hour, which would be highest reading yet recorded—about 310 millirems/hour. For comparison, the U.S. EPA allowable dose to a member of the public from a single reactor is 25 millirems/year, the U.S. NRC’s allowable dose is 100 millirems/year from all nuclear sources. UPDATE 10:00 am, Monday, An explosion has occurred at Fukushima Daiichi Unit 3.

UPDATE 2:30 pm, Sunday, Tokyo Electric Power is reporting that some six feet of the core of Unit 3 remains uncovered and has been for some time despite efforts to pump water into the core. Tepco speculates there may be leaking pipes and water is not remaining in the core. A translation of part of the statement from our Japanese colleagues: "The fuel’s integrity has been considerably compromised. We are assessing a considerably serious situation."

UPDATE, 1 pm, Sunday, The International Atomic Energy Agency reports that a first-level (lowest-level) emergency has been declared at the three-unit Onagawa nuclear station north of Fukushima. Immediately after Friday’s earthquake a fire broke out at this facility, but it was said to have been extinguished fairly quickly. However, on Saturday, elevated radiation levels were measured at the Onagawa site (about 10 microSievert/hour or about 1 millirem/hour) for a few hours. The emergency was declared due to these elevated levels, but utility officials say the reactors there are under control.

Statement of Citizens’ Nuclear Information Center,

UPDATE, 3:25 pm, Saturday, The International Atomic Energy Agency reports that a total of 140,000 people have now been evacuated from zones 20 kilometers around Fukushima Daiichi and 10 kilometers around Fukushima Daini. The IAEA says that evacuation has not been completed.

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.