Reports from various sources and the video above indicate there has been another explosion at the Fukushima Daiichi Nuclear Station, this time at Unit 3. The video shows a dark plume rising hundreds of feet in the air, a bad sign if there is significant radiation involved.

An earlier report from Tokyo Electric indicated the water level in the reactor dropped 3 meters, exposing the core. The most ominous news, as reported by Union of Concerned Scientists (h/t commenter 4cdave):

… after multiple cooling system failures, the water level in the Unit 3 reactor vessel dropped 3 meters (nearly 10 feet), uncovering approximately 90 percent of the fuel in the reactor core. Authorities were able to inject cooling water with a fire pump after reducing the containment pressure by a controlled venting of radioactive gas. As they did with Unit 1, they began pumping sea water into Unit 3 … the water level in the Unit 3 reactor still remains more than 2 meters (6 feet) below the top of the fuel, exposing about half the fuel to air, and they believe that water may be leaking from the reactor vessel. … In September 2010, 32 fuel assemblies containing MOX fuel were loaded into this reactor. This is about 6% of the core.

The use of MOX generally increases the consequences of severe accidents in which large amounts of radioactive gas and aerosol are released compared to the same accident in a reactor using non-MOX fuel, because MOX fuel contains greater amounts of plutonium and other actinides, such as americium and curium, which have high radio-toxicities.

The utility, Tokyo Electric is reporting there are three persons injured and seven person missing. [Update: they've reportedly been found] BBC says there may be 11 persons missing.

Unit 3 is at the same Station as Unit 1 where the earlier hydrogen explosion occurred two days ago. There are also reports of smoke rising from Unit 1, but this may be from the same blast.

The earlier explosion blew the roof and upper walls off of the reactor building at Unit 1. At this point, we don’t have any indication of the damage done to Unit 3 reactor, but utility/govt officials are claiming the reactor vessel was not damaged. As for the building housing the reactor, before/after images captured by Jim White indicate this latest explosion blew the roof and walls off the reactor building. See below.

The plant operators have been struggling with the loss of the cooling system and a resulting rise in pressure in the reactor. They have been injecting sea water, using fire pumps, to sustain at least some cooling and may have attempted another controlled release to relieve the rising pressure. That procedure was also presumably involved in the first explosion at Unit 1.

We’ve captured a screen grab, below, that shows before/after. This new explosion appears to have caused substantially more external damage to the outer building than the first explosion. The NYT also reports the same thing.

We will update as we get more details and confirmation.