Stratfor calls it a full meltdown, although still contained: "Red Alert: Nuclear Meltdown at Quake-Damaged Japanese Plant." (Via Mememorandum.) And a roundup at POWIP, "Video of Fukushima Nuclear Plant Explosion."
See also New York Times, "Explosion Rocks Japan Nuclear Plant After Quake," and Los Angeles Times, "Japan's fears mount with nuclear plant blast":
Officials try to calm residents wary of a possible radiation leak -- or worse -- at the Fukushima power plant, which lost its cooling system in Friday's massive earthquake. Nationwide, the death toll from the quake and tsunami could top 1,700.RELATED (WITH additional links and information): "Meltdown at Japan's Fukushima Nuclear Plant?"
Reporting from Tokyo and Beijing — A day after responding to one of the worst earthquakes on record and a massive tsunami, the Japanese government sought to allay fears of a radioactive disaster at a nuclear power plant on the country's battered northeastern coast.
The outer walls of the Fukushima power plant's No. 1 reactor were blown off by a hydrogen explosion Saturday, leaving only a skeletal frame. Officials said four workers at the site received non-life-threatening injuries.
The inner container holding the reactor's fuel rods is not believed to be damaged, said Japan's Chief Cabinet Secretary Yukio Edano, and workers were cooling the facilities with seawater.
In a press conference shortly after the explosion, which left the facility shrouded in plumes of gray smoke, Edano explained that the reactor is contained within a steel chamber, which in turn is surrounded by a concrete and steel building. Although the explosion destroyed the building, it did not occur in the chamber.
"The escape of hydrogen mixed with the air between the chamber and the concrete-and-steel building and led to the explosion," Edano said.
"Tokyo Electric Power Co. has confirmed that the inner reactor is undamaged," he added. "There was no massive release of radiation."