The schematic diagram above shows the GE Mark I Boiling Water Reactor reactor building structure, the Fukushima Dai-ichi Unit 1.

The explosion at Fukushima has apparently disintegrated the upper third of the reactor building. The video and pictures currently available indicate that the "blow out panels" of the reactor building and roof cover were blown away by an energetic explosion likely due to a hydrogen gas detonation. The reactor core refuelling deck and the surface of the elevated irradiated nuclear fuel pool are now exposed to the atmosphere. Essentially, the photos show the remaining steel I-beam structure for the weather cover that was over the refueling deck and the top of the "spent fuel" pool. These panels are designed to "blow out" at overpressure.

The actual "pressure suppression system" structures credited for containment sit below this structure inside the concrete reactor building, namely the drywell and wetwell or "torus." The drywell is the large inverted lightbulb steel structure which is 100 feet tall and a nominal wall thickness of 1.5 inches. The reactor vessel sits inside this structure. In the event of a coremelt accident involving high pressure and high temperature, the highly radioactive steam and pressure would be vented into the drywell and then routed through the large diameter pipes to the "wet well" or "torus" which is the large 18 foot diameter hollow doughnut-shaped structure that surrounds the drywell. The torus contains approximately 1 million gallons of water and designed to receive the pressurized radioactive steam where it is supposed to be quenched and contained.

The status of the reactor containment in the reactor building remains unclear, but apparently remains intact. Fuel damage has apparently occurred because elevated levels of radioactive iodine and cesium are being monitored outside of reactor containment.

What is additionally unclear is how much cooling water is left in the fuel storage pools and whether or not there has been damage to irradiated fuel stored in that pool. There are reports of sea water being brought in to cool this facility.

Radiation levels are reported to have fallen following the explosion. David Lochbaum, Senior Reactor Safety Engineer for the Union Concerned Scientists, has reported that the explosion may acutally have occurred in the turbine hall building adjacent to the reactor building.

An anonymous Japanese government spokesperson has attributed the explosion to the buildup of a combination of hydrogen and oxygen that detonated inside the concrete reactor building but not the crdedited containment structure. Industry reports that the containment structure itself was not compromised or breached by the explosion.