According to an update from the Nuclear Information and Resource Service (NIRS) an explosion has occurred at Fukushima Daiichi Unit 1.
Video of the aftermath of the explosion shows that the containment building
has been destroyed.
The NIRS update goes on to explain, 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.
Radiation levels at the site are reported to be 1,015 micro/Sieverts per hour. This is roughly equivalent to 100 millirems/hour. The allowable annual dose for members of the public from nuclear facilities in the U.S. is 100 millirems/year. The allowable annual dose for nuclear workers is 5,000 millirems/year. The average annual background dose from all radiation sources in the U.S. is about 360 millirems/year.
The explosion in Unit 1 was almost surely a hydrogen explosion. Pressure has been building up in the containment since offsite power was lost to the reactor because of the earthquake/tsunami. The GE Mark I reactor design is called a “pressure suppression” design. Rather than be built to withstand large pressure increases, General Electric sought with this design to attempt to reduce such increases in an accident scenario. The design has been criticized by independent nuclear experts and even Nuclear Regulatory Commission staff for many years (see: http://www.nirs.org/factsheets/bwrfact.htm). In this case, the design clearly did not work. 24 U.S. reactors use the GE Mark I design.
The evacuation zone around the site has been expanded to 20 kilometers (about 12 miles). Another reactor at Fukushima Daiichi, Unit 2, is reported to be without cooling capability at this time. Three reactors at the nearby Fukushima Daini site are reported to be without cooling capability. These are GE Mark II designs, which are considered a mild improvement over the Mark I design. Both sites are on the Pacific Ocean, about six miles apart.