Hiroshima to Fukushima

http://www.springer.com/chemistry/book/978-3-642-38726-5

Excerpt from ‘The Accident at Fukushima Dai-ichi Nuclear Power Plant‘

The unit 3 reactor exploded on March 14, and the cause of the explosion is still being debated. A very strong flash of light was seen at the beginning and several detonation sounds were heard. The smoke due to the explosion was not quite the same as that of the unit 1 reactor […] In the case of the hydrogen explosion at unit 1, the vapor spread horizontally. The smoke from the unit 3 reactor was actually gray to black, rather than white. It has been suggested that a water vapor explosion was immediately accompanied by a nuclear explosion. The TEPCo have now officially called it “detonation”. The hot fuel rods caused a rapid expansion of the water added (water vapor explosion). This caused the collapse of the water shield between the fuel rods stored in the storing pool, and this may have caused a supercritical condition momentarily and, hence, led to a nuclear explosion […] The nuclear explosion was not very extensive, but there is a sign that it may have occurred at the storing pool […] If this is, indeed,the case, then not only fission products but also U-238. U-235, Pa-239, and neutrons would have been released. Pu-239 (and also other Pu isotopes) has been detected around the plant and up to several tens of kilometers away, though not as high as in the Chernobyl case, and, also, neutrons were observed in the premises of the plant. Data from several of the EPA’s radiation monitoring stations (in the USA) recorded sudden increases of Pu-239, U-234, and U-238 after the Fukushima accidents, particularly in Hawaii, Alaska, Guam, and California. These data suggest the release of fuel rod material itself in addition to fission products, and are consistent with the notion that a nuclear explosion, indeed, took place. This description is entirely a conjecture based on what has been observed, but much uncertainty still exists regarding the explosion at unit 3.