Japanese news agency NHK and Chinese news agency Xinhua are reporting that a little over three hours ago as of this posting — shortly after 11 a.m. Japan time on Monday — there was a second explosion at the Fukushima I nuclear power plant, injuring eleven.
This explosion was at #3 reactor at the Fukushima I site (the first explosion was at #1 at the same site). The Japanese government is reporting that the containment vessel for the nuclear fuel at #3 reactor is intact. A breached containment vessel would have potentially meant that the nuclear fuel was exposed to the air. I haven’t heard the specific word on whether the containment vessel is
Like the #1 explosion, this one was caused by the buildup of hydrogen from the reaction of the zirconium cladding of the fuel rods, under extreme heat, with water.
Huh!?!? Basically, here’s the short version as I understand it: For operational practicality, because it doesn’t absorb neutrons, the element zirconium is used, as one of several alloys called zircaloy, to create the matrix that holds the fuel rods and control rods in a nuclear reactor. Yes, it’s the same element in cubic zirconia, aka zirconium dioxide, ZrO2 or fake diamonds, for those of you who watch a lot of late-night TV and/or remember the ’70s.
When zirconium encounters extreme heat, like that in a reactor core meltdown, it reacts with water (the coolant) by creating hydrogen, which is highly flammable. At Three Mile Island, a hydrogen bubble formed, but did not ignite.