The cores were exposed about a foot above the cooling water at this point. Core temps reached sufficient heights to cause the cooling water to dissociate into H2 and O2 gas. It's claimed the gas explosion didn't destroy the containment vessel, but the source of the H2 and O2 gas was the core melting down. Infinite evils stars, this.
I didn't read the link either, but that argument is silly. I'll buy that the singlet O-'s recombine with the zirconium to form an oxide preferentially to forming O2 though. But it's the cooling H2O that is the source of the H2. Jeeze, you people.