"The world's first commercial nuclear power station opened in the UK in 1956 at the Sellafield site on the Cumbrian coast, and it ran for almost half a century before closing in 2003. The four Calder Hall reactors were of the Magnox type, which means they used a magnesium 'no-oxidation' alloy to encase the uranium fuel rods.
"France initially followed the UK's lead by building reactors similar to the Magnox design during the 1960s. Meanwhile, the US realized that the most economical reactors are those that are collectively referred to as light-water reactors (LWRs).
"LWRs use ordinary water as a moderator and as a coolant, running on uranium-oxide fuel enriched with up to 5% uranium-235 and contained in a zirconium alloy cladding." (Physics World, July 2007).
"The core temperature rose to the point at which the zirconium alloy cladding began to react with steam to produce hydrogen, some of which escaped into the reactor building. Early in the afternoon of the first day, sufficient hydrogen had accumulated in the reactor building to result in a low-level explosion." (Environmental Radioactivity, Eisenbud, p370-371).
Three Mile Island, March 1979.
"Two explosions occurred at 01:24. The steam released by the failure of one or more pressurized tubes had reacted exothermally with the zirconium, producing hydrogen that exploded. Parts of the core were scattered about the building as well as on the roofs of the reactor, turbine, and auxiliary buildings." (ibid, p378).
Chernobyl, April 1986.
"The cladding, which is just the outside of the tube, at a high enough temperature interacts with the water. It's essentially a high-speed rusting, where the zirconium becomes zirconium oxide and the hydrogen is set free. And hydrogen at the right concentration in an atmosphere is either flammable or explosive.
"Hydrogen combustion would not occur necessarily in the containment building, which is inert—it doesn't have any oxygen—but they have had to vent the containment, because this pressure is building up from all this steam. And so the hydrogen is being vented with the steam and it's entering some area, some building, where there is oxygen, and that's where the explosion took place."
Fukushima, March 2011.