IANANS, however from what I understand the problem is this: -> The reactor is a BWR type, hence even though it is scrammed it is still generating some amount of

IANANS, however from what I understand the problem is this: -> The reactor is a BWR type, hence even though it is scrammed it is still generating some amount of power from fission products. -> The ECCS failed, meaning that the core can't maintain sufficient cooling. -> So what water was left in the core is then exposed to very high temperatures and pressures. -> That apparently then creates conditions sufficient to cause the water to react with the zirconium alloy which is used in the fuel rods, forming zirconium dioxide and hydrogen. (2H2O + Zn -> ZnO2 + 2H2). -> As reactors generally don't create much gaseous waste (The only waste products that I can think of would be, for example, radon, and that would be in small quantities), and the core isn't expected to be above the autoignition temperature of whatever gas that is being vented. What could be happening is that the gas is being vented but is being concentrated at some point where it comes into contact with something hot enough to ignite it thus causing an explosion. For example, if the gas is being vented out of the core, but into a secondary containment vessel, if the vessel is filled with air and if it contains say a piece of metal hot enough to ignite the hydrogen, it would simply fill with gas until it reaches a point where the mixture is explosive, before then it then explodes. Disclaimer: I'm not a nuclear scientist, any corrections/additional information would be very welcome.