The reactor filled up with steam and went into a runaway power generation loop, which first caused a steam explosion, then a more powerful nuclear explosion. The nuclear explosion itself wasn't very impressive. It had as much power as about 10 tons of TNT, which was several orders of magnitude lower than the Hiroshima and Nagasaki explosions.
source? everything I know about nuclear reactors tells me that they are 100% incapable of any type of fission explosion; at most you could get some kind of prompt criticality accident from fuel rods bashing into eachother, but nothing that could possibly release 10t equivalent energy.