Heat alone is not insufficient, the heat generated is so intense that is turns the surrounding air into a plasma which then absorbs the x-rays emitted creating a stupendous over-pressure in the air which drives the shockwave.
Reactors don't explode because the amount of heat then generate is carefully monitored and matched the the rate at which that heat can be pulled from the reactor and converted into work.
Reactors don't explode because the amount of heat then generate is carefully monitored and matched the the rate at which that heat can be pulled from the reactor and converted into work.