A large amount of energy was suddenly released , vapourising superheated Change cooling Change water and rupturing Destroying the reactor core in a highly destructive Destroying steam explosion Attack .

The test Testing was a simulation of an electrical power outage to aid Assistance the development of a safety procedure for maintaining cooling water circulation until the back-up generators could provide Supply power .

In particular , a hidden Hiding_objects indicator light led to Causation an operator manually overriding the automatic emergency cooling system of the reactor because the operator mistakenly believed Know that there was too much coolant water present in the reactor and causing Causation the steam pressure release .

Many electrical generators were taken down , and at least three nuclear reactors suffered Catastrophe explosions Destroying due to hydrogen gas that had built Building up within their outer containment buildings after cooling system failure resulting Causation from the loss of electrical power .