The accident began Process_start with failures in the non-nuclear secondary system , followed Motion by a stuck-open pilot-operated relief valve in the primary system , which allowed large amounts of nuclear reactor coolant to escape .

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 .

Critically , they were powering Supply the pumps that circulated coolant through the reactors ' cores to remove Removing decay heat , which continues after fission has ceased Process_end .