he accident occurred during a safety test on the steam turbine of an RBMK-type nuclear reactor. During a planned decrease of reactor power in preparation for the test, the power output unexpectedly dropped to near-zero. The operators were unable to restore the power level specified by the test program, which put the reactor in an unstable condition. This risk was not made evident in the operating instructions, so the operators proceeded with the test. Upon test completion, the operators triggered a reactor shutdown. But a combination of operator negligence and critical design flaws had made the reactor primed to explode. Instead of shutting down, an uncontrolled nuclear chain reaction began