Chernobyl Part 2: The Path to Disaster

While many people know the date of the Chernobyl explosion, few understand the specific chain of failures that made it inevitable. This quiz focuses on the technical and human factors that converged in the Reactor 4 control room during the early hours of April 26, 1986.

The RBMK-1000 reactor at the heart of the disaster was a graphite-moderated, water-cooled design unique to the Soviet Union. It was never built in the West, partly because of an inherent design flaw: a positive void coefficient. When the coolant water began to boil and turn to steam, the reactor did not slow down — it accelerated. This is the opposite of how Western reactor designs behave, and it made the RBMK fundamentally unstable at low power levels.

The night of the disaster was built around a safety test — an attempt to prove that the reactor's turbines could generate enough electricity during a shutdown to power the emergency cooling pumps during the 60–75 second gap before backup generators came online. The test had already been delayed twice. When it finally ran, the reactor had been poisoned by a buildup of Xenon-135, a fission byproduct that absorbs neutrons and suppresses power. Operators withdrew nearly all control rods to compensate.

Anatoly Dyatlov, the deputy chief engineer overseeing the test, pressured his operators to continue despite the warnings. When the emergency shutdown button was pressed at 1:23:40 AM, the graphite tips at the bottom of the control rods caused a brief, massive power surge before the rods could do their job. The reactor produced approximately 30,000 megawatts of thermal power — ten times its design limit — in the seconds before it destroyed itself.