By Robby Kile
The Accident
On April 25, 1986, early in the morning, operators at the Chernobyl nuclear power plant near Pripyat, Ukraine, began a test of their backup power supply during a planned outage of Unit 4.  The first step of the test was to reduce the power in the reactors to about 20 percent power. By 2:00 pm, power had been reduced to 50 percent, and grid operators ordered the reactor to hold at a steady power to meet demand. At 11:10 pm, the power reduction resumed and, by 12:05 am on April 26, the reactor was at an appropriate power to conduct the test. At 12:28 am, the power dropped to one percent, far too low to conduct the intended test. Rather than abandon the test, operators decided to bring the reactor back up to power, and after about 45 minutes, power was stabilized near 6 percent, and emergency shutoff signals were disabled, in violation of operating procedure. Finally, at 1:23 am on April 26, 1986, the test began. Due to conditions beyond the test design, power began to rise rapidly, and control rods had been removed too far to compensate for unanticipated conditions, and by the time rods could be reinserted, the power had already increased to an estimated 100 times normal operating levels, leading to rapid boiling and a sudden steam explosion (not a nuclear explosion).
It’s well worth noting that only the RBMK reactors designed in the Soviet Union could undergo this exact progression of events and lead to the rapid power increase that ultimately blew the roof off the reactor building. And those reactors – and their operating procedures – have since been modified to ensure nothing like the Chernobyl accident ever happens again.
A key feature of most nuclear power plant designs is the containment dome,

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