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In the Chernobyl they tried to run an experiment to see how long they could provide power to the plant’s control and cooling systems from the idling down turbine if they experienced an emergency loss of power.

The reason for this was that their backup diesel generators took up 60 seconds to ramp up to full power, which was way too long. They were trying to see if the idling down turbine could provide enough energy power the control and cooling systems in the time before the backup generators kicked in.

RBMK type reactors (the design that was used there) are inherently unstable under very low power levels. They also have something called a “positive void coefficient”, in layman’s terms basically as temperature and pressure inside the reactor rises, so does the rate of the reaction. All modern reactors are designed such that if they get hotter and built up pressure, the reaction slows down by virtue of physics. That way you can’t have a runaway reaction.

Anyway, they disabled (hard shutoff) the main cooling valve and disabled the emergency control cooling system, so it would not kick on during test (a big violation of their own procedures)

Then they proceeded with the test but let the reactor power drop down too much in the region where the reactor was inherently unstable. They were trying to bring up power and miscalculated the how fast the reaction was rising. If the emergency cooling system was active, it would have shut the reactor down then and there.

But they were trying to do things manually and miscalculated. The reactor started to build up power too rapidly. They panicked and hit the SCRAM button (bring all the control rods down) but due to the design/manufacturing defect of the control rods that actually caused a surge.

The rest is history.