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Reactor cooling after shutdown

In nuclear reactor operation, most heat is generated by nuclear fission, but over 6% comes from radioactive decay heat, which continues after the reactor shuts down. Continued coolant circulation is essential to prevent core overheating or a core meltdown. RBMK reactors, like those at Chernobyl, use water as a coolant, circulated by electrically driven pumps. Reactor No. 4 had 1,661 individual fuel channels, requiring over 12 million US gallons per hour for the entire reactor.

In case of a total power loss, each of Chernobyl's reactors had three backup diesel generators, but they took 60–75 seconds to reach full load and generate the 5.5 MW needed to run one main pump. Special counterweights on each pump provided coolant via inertia to bridge the gap to generator startup. However, a potential safety risk existed in the event that a station blackout occurred simultaneously with the rupture of a coolant pipe. In this scenario the emergency core cooling system (ECCS) is needed to pump additional water into the core.

It had been theorized that the rotational momentum of the reactor's steam turbine could be used to generate the required electrical power to operate the ECCS via the feedwater pumps. The turbine's speed would run down as energy was taken from it, but analysis indicated that there might be sufficient energy to provide electrical power to run the coolant pumps for 45 seconds. This would not quite bridge the gap between an external power failure and the full availability of the emergency generators, but would alleviate the situation.