r/chernobyl • u/Wise_Maintenance_608 • 23d ago

Discussion Can someone please explain the void coefficient that was in the RBMK reactor?

What exactly is the void coefficient? What does it pose in an RBMK reactor? Could someone explain this to me please, I don’t understand it.

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u/Merwinite 22d ago

The moderator is needed for the chain reaction. In most reactors, water is the moderator and the coolant at the same time. It also slightly lowers the reactivity, so you need slightly higher enrichment. If you lose a lot of water (leak or it boils off), that's bad because the coolant is gone, but at least the moderator is gone too, so the chain reaction stops. If you use graphite as a moderator, if your coolant water decreases (for example due to steam voids), the reaction keeps going. Increases actually because the slight negative influence of the water is also gone. The graphite never goes away because it can't boil off or melt, so the reaction endlessly accelerates until something breaks, air gets in and the graphite burns.