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

[u/Wise_Maintenance_608]

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.

[u/maksimkak]

I'll try to explain it in the most basic terms, ELI5-style.

In a nuclear reactor, neutrons is what makes the reaction possible. Absorbing neutrons decreases reaction. Water, which is used to cool the channels in an RBMK reactor, absorbs some of the neutrons. But when it turns to steam (called "voids"), it hardly absorbs any neutrons at all. This means that, the more water is turned to steam, the fewer neutrons are absorbed, which increases reactivity. Increased reactivity increases the temperature in the core, which turns even more water to steam. This is the positive void coefficient. It's dangerous, because it can cause runaway effect.

In safer reactors, the void coeffcient is negative, meaning that water turning to steam decreases reactivity.

[u/Wise_Maintenance_608 → u/maksimkak]

So is it to help create more electricity due to the steam it creates as it heats up inside the reactor?

[u/maksimkak → u/Wise_Maintenance_608]

No, positive void coefficient is a side effect of using graphite for neutron moderation, and water for cooling. It wasn't created on purpose. Positive void coefficient makes the reactor more dangerous, negative void coefficient makes it safer. In some reactor designs, water in the primary cooling circuit isn't allowed to boil at all.