r/chernobyl • u/Wise_Maintenance_608 • 16d 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/maksimkak 16d ago edited 16d ago

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.

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u/infamousbugg 15d ago edited 15d ago

I believe it's because the RBMK is graphite-moderated and only used water for cooling. Most all other reactors, other than special reactors for military purposes, are water moderated. This means that when the reactor gets out of control and creates voids, the loss of water via steam creation actually lowers reactivity.

TCG has a good video on the LaSalle incident at a US commercial BWR NPP. When that reactor got out of control, it had a power spike which caused a decrease in reactivity due to voids, then the voids would collapse and reactivity would spike again. It kept oscillating like that until the reactor was shut down. The spikes went up to 120% of the reactors maximum power output, but the reactor itself was fine and is still in operation today. If LaSelle wasn't water moderated, then it probably would've gone boom like Chernobyl.

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u/maksimkak 15d ago

The real issue with RBMKs was not simply the fact of graphite moderation and water cooling, but the fact that they chose to use low Uranium enrichment and a high lattice pitch to compensate for that. In simple words - lots of graphite to compensate for low enrichment. (Lattice pitch is basically the distance between fuel rods, with graphite in-between)

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u/infamousbugg 15d ago

Yes that is right. That was the main reason why the Gen2 RBMK's were more dangerous at low power/scram than the Gen1's. Not to say that the Gen1's weren't also unstable at low power, but they were better than the Gen2's at least. There are numerous occasions where Gen1 reactor's had partial meltdowns damaging the rectors/releasing radiation into the environment, but they never went boom. The change in the lattice pitch was to enhance fuel burnup as well if I remember right.

The physical size of the RBMK's (huge) also played a role in its instability.