Void Coefficient
When coolant boils into steam, what happens to reactivity? In the RBMK, it increases. In a PWR, it decreases. This single difference is why Chernobyl was possible.
Normal Operating Range
At 14% steam quality, the RBMK produces 0.70$ of positive reactivity from voiding. This is compensated by control rods during normal operation. The PWR sees -2.10$, inherently self-stabilizing.
Why the Difference?
In a PWR (Pressurized Water Reactor), water serves as both coolant and moderator. When water boils away, neutrons lose their moderator and the chain reaction slows. This is inherently safe, the reactor stabilizes itself.
In the RBMK, graphite is the moderator and water is primarily a coolant and neutron absorber. When water boils away, you lose the absorber but keep the moderator. Neutron absorption decreases while moderation continues , reactivity increases. More power creates more steam, which adds more reactivity, which creates more power. A runaway positive feedback loop.
The Chernobyl Connection
At 01:23:40 on April 26, 1986, the combination of low coolant flow (pump coast-down), a positive void coefficient, and the graphite displacer tip effect created a power surge from 200 MWt to over 30,000 MWt in less than 4 seconds. The positive void coefficient is the fundamental reason the RBMK design was unstable at low power, and why Western reactors, with their negative void coefficients, cannot experience the same catastrophe.