RBMK vs PWR
Apply the same perturbation, "coolant begins boiling", and watch the opposite responses. The RBMK amplifies; the PWR self-corrects.
The Key Insight
In the PWR, water is both moderator and coolant. When it boils away, you lose your moderator, the chain reaction slows and power drops. The reactor protects itself.
In the RBMK, graphite is the moderator and water is primarily an absorber/coolant. When water boils away, you still have graphite moderating neutrons but you've lost the absorber. Reactivity increases, power surges, more water boils, a positive feedback loop.
Why the Separation Matters
The RBMK's fundamental difference from Western reactors is the separation of moderator and coolant. In a PWR, losing coolant means losing moderation, an inherent safety feature called "negative void coefficient." In the RBMK, the graphite moderator remains regardless of coolant state, so losing coolant only removes neutron absorption.
After the Chernobyl accident, remaining RBMK reactors were modified to reduce the positive void coefficient (additional absorber rods, enriched fuel). But the fundamental design philosophy, separating moderator from coolant, remains a studied case in reactor safety.