Doppler Broadening
As fuel heats up, U-238 absorption resonances widen, capturing more neutrons. This is the primary stabilizing feedback in any reactor, but in the RBMK, the positive void coefficient overwhelms it.
Normal Operating Range
At 600°C, Doppler feedback provides 0.000$ of negative reactivity. This stabilizes the reactor against small power perturbations.
How Doppler Broadening Works
Uranium-238 has many narrow resonance peaks where it strongly absorbs neutrons at specific energies. As fuel temperature increases, thermal vibration of U-238 nuclei causes these resonance peaks to broaden. Broader peaks capture more neutrons across a wider energy range, reducing the number available for fission.
This is called Doppler broadening and provides instantaneous negative feedback, it acts as fast as the fuel heats up, without any delay from coolant or control rod movement.
The RBMK Problem
In a PWR, Doppler broadening is the first line of defense and is sufficient to stabilize the reactor during transients. But in the RBMK, the positive void coefficient (+5$ at full void) is far larger than the Doppler feedback can compensate. The net power coefficient remains positive at low power, meaning any perturbation is amplified rather than dampened.