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Jul 19, 2022Liked by Jacob Bayless

> The actual operation of control rods in a nuclear reactor is a bit more complicated than a standard linear feedback controller, because rather than just directly subtracting neutrons, the control rod position modulates the (average) gain of the reaction feedback loop itself, so they can shift the reaction from being intrinsically stable to intrinsically unstable.

Nuclear reactors are also frighteningly complicated control systems because of delayed neutrons. The classic image of a bomb-like chain reaction is one of "prompt criticality," where nuclear fission produces new neutrons near-instantly that (can) cause more fission reactions, and so on. The timescales of this reaction are so quick that physical control would be impossible — nuclear core detonations are faster than the speed of sound in the core, after all.

However, nuclear reactors are saved through delayed neutrons; a small fraction of neutrons flying around the core come not from the fission itself, but from the decay of short-lived fission products. This timescale is still relatively fast (a few fractions of a second), but it's slow enough for control systems (passive or active) to maintain the reactor in a stable state. The reactor is "prompt subcritical" with respect to the direct fission neutrons, but held at criticality after the contribution of delayed neutrons.

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You explained what the distance along the real axis implies, but what about the distance along the imaginary axis?

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