Reactivity coefficients (Doppler, moderator temperature and void)

The feedbacks that describe how a reactor's reactivity changes on its own as conditions change, without any operator action. The Doppler, or fuel temperature, coefficient: hotter fuel absorbs more neutrons in uranium-238 without fissioning, which pushes reactivity down almost instantly; it is negative in fuel with a large share of uranium-238. The moderator temperature coefficient: in a water-moderated reactor, hotter water is less dense and slows neutrons less well, which usually lowers reactivity. The void coefficient: what happens to reactivity if coolant boils or is lost. It is negative in light-water reactors, where losing water removes moderator. It was large and positive in the RBMK reactors at Chernobyl, where boiling raised power, which caused more boiling, and that contributed to the 1986 accident. CANDU heavy-water reactors have a small positive void coefficient that their control and shutdown systems are designed to handle, and large sodium-cooled fast reactor cores can have a positive sodium void effect, which designers reduce with core shape and rely on other, negative feedbacks such as fuel and structural expansion to outweigh. None of these signs is automatic: they depend on the specific design and on where the core is in its fuel cycle, which is why a passive safety claim should come with the actual numbers.

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concept:reactivity-coefficients

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