Reactivity

A measure of how far a reactor's chain reaction is from exactly holding steady: how much the effective multiplication factor, k, differs from 1. Positive reactivity means power is rising, negative means it is falling. It is quoted in pcm (per cent mille, a thousandth of a percent) or in dollars, where one dollar equals the delayed neutron fraction: a reactor given a dollar of reactivity or more is prompt critical. Reactivity coefficients describe how reactivity responds automatically to conditions, such as fuel or moderator temperature or coolant voids, and a coefficient that responds by pushing power back down is what a claim of 'negative feedback' or a 'self-regulating' reactor rests on.

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