Each nuclear fission produces several neutrons that can be absorbed, escape from the reactor, or go on to cause more fissions in a chain reaction. When an average of one neutron from each fission goes on to cause another fission, the reactor is just barely "critical" and the chain reaction proceeds at a constant power level. Most neutrons produced in fission are "prompt", i.e., created with the fission products in less than about 10 nanoseconds. But certain fission products produce additional neutrons when they decay up to several minutes after their creation by fission. These delayed-release neutrons, a few percent of the total, are key to stable nuclear reactor control. Without delayed neutrons, in a reactor that was just barely above critical, reactor power would increase exponentially on millisecond or even microsecond timescales – much too fast to be controlled. Such a rapid power increase can also happen in a real reactor when the chain reaction is sustained without the help of the delayed neutrons. This is prompt criticality, the most extreme example of which is an exploding nuclear weapon where considerable design effort goes into keeping the core deep into prompt criticality for as long as possible until the greatest attainable percentage of material has fissioned. By definition, a reactivity of one dollar is just barely on the edge of criticality using both prompt and delayed neutrons. A reactivity less than one dollar is subcritical; if not already one, the power level will decrease exponentially and a sustained chain reaction will not occur. Two dollars is defined as the threshold between delayed and prompt criticality. At prompt criticality, on average each prompt neutron will cause exactly one additional fission, and the delayed neutrons will then increase power. Any reactivity above $1 is supercritical and power will increase exponentially, but between $1 and $2 the power rise will be slow enough to be easily and safely controlled with mechanical control rods because the chain reaction partly depends on the delayed neutrons. A power reactor operating at steady state will therefore have an average reactivity of $1, with small fluctuations above and below this value. Reactivity can also be expressed in relative terms, such as "5 cents above prompt critical". While power reactors are carefully designed and operated to avoid prompt criticality under all circumstances, many small research or "zero power" reactors are designed to be intentionally placed into prompt criticality with complete safety by rapidly withdrawing their control rods. Their fuel elements are designed so that as they heat up, reactivity is automatically and quickly reduced through effects such as doppler broadening and thermal expansion. Such reactors can be "pulsed" to very high power levels for a few milliseconds, after which reactivity automatically drops to $1 and a relatively low and constant power level is maintained until shut down manually by reinserting the control rods.
