During the design and testing of a nuclear reactor, each component will be scrutinized to determine its reactivity worth, often at different temperatures, pressures, and control rod heights.
[2] Likewise, an InHour (inverse hour) is another small measurement of reactivity that takes into account the time of multiplication.
The unitless, pcm, percent, and inverse-time-based versions of reactivity can all be converted to dollars with the formula above and the InHour equation.
These delayed-release neutrons, a tiny fraction of the total, are key to stable nuclear reactor control.
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.
At or above 1$, the chain reaction proceeds without them, and reactor power increases so fast that no conventional controlling mechanism can stop it.
A reactivity less than zero dollars is subcritical; the power level will decrease exponentially and a sustained chain reaction will not occur.
Any reactivity above 0$ is supercritical and power will increase exponentially, but between 0$ and 1$ the power rise will be slow enough to be safely controlled with mechanical and intrinsic material properties (control rod movements, density of coolant, moderator properties, steam formation) because the chain reaction partly depends on the delayed neutrons.
[5] 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 (greater than 1$) 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.
[6] Subcritical reactors, which thus far have only been built at laboratory scale, would constantly run in "negative dollars" (most likely a few cents below [delayed] critical) with the "missing" neutrons provided by an external neutron source, e.g. spallation driven by a particle accelerator in an accelerator-driven subcritical reactor.