A dollar is a rooted measurement of the amount of neutron activity in a nuclear reactor. The cent is 1/100 of a dollar.
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Meaning and use
When nuclear fission occurs, neutrons are released in a stochastic process which is determined by the nature of the fissionable. Most of the neutrons are "prompt"; they are created at the same time as the fission products and are released in less than 10 nanoseconds (a "shake" of time). Others are released from the fission product nuclei by decay in anywhere from a shake to minutes (or even longer) after the fission. These delayed-release neutrons allow nuclear reactors to be controllable by humans and automated machinery, which cannot operate in less than millisecond reaction times. If there were no such delayed neutrons (all neutrons were prompt), then reactors that went even barely critical would multiply their reactivity rates into huge values much quicker than the reactor could possibly be controlled.
Devices which depend upon neutron-producing chain reactions use dollars and cents to describe the amount of reactivity margin within the device. Like the Fahrenheit and Celsius temperature scales, there are two points of reference. The zero point of a measurement of dollars is rooted at a particular value; in this case, the point where the fissionables in a nuclear device are producing just enough neutrons to balance the amount causing fissions with those which are lost by absorption or which are lost to the outside environment; the point of criticality. The region of reactivity less than zero dollars is termed sub-critical; the region between 0 and 1 dollar (100 cents) is critical controllable reactivity, where nuclear power reactors are designed to operate, while the region at the second point of reference, 1 dollar, and above is prompt critical; in the prompt critical regime, so many neutrons are being created that the fission process does not require delayed neutrons, and it becomes uncontrollable. Nuclear explosives and out-of-control reactors operate in this regime.
Like degrees of temperature, cents (and sometimes dollars) of reactivity are used relatively, as in "5 cents above prompt critical".
History
According to Alvin Weinberg and Eugene Wigner, Louis Slotin was the first to propose the name "dollar" for the interval of reactivity between barely critical and prompt criticality, and "cents" for the decimal fraction of the dollar.