per cent mille
Dimensionlessexact by definition
Per cent mille is one part in one hundred thousand, exactly 1e-5. It is reactor physics' unit of reactivity, where the delayed neutron fraction of uranium-235 is about 650 pcm and a control rod worth or a temperature coefficient is quoted in pcm or pcm per degree. The factor is exact by definition.
Watch out: Reactivity is also quoted in dollars and cents, where one dollar is the delayed neutron fraction for the specific fuel and so is worth a different number of pcm in every core. Dollars are not convertible here, since the conversion is a property of the reactor rather than of the units.
| 1 pcm | 0.00001 — |
The name is a compound of percent and mille, meaning a hundredth of a thousandth. It exists because reactivity changes that matter operationally are far too small to read in percent and the unit needs to be countable in whole numbers on a console.