# Per cent mille

Short description: One-thousandth of a percent

A per cent mille or pcm is one one-thousandth of a percent. It can be thought of as a "milli-percent". It is commonly used in epidemiology, and in nuclear reactor engineering as a unit of reactivity.

## Epidemiology

Statistics of crime rates, mortality and disease prevalence in a population are often given in "per 100 000".

## Nuclear Reactivity

In nuclear reactor engineering, a per cent mille is equal to one-thousandth of a percent of the reactivity, denoted by Greek lowercase letter rho. Reactivity is a dimensionless unit representing a departure from criticality, calculated by:

$\displaystyle{ \rho=(k_{\text{eff}}-1)/k_{\text{eff}} }$

where keff denotes the effective multiplication factor for the reaction. Therefore, one pcm is equal to:

$\displaystyle{ 1~\text{pcm} = \rho \cdot 10^5 }$

This unit is commonly used in the operation of light-water reactor sites because reactivity values tend to be small, so measuring in pcm allows reactivity to be expressed using whole numbers.