A per cent mille, or pcm, is a hundred-thousandth, or a thousandth of a percent.SCALE: A Comprehensive Modelling and Simulation Suite for Nuclear Safety Analysis and Design. Available from Radiation Safety Information Computational Center at Oak Ridge National Laboratory as CCC-785. Oak Ridge, Tennessee: Oak Ridge National Laboratory, June, 2001. Version 6.1. ORNL/TM-2005/39 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
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:ana
where keff denotes the effective multiplication factor for the reaction. Therefore, one pcm is equal to:
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.
Related units Percentage point difference of 1 part in 100 Percentage (%) 1 part in 100 Per mille (‰) 1 part in 1,000 Basis point (bp) difference of 1 part in 10,000 Permyriad (‱) 1 part in 10,000 Parts-per notation including parts-per million, parts-per billion etc
See also InHour (another unit of reactivity) Dollar (reactivity) Parts-per notation Per-unit system Percent point function
Notes