# Per cent mille

__: One-thousandth of a percent__

**Short description**A **per cent mille** or **pcm** is one one-thousandth of a percent.^{[1]} 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".^{[2]}^{[3]}

## 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:^{[4]}

- [math]\displaystyle{ \rho=(k_{\text{eff}}-1)/k_{\text{eff}} }[/math]

where k_{eff} denotes the effective multiplication factor for the reaction. Therefore, one pcm is equal to:^{[5]}

- [math]\displaystyle{ 1~\text{pcm} = \rho \cdot 10^5 }[/math]

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.^{[6]}

## 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

- ↑ 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, TN: Oak Ridge National Laboratory, June, 2001. Version 6.1. ORNL/TM-2005/39
- ↑ "Number of Infections and Incidence* per 100,000 Persons | FoodNet | CDC". 28 September 2018. http://cdc.gov/foodnet/reports/data/infections.html.
- ↑ "GHO | by category | Homicide - Estimates by country". http://apps.who.int/gho/data/node.main.VIOLENCEHOMICIDE.
- ↑ Merljak, Vid. "Reactivity measurements". University of Ljublj. http://mafija.fmf.uni-lj.si/seminar/files/2012_2013/Reactivity_Measurements_(Vid_Merljak).pdf.ana
- ↑ "Reactivity". http://www.nuclear-power.net/nuclear-power/reactor-physics/nuclear-fission-chain-reaction/reactivity/.
- ↑ "percent mille – pcm – unit of reactivity". http://www.nuclear-power.net/nuclear-power/reactor-physics/nuclear-fission-chain-reaction/reactivity/percent-mille-pcm-unit-of-reactivity/.

Original source: https://en.wikipedia.org/wiki/Per cent mille.
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