Per cent mille
{{Short description|One-thousandth of a percent}}
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A per cent mille or pcm is one one-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, TN: 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 {{nowrap|"per 100 000".}}{{Cite web|url=http://cdc.gov/foodnet/reports/data/infections.html|title=Number of Infections and Incidence* per 100,000 Persons | FoodNet | CDC|date=28 September 2018}}{{Cite web|url=http://apps.who.int/gho/data/node.main.VIOLENCEHOMICIDE|title = GHO | by category | Homicide - Estimates by country}}
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:{{cite web |url=http://mafija.fmf.uni-lj.si/seminar/files/2012_2013/Reactivity_Measurements_(Vid_Merljak).pdf|title=Reactivity measurements|last=Merljak|first=Vid|publisher=University of Ljublj|access-date=September 17, 2017}}ana
:
where keff denotes the effective multiplication factor for the reaction. Therefore, one pcm is equal to:{{cite web|url=http://www.nuclear-power.net/nuclear-power/reactor-physics/nuclear-fission-chain-reaction/reactivity/|title=Reactivity|website=nuclear-power.net|access-date=September 17, 2017}}
:
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.{{cite web |url=http://www.nuclear-power.net/nuclear-power/reactor-physics/nuclear-fission-chain-reaction/reactivity/percent-mille-pcm-unit-of-reactivity/|title=percent mille – pcm – unit of reactivity|website=nuclear-power.net|access-date=September 17, 2017}}
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
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