Per cent mille
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 nuclear reactor engineering as a unit of reactivity.
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:[2]
where keff denotes the effective multiplication factor for the reaction. Therefore, one pcm is equal to:[3]
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.[4]
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
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
- ^ Merljak, Vid. "Reactivity measurements" (PDF). University of Ljublj. Retrieved September 17, 2017.ana
- ^ "Reactivity". nuclear-power.net. Retrieved September 17, 2017.
- ^ "percent mille – pcm – unit of reactivity". nuclear-power.net. Retrieved September 17, 2017.
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