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]

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

where k_eff denotes the effective multiplication factor for the reaction. Therefore, one pcm is equal to:[3]

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

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.[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

1. 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
2. Merljak, Vid. "Reactivity measurements" (PDF). University of Ljublj. Retrieved September 17, 2017.ana
3. "Reactivity". nuclear-power.net. Retrieved September 17, 2017.
4. "percent mille – pcm – unit of reactivity". nuclear-power.net. Retrieved September 17, 2017.