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]

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gollark: Diversity of nulls for, what, multiple error signal purposes.
gollark: Which is a great* benefit.
gollark: All is floats none are safe.
gollark: Backward compatibility requires that some octachoron make it START that way.

See also

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.


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