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    • Source: Per cent mille

      • A per cent mille or pcm is one one-thousandth of a percent. 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".


      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:




      ρ
      =
      (

      k

      eff


      −
      1
      )

      /


      k

      eff




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


      where keff denotes the effective multiplication factor for the reaction. Therefore, one pcm is equal to:




      1


      pcm

      =
      ρ
      ⋅

      10

      5




      {\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.


      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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