2.1 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Per cent mille | 1/1 | https://en.wikipedia.org/wiki/Per_cent_mille | reference | science, encyclopedia | 2026-05-05T11:52:18.992940+00:00 | kb-cron |
A per cent mille, or pcm, is a hundred-thousandth, or a 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 ==