Units of Reactivity

Reactivity is a dimensionless number. It does not have dimensions of time, length, mass, or any combination of these dimensions. It is simply a ratio of two quantities that are dimensionless. As shown in the calculation in the previous example, the value of reactivity is often a small decimal value. In order to make this value easier to express, artificial units are defined.
By definition, the value for reactivity that results directly is in units of Δk/k. Alternative units for reactivity are %Δk/k and pcm (percent millirho). The conversions between these units of reactivity are shown below.

Another unit of reactivity that is used at some reactors is equivalent to 10^-4 Δk/k. This unit of reactivity does not have a unique name. Special units for reactivity that do have unique names are dollars and cents.

If the reactivity is known, the effective multiplication factor can be determined by solving for k_eff in terms of the reactivity. This results in the following relationship.

(1-1)

Reactivity must be in units of Δk/k for use in Equation (1-1).