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 from the calculation of Equation (3-5) is in units of . Alternative units for reactivity are % 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.            This unit of reactivity does not have a unique name. Special units for reactivity that do have unique names are dollars and cents. These units and their applications will be described in a later chapter.

Example:

Convert the values of reactivity listed below to the indicated units.

Solution:      

If the reactivity is known, the effective multiplication factor can be determined by solving Equation (3-5) for k_eff in terms of the reactivity. This results in the following relationship.

k_ef _

(3-6)

Reactivity must be in units of  for use in Equation (3-6).

Example:

Given a reactivity of , calculate k_eff. Solution: