Stable Period Equation

For normal reactor operating conditions, the value of positive reactivity in the reactor is never permitted to approach the effective delayed neutron fraction, and the reactor period equation is normally written as follows.

(1-1)

Equation (1-1) is referred to as the transient period equation since it incorporates the term to account for the changing amount of reactivity in the core. The */ term (prompt period) is normally negligible with respect to the remainder of the equation and is often not included. For conditions when the amount of reactivity in the core is constant  and the reactor period is unchanging, Equation (1-1) can be simplified further to Equation (1-2) which is known as the stable period equation.

(1-2)