The reactivity worth of control rods (control rod worth), pW, can be defined as

Pw = Po~Pi (1.49)

where p0 and p, are the system reactivity at control rod withdrawal and insertion, respectively. This definition is mainly used when measuring the insertion depth of control rods required to change the reactor power level. Another definition of control rod worth can be used to compensate for the reactivity change due to a long-time fuel burnup. In the former definition, there is no variation in physical properties of the core materials even though the control rods are inserted. Mean­while, the physical properties vary with the fuel burnup in the latter definition. But, it was eventually found that the two definitions of control rod worth were equiva­lent. It is now common practice to use the first definition [11].

The calculation of control rod worth is one of the most difficult problems in the core calculation of nuclear reactors. Recently, the control rod worth has been directly evaluated through numerical calculations with a three-dimensional core representation. Many theoretical evaluations were conducted in early designs because of limitations in calculation capability. One approach was to determine the reactivity worth by using the changes in the bucklings of the system with a boundary condition at the control rod surface [12]. Another one was to calculate the reactivity worth of a large number of control rods in a regular array or cruciform control rods by applying the Wigner-Seitz method [13]. At present, such theoretical approaches are seldom applied.