CONTROL RODSDOE-HDBK-1019/2-93Reactor Theory (Nuclear Parameters)Not all reactors have different control rods to serve the purposes mentioned above. Dependingupon the type of reactor and the controls necessary, it is possible to use dual-purpose or eventriple-purpose rods. For example, consider a set of control rods that can insert enough reactivityto be used as shim rods. If the same rods can be operated at slow speeds, they will function asregulating rods. Additionally, these same rods can be designed for rapid insertion, or scram.These rods serve a triple function yet meet other specifications such as precise control, range ofcontrol, and efficiency.ControlRodEffectivenessThe effectiveness of a control rod depends largely upon the value of the ratio of the neutron fluxat the location of the rod to the average neutron flux in the reactor. The control rod hasmaximum effect (inserts the most negative reactivity) if it is placed in the reactor where the fluxis maximum. If a reactor has only one control rod, the rod should be placed in the center of thereactor core. The effect of such a rod on the flux is illustrated in Figure 8.If additional rods are added to this simple reactor, the most effective location is where the fluxFigure 8 Effect of Control Rod on Radial Flux Distributionis maximum, that is, at point A. Numerous control rods are required for a reactor that has alarge amount of excess reactivity (that amount of reactivity in excess of that needed to becritical). The exact amount of reactivity that each control rod inserts depends upon the reactordesign. The change in reactivity caused by control rod motion is referred to as control rodworth.NP-03Rev. 0Page 50
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