Control rods are arranged in symmetrical patterns within the core structure and around the core periphery The total absorption capability in all the rods largely determines the shutdown reactivity of the core Shutdown reactivity is defined as the increase in reactivity required to bring the reactor to critical from a fully shutdown condition

Safety rods are specifically designed to effect rapid shutdown or scram should a hazardous reactor or plant condition occur These rods normally contain poison material and are withdrawn to a maximum degree before start-up and are kept in that position during power operation They are designed for rapid release and accelera­tion into the reactor Separate safety rods are seldom used in the present generation of power reactors, but their scram function is combined into the shim rods (Vol 2, Chap 12)

Shim rods comprise the greatest number of rods and control the greatest amount of reactivity Shim rods are used to remove shutdown reactivity during start up and to offset the effects of temperature, xenon, samarium, and fuel depletion during power operation Controlled shim-rod motion is very slow during power operation and may be only a few inches or tenths of inches per day Shim rods are usually arranged in groups or banks, only one of which can be moved at a time In any group, one or several rods can be moved at the same time, at the option of the operator

The regulating rod, similar in design to a shim rod, is used for fine control of reactor power level Small changes of reactivity are needed Regulating-rod motion may be

manually controlled, but usually the regulating rod is driven by a servomechanism in a feedback control loop that finely regulates power or flux To satisfy the performance requirements of automatic control, the drive mechanism for the regulating rod has more stringent performance require ments than the drives for shim or safety rods