The power-increase phase is normally started by estab­lishing coolant flow, adjusting power and flow to equal the demanded values, and closing the automatic control loops As the coolant flow is then increased, the power level is also increased, and the reactor temperature rises to the operat­ing value. The reactor control system responds to the plant control demand by causing appropriate motion of the control rods. The coolant flow and power level may be
increased simultaneously or separately, with the flow reach­ing full value before power.

The primary requirements on the control system during this phase are

1 The control rods must smoothly adjust the reactivity in accordance with the plant controller demands. The amount of rod motion depends on the reactivity change associated with the incremental rod worth and the reactor temperature and pressure conditions. Particular attention must be given to the transient conditions during changes of reactor power and flow, since excessive overshoots in temperature or pressure might cause intolerable damage to the reactor core.

2. Since full-power conditions are being approached, the relative control-rod position becomes important to ensure uniform power distribution throughout the core.

6- 2.3 Power Operation Phase

During this phase the reactor must respond to the plant control-system demands to deliver the desired power, main­tain the reactor operating conditions, and remain within predetermined reactor parameter limits. The required dy­namic characteristics of the control system are different for each reactor type (PWR, BWR, and gas-cooled or liquid- metal-cooled reactors).

Some of the important factors that must be considered in the design of the control system for this phase of reactor operation are

1 Since the reactor in this phase is at or near full power, the control system must respond to the plant
power system demands rapidly enough to meet plant re­quirements and yet maintain the core temperatures within prescribed limits (such as the hot-spot temperature limit) To derive the maximum power, the designer faces a trade off between the desirability of operating close to the maximum temperature limits of the reactor and the as sociated requirements of more accurate temperature mea­surement and dynamic response of the control system

2 In addition to the requirement that the reactor rod-actuation system be accurately positioned m response to a demand, the banks of rods must also be positioned accurately relative to each other Inaccuracies in relative positioning of control rods causes local power increases in the region adjacent to those rods that are farthest with drawn Positioning inaccuracies can result in a smaller margin between the limiting temperatures and the normal operating conditions of the reactor core

3 Increments of control rod motion must be of such magnitude that thermal transients do not increase the temperatures or temperature gradients to undesirable values For example, the reactivity insertion steps resulting from unit rod motion in stepping-motor systems must be kept within allowable limits Excessive friction and control deadbands must also be considered in designing the overall control system

4 Some of the fission products produced during power operation absorb neutrons and necessitate the addition of reactivity by control-rod movement The most important of these are 13sXe and 149Sm (see Sec 1 3, Chap 1) The reactivity, 5k/k, to overcome the fission-product poisoning under equilibrium operating conditions is a function of reactor design but normally varies from 0 3 to 3 0% The control rods must be capable of compensating for the buildup of these neutron absorbers About 10 hr after shutdown, the 1 5 Xe poisoning increases to a value higher than the equilibrium value at full power The poisoning peaks at about 10 hr after shutdown and then diminishes This leads to the requirement that the control rods be capable of introducing sufficient neutron absorber at a rate that will maintain the reactor in a shutdown condition as the 1 35Xe poisoning is reduced

5 The reactivity must be compensated for the reduc tion of fissionable material attributable to burnup during power operation This compensation is normally provided by the control system, which automatically positions the control rods to maintain full power The control-rod worth designed into a reactor is a function of the amount of fuel depletion anticipated during the interval between reload ings In some reactors a burnable poison, such as 10B, is introduced into the core As neutrons are absorbed by the boron, the number of remaining boron atoms available for neutron absorption decreases The amount of burnable poison introduced into a reactor is governed by the desire to have this effect compensate for fuel depletion In this manner the total reactivity control requirement of the rod system is reduced

6- 2.4 Shutdown Phase

The shutdown phase of operation is usually accom plished by a controlled insertion of the control rods in response to a reduction in power demand A second type of shutdown is a scram[17] in which the reactivity and reactor power must be reduced in a short time to prevent exceeding the reactor or plant limits The control system must be designed such that sufficient negative reactivity is available to shut down the reactor under all conditions

To simplify the control rod drive system for normal power operation, the rate of control-rod motion is normally the same in either direction However, the rate of change of reactivity can still be varied significantly by using only one or a few rods at a time when increasing the reactivity and using all rods simultaneously when decreasing the reac­tivity