A basic reactor control system is shown schematically in Fig. 7.1 During operation at power, the demand signal is an output of the plant control system. This demand signal is compared with the measured neutron level, and the reactivity is correspondingly adjusted by programming the control-rod actuators to increase or decrease reactor power.

In reactors that are inherently stable, it is possible, though not necessarily preferable, for an operator to keep the power at the demand level by manually adjusting the control-rod position. When a reactor is not inherently stable, a continuous feedback control system, usually a servo-controlled rod, is essential.

There are four distinct phases of reactor operation the approach to criticality, power increase or decrease, power operation, and shutdown Each phase imposes different requirements on the reactor control system.

7- 2.1 Approach to Criticality

The reactor is manually controlled by an operator during the approach to criticality. The control rods are

moved intermittently to add reactivity until the reactor is critical. At this juncture the reactivity is zero and the rate of neutron production from fission is just equal to the rate of neutron loss The magnitude and rate of rod motion is governed by the need for maintaining the reactor period longer or slower than some predetermined value

The following requirements are imposed on the control system in this phase of reactor operation

l. The control rods must be capable of motion in increments small enough to insert very low values of 5k. Rod drives subject to uneven motion, e g, because of fric­tion, must be avoided

2 The rod actuation system must be capable of measur­ing necessary reactor performance data during the initial start-up. Typical measurements required are total and in­cremental rod worths. (Ganging several control rods which are then driven by one actuator sometimes makes this measurement difficult.)

3. The control system must be able to insert reactivity at a maximum rate consistent with the start-up time requirements.