7- 3.1 System Requirements

The requirements that must be met by the reactor control system, including sensors and control rods, may be divided into the following categories

1 Amount of reactivity controlled and rod positioning accuracy

2 Rate of change of reactivity for normal operation, including initial start-up, planned shutdown, and restart

3 Emergency shutdown

4 Reliability

The excess or maximum reactivity requirements of a reactor are dependent on the planned rate of fuel depletion, fission-product buildup, inherent reactivity effects (e g, temperature), and control range desired Table 7 1 shows that the range of total rod reactivity for a number of typical power reactor plants varies from 6 to 25% in 5k/k

The control rod drive must be capable of making small changes in reactivity to maintain a flat neutron flux dis­tribution (uniform generation of heat throughout the core) and to be able to adjust the power level of the reactor with sufficient precision throughout full-power operation Typically, changes in 5k/k £ 10 s are required In terms of positioning accuracy, this means that the control rod drive system must be able to position a rod to an accuracy of about 0 02 in This value can vary widely, however, depend mg on the particular reactor design and the individual rod worth, values from 0 01 in to several inches are possible 2

The reactor designer determines the required rate of change of reactivity by examining how rapidly power must be changed to maintain proper operation during both

normal and emergency operation For conventional start up and power phases, periods of 30 sec or longer are normal This requires reactivity adjustments in the range of 10-3 to 10 5 Sk/sec, with an average value of 2 X 10 4 5k/sec * This same rate is usually satisfactory for steady-state con­trol For linear control rods, this corresponds to about 10 m /mm as an average, although it can vary2 (with reactor design) from 3 to 300 in /min For a directly coupled rotational system, where the control drum may rotate 180°, this rate corresponds to about 0 2°/sec During shutdown the reactivity (and thus the reactor power) is usually decreased more rapidly than its rate of increase during start up As noted earlier, this requirement is often satisfied by moving the control rods at the same rate used for start-up, but moving all rods simultaneously rather than a few at a time

For scram or emergency shutdown, the required rate of reactivity reduction normally exceeds the insertion rate for power control by a factor of 10 to 100 Higher rates of reactivity reduction do not yield significant benefits since, after the reactivity becomes about 1% below critical (p = 6k/k = — 0 01), the reactor power decreases as the delayed-neutrons decrease Of more importance in shut­down is the release time or turnaround time following receipt of a shutdown command Small delays in beginning the reduction of reactivity can result in significant power

•The expression 2 x 10 4 6k/sec means 2 x 104/sec = Sk/sec This notation is commonly used in nuclear engineering excursions The usual practice is to design for release times of about 10 to 50 msec When this rapid initiation is coupled with a reactivity insertion rate of about 5 X 10 2 5k/sec, the reactor can be shut down on a nominal negative period of 5 sec or less

It is common practice to satisfy both normal perfor­mance and safety requirements with one reactivity adjust ment mechanism It is also common practice to design the scram mechanism to operate in a fail-safe mode, і e, to operate in the event of loss of primary power (The primary-power loss may be inadvertent or it may be initiated by another part of the scram system ) There are many ways of designing the rod-drive mechanism to fail safe One general practice is to use gravitational force to store energy for scram A simple example of this practice is to place a coupling device between the control rod and its drive mechanism that has the same primary-power source as that which supplies the control-rod actuator If the primary power is lost, the control rod is released and gravity forces the rod into the reactor Springs and hydraulic devices can also be used to store energy and to release it on power failure If higher scram velocities are desired, a spring may be incorporated to increase the acceleration of the control rod into the reactor If springs are used, the actuator may be designed such that the rod is held against force, and, if the primary power is lost, the spring returns the control rod rapidly to the shutdown position The spring can also serve to eliminate backlash and thus reduce the deadband in the control loop

The reliability requirements for the control-rod drive system are influenced by considerations of safety and maintainability For safety reasons there must be a high level of confidence that the scram system will operate correctly, і e, it will be reliable (see Vol 2, Chap 12) The required confidence that this system will work is signifi­cantly higher than that required of the control system during normal operation In addition, the availability of the control-rod drive system is essential This means the system must be available for operation during all scheduled operational periods, except during normal preventive — maintenance periods when the reactor is shut down A failure in the control-rod drive system normally requires that the reactor be shut down in view of the possibility of unsafe operation (loss of control of output power or loss of scram capability) The reactor is then unavailable for the total time required to shut down, correct the failure, and restart Hence, unscheduled maintenance must be avoided Nuclear power plants are designed for a life of about 30 years, with scheduled shutdowns for maintenance at in­tervals of 6 to 18 months Since the control-rod drive systems can be serviced during the scheduled maintenance periods, their reliability requirements are correspondingly reduced In essence, control-rod drive systems must be highly reliable, but only for relatively short periods of time For an increase in overall reactor reliability, some systems are designed so a failure of one control-rod drive does not require shutdown In these systems failures may occur during operation, but corrective maintenance is not re­quired until the next scheduled maintenance period If unscheduled maintenance is required because a rod-drive mechanism fails and forces a shutdown, it is very desirable to minimize the time required for corrective maintenance This time can be reduced if the designer has considered this requirement during the initial design phase

The requirements discussed above result from nuclear design operational considerations and are applicable to reactors controlled through a primary control loop on nuclear power In establishing the requirements for a control-drive mechanism, the designer must first consider the reactivity span and the rate of change needed, these are used to calculate the desired period and steady-state operating conditions However, since an automatic control loop is normally used, the designer must also ensure that the control system can operate in a stable closed-loop manner The requirements on the control-drive mechanism that must be met to provide stable closed loop operation during all feasible transients and perturbations may be more severe than those for satisfactory period and steady-state operation These requirements are identified by dynamic control analysis of the complete reactor system

As shown in Fig 7 1, the control-rod position loop is usually an inner loop of the automatic power control loop Although the speed of response of the power-control loop is selected to provide the desired reactor performance, a dynamic control-systems analysis would indicate that the speed of response of the rod-position loop must be from 2 to 10 times more rapid to result in stable (nonoscillatory and nondiverging) operation when all control loops are closed

The basic power-control loop, as shown in Fig 7 1, is sometimes supplemented by a trim loop that controls the temperature of the primary-coolant flow This latter would be an outside loop on power control which compares a measured temperature with a demanded value and produces a supplemental power-demand signal More stringent re­quirements are placed on the drive mechanism when the reactor controllers are complicated by introducing coolant temperature, or variables from the steam side of the power plant loop, because of the interaction of these parameters and the normal dynamic requirement to make all inner loops respond about six times faster than outer loops

The requirements of the amount of reactivity con­trolled, the rod-positioning accuracy, reactivity rate of change under various situations, and the reliability and availability have been met in many power reactors by using low-maintenance, high-reliability a-c motors for the control-rod drive The coupling to the control rod is usually by a rack-and-pinion mechanism or a lead screw and nut However, other techniques have been used, including d-c motors, hydraulic cylinders and motors, linear induction motors, and magnetic jacks The designer must establish the requirements for a given reactor design and then review all available systems and components to select the most appropriate The advantages and disadvantages of a number of systems and typical applications are discussed in the following sections

7- 3.2 Means of Control

The control rods selected for water reactors are usually linear structures of a neutron-absorbing material designed to be moved vertically into and out of the core The amount of neutron absorber in the core is determined by the position of the rods with respect to the core At shutdown, the rods are positioned with the maximum amount of neutron absorbing material within the core As the rod is withdrawn from the core, the reactivity and neutron population increase by an amount generally pro portional to the amount of neutron-absorbing material removed

Although control rods of neutron-absorbing material are used in most reactor installations, in some instances neutron-reflecting and neutron-moderating or fuel-bearing rods are used Another type of control, a cylindrical device called a drum, has its surface comprised partly of neutron absorbing material and partly of neutron-reflecting ma tenal, or there can be combinations of absorber—fuel or absorber—moderator Several of these drums are located in a vertical position around the core periphery and have rotary control motion, the extent of drum rotation determining the core reactivity

Reactivity control by a neutron-absorbing liquid may be heterogeneous, with the liquid flow in a sealed pipe or pipe system adjacent to the reactor core, or homogeneous, with the liquid mixed with the reactor coolant water and extracted by ion-exchange equipment For example, mercury can be used in a heterogeneous system and boric acid can be introduced into the reactor coolant water system in a homogeneous system