The preceding sections were concerned with the processes in a facility that were placed under computer direction There remains the question of how extensively should the control system cover the full range of operating conditions

If our only worry were to keep a reactor plant going in stable fashion at full power, it would be difficult to justify computer control The task of regulating processes under minor perturbations usually can be done adequately by analog controllers Even operation at different predeter mined power levels can be provided, although m a large plant this begins to reach the practical limit of analog capability An example is automatic power reduction, sometimes called “power setback,” where several combina­tions of out-of limits measurements can automatically cause the plant to go to a selected lower power, thus avoiding the stresses of a full scram and the consequent restart troubles

The greatest need for computer control arises during operation at reduced or changing power levels Changes in power level may be planned, as in start-up or shutdown, or unforeseen, as in recovery from transients and response to equipment failure The superiority of a computer-based system under such conditions is due to one or more of the following

1 There are a large number of interdependent proce­dural steps to be taken that a computer can execute in considerably less time and with a lower probability of erro than with human operators using conventional controls Reactor start up and steam turbine run-up are in this class 1 0 Both are being included in current reactor plant designs

2 The controlled variables are changed at different operating levels One way to start up a reactor is to maintain a fixed and safe period up to about 1% of power, then to raise the level while keeping below a predetermined maximum power rate of rise to avoid damaging thermal stresses, and finally to control at full power with power level as the input to the control program The decision­making ability of the computer can be used to make these changes in control organization at the optimum points in the ascent-to-power routine

3 Control elements, processes, and sensors are non­linear Fixed controller settings can provide good regulation over a limited part of the plant operating range but produce inefficient or unstable behavior at others Again the ability of the computer to make decisions allows it to adjust its

own transfer function automatically to fit changing plant characteristics and to provide nearly optimum control under all conditions

4 Corrective actions often require a faster speed of response than human operators can provide Automatic power reduction is a good example The combination of fast logical analysis of a large amount of data and the ability to take quick remedial action permits the computer system to reduce reactor power or to shut down the plant in a controlled manner and to avoid situations that would cause reactor scrams This results in two very real benefits (1) reducing reactor scrams lessens the chance of damage by thermal and mechanical stresses, thereby reducing mainte­nance and increasing the useful plant lifetime, and (2) lowering power only as far as is necessary allows rapid recovery to full power and raises the plant factor