To complete a description of the extent of plant automation, one must discuss the proportion of facility operation that is under automatic control This is here called “scope” and comprises two distinct aspects (1) the physical portion of the facility involved and (2) the number of different plant operating conditions included

Reactor safety circuits (see Vol 2, Chap 12) are not treated in this chapter since they are not considered a part of the control system

8- 4.1 Conventional Processes

A reactor facility can usually be divided into two parts according to whether or not the operation of the compo­nent processes is markedly affected by using a nuclear reactor for a source of heat Thus the generators, turbines, and steam loops in a pressurized water-reactor power­generating unit could be considered conventional equip­ment Their operation is not substantially different from that of a fossil fuel plant, so automatic control systems that have proven successful for this part of a nonnuclear station can be applied to a nuclear plant Nevertheless, extensive computer control of only the conventional part of a nuclear plant is uncommon There are several interrelated reasons for this (1) computers are installed primarily for data acquisition and display, (2) basic feedback control by computer to date has shown little or no cost advantage over analog controllers, and (3) the more serious control problems involve complex modes (such as multivariable, feed-forward, or cascade) which include controlling a part of the nuclear plant as well, again introducing concern by the designer over acceptance by licensing authorities

8- 4.2 Nuclear Processes

When computer control extends into the nuclear part of the facility, the engineer becomes involved with design procedures not common to a conventional plant Radiation and radioactive materials reduce access to many compo­nents of the control system and constitute potential hazards that demand extra care in system design and strict conformance to safety criteria The division between conventional and nuclear processes is made to emphasize those control functions requiring the most thorough reli ability analysis 9

8- 4.3 Auxiliary Processes

During the early planning of a process computer system, the following question arises “How much of the facility is going to be under computer control5” One must decide whether or not to include equipment not directly related to the main plant process, such as coolant makeup units, coolant-purification loops, and standby power plants Added functions of this kind are not being included in current plant designs The hardware cost savings are outweighed by the expense of greater system size and complexity

An exception to the above is the control of a certain class of on-line instrumentation comprising coolant samplers, chromatographs, mass spectrometers, neutron — flux scanners, etc These are characterized by their need for precisely timed control signals with a resulting data input to the computer The control signals are normally of a fixed-sequence kind, independent of the reading of the instrument, and do not effect closed-loop control actions in the usual sense

A second exception is monitoring and procedural control of reactor refueling operations If so programmed, the control system keeps track of the reactivity status of the reactor at all times Given the incremental reactivity of a replacement fuel element, the computer can predict the new reactivity status at each stage in the refueling sequence This, coupled with a step-by-step comparison between fuel-handling machine control actions by an operator and a prescribed checklist in the computer, can substantially improve the charge—discharge process