The time-shared nature of a process computer system makes it capable of doing nonessential off-line tasks at the same time that it is controlling the plant The central processor idle time could be used to compile and run FORTRAN calculating routines, assemble new subroutines, or modify and expand the process control programs 1 1 However outweighing the benefit of greater machine utilization are the following drawbacks

1 The operating speed of a nuclear-plant control system does not permit interchanging whole programs between core and drum (or disk), so the core memory must be larger

2 Greater keyboard—printer capacity is needed

3 A much more complex monitor is required

4 There is a finite probability that the time-shared systems software will derail the control program, although this probability can be made small through a computer memory-protect feature.

Taking all these into consideration, with nuclear safety to emphasize the last item, power-reactor control engineers have not included on-line program preparation as a system function An exception, discussed in Sec 8-5 4, is a configuration of redundant central processors where one is on standby and may be used by a programmer until interrupted to take over the control task 1 2

8- 4.6 Typical Applications

Examples of computer applications to specific power — reactor functions are given in the chapters on instrumenta­tion systems in boiling water reactors (see Vol 2, Chap 16, Sec 16-9), sodium-cooled reactors (see Vol 2, Chap 17, Sec 17-5 5), and gas-cooled reactors (see Vol 2, Chap 18, Sec 18-6 3)