К J Monarty, F H Just, and L J Christensen 5-1 INTRODUCTION chapter contents

Neutron flux measuring channels in nuclear power plants are generally classified according to the neutron flux range involved

Start up channel 10° to 10s neutrons cm 2 sec 1

Intermediate channel 104 to 1010 neutrons cm 2 sec 1 Power channel 106 to 1010 neutrons cm 2 sec 1

The channels successively overlap one another (Fig 5 1) to provide continuous measurements of the neutron flux level at all power levels (see also Chap 2 Sec 2 2 3)

Control circuits are provided in conjunction with one or more of the channels Reactor power is varied by the action of circuits associated with the neutron flux measuring equipment High power operation is controlled with the

power channel The neutron-measuring systems must be capable of providing alarm set points and annunciator units to warn the reactor operator

In conjunction with the alarm features are the reactor — protection mechanisms When an alarm set point is ex­ceeded, appropriate automatic action must be taken to reduce the reactor operating power to a safe level The type of protective circuits and action (see Chap 12) varies from reactor to reactor, from a simple reduction in reactivity (cutback) to a scram where essentially all the negative reactivity available is inserted.

The neutron-measuring equipment and circuits must be designed to minimize reactor downtime caused by unit maintenance, component failures, and spurious noise alarms Redundancy and independent systems are used for each power-level channel