In-core neutron sensors are most important because of the direct relation between the neutron-flux distribution and the thermal-power distribution in the reactor core.

Systems for determining neutron-flux distribution fall into two broad categories: systems using fixed sensors at a large number of fixed locations to provide data for generating one-, two-, or three-dimensional power — distribution information, and systems using traveling (mobile) neutron-sensing devices to provide a large number of neutron-flux scans of the core from which the desired power-distribution information can be derived. There are advantages and disadvantages to each system.

Fixed sensors can provide the operator with neutron — flux data at all times during reactor operation. They can also be adapted to sound an alarm or to control or protect against anv power-distribution anomaly that develops during the time interval between successive scans of a traveling sensor. Because the sensors are fixed in position, they must be made so they require no maintenance; in fact, generally, no maintenance or replacement can be performed on a fixed in-core sensor without shutting down the reactor. However, because fixed sensors are continuously exposed to the in-core environment during plant operation, they suffer radiation degradation or damage and must be replaced at planned intervals during refueling periods. Fixed sensors distributed throughout the reactor volume provide data at discrete points; data at all other points must be obtained by interpolation through curve fitting, usually with a computer (either on-line or off-line). The errors in the interpolated data depend on the sensor spacing and the precision of the computer curve-fitting routines.

Traveling or mapping flux-sensor systems, although unable to provide flux-distribution information at all times for alarm, control, or protection, can provide a spatially continuous flux plot along the entire path over which they travel. Traveling sensors thus can detect flux perturbations not picked up by fixed sensors, such as the flux distur­bances at fuel spacer grids and at the ends of control rods. Although these perturbations are seldom of great signifi­cance in reactor operation, since there is not much one can

do about them, the ability to sense them can lend confidence that the entire neutron-flux distribution is being observed, і e, an accurate one-dimensional picture of the real flux distribution is being observed The other two dimensions must still be filled in by interpolation through computer calculations unless, of course, there are traveling flux sensors in the other dimensions as well (which is not the case in present-day power reactors)

Although traveling or mapping neutron flux sensing systems incorporate motors and gear boxes that may require periodic maintenance, they are located where maintenance can be performed with minimum difficulty The flux sensors themselves may last the life of the reactor since flux maps are run only at relatively infrequent intervals and since the sensors are withdrawn from the core when not in use

All neutron-flux sensing systems measure the properties of the products of interactions between neutrons and the sensor materials (see Chap 2) When a neutron sensor is exposed for a long time to a neutron flux, its neutron sensitivity (output signal per unit neutron flux) usually decreases and its gamma sensitivity (output signal per unit gamma flux) remains unchanged This results in a steady decrease in the signal-to noise ratio with neutron exposure When the signal to-noise ratio decreases below a specified value, the lifetime of the neutron sensor is ended (by definition) For a given neutron sensor exposed to a mixed neutron and gamma flux, it follows that any design action that increases the initial value of the neutron to gamma signal ratio also increases the lifetime of the sensor