The reactor control is based on physical quantities which are measured with various accuracy, delay and reliability. The quantities of interest are usually: neutron flux, temperature of fuel, coolant and structures, coolant mass flow, coolant pressure, coolant activity.

Core-temperature measurements are rather unreliable in HTRs so that they can be used for providing information on the operating condition of the reactor, but cannot be relied upon for control and safety purposes.

The coolant outlet temperature measurement is sufficiently reliable and provides information about core temperatures, but with considerable delay.

This means that this measurement can be used for the regulation of slow transients, but cannot be used for counteracting fast excursions. The neutron flux measurement is very fast. The proportionality factor between the flux in the neutron detectors and the average core power may depend on the control-rod configurations so that it may be necessary to build a mean value over the signal of various detectors or place the detectors rather far away from the core.

Because of the great range of variation of the neutron flux between approach to criticality and full reactor power, it is necessary to have different sets of instruments each of which is used for a limited range. Usually three different sets are used. In order to follow the neutron flux variation during start-up it is necessary to have a neutron source in the core so that a neutron flux can be measured also in undercritical conditions. Particular problems may be posed to the instrumentation of HTRs in case of spatial instability. Informations on the spatial variations of flux and temperatures must in this case be obtained from measurements made from outside the core.