Monitoring of power distribution in nuclear reactor cores

Abstract

One of the most characteristic features of modern reactor design is the steady growth of the overall capacity and specific power density of reactor cores; this requires operation of the individual fuel elements at close to their maximum permissible power and optimization of power distribution in the core for most economical and safe operation. This in turn demands accurate and reliable measurement of power fields in the core and continuous perfection of theoretical and practical monitoring techniques. Obviously, most accurate power distribution measurements require fitting of every single fuel assembly or fuel channel with a detector. However, such an approach is justified only if the detectors are placed outside the core as, for example, in thermal monitoring of channel-type nonboiling-water reactors. In most modern reactors the power distribution is measured with the aid of detectors located at discrete points inside the core. When using this technique one must take into account the fact that a large number of detectors, which are neutron absorbers and as a rule present an additional hydraulic resistance to coolant flow, is liable to deterioriate the reactor performance. In-core measurements meet with considerable difficulties as a result of the rigorous radiation, temperature, and other conditions under which in-core detectors must operate. This probably explains the relatively late appearance of reliable, in-core power-field measuring systems. The first systems for measuring neutron fluxes in reactor cores were based on periodic activation of indicators and subsequent counting of their induced activity outside the core with the aid of special semiautomatic devices [1-4]. Such periodic monitoring systems were later replaced by continuous monitoring systems with r~eutron or gamma detectors permanently left in the core (see Table I).