During the first charging of a nuclear reactor with fuel, as well as in experiments at the physical power level and during the initial use of the reactor at full power, safety is largely determined by the incorporation of reasonably sensitive and reliable measuring instruments in the neutron-power monitoring system. The sensitivity of the standard starting equipment of power reactors is insufficient for a number of reasons (the difficulty of creating a powerful starting source, the high level and broad spectrum of industrial noise). The laboratory adjustment of the standard apparatus is ineffective, since under laboratory conditions it is hard to reproduce all forms of interference. The creation of screened rooms for the standard measuring apparatus, which involves extra laying of cables, the use of independent supply sources, and so on, are not justified because of the "one-off" nature of the experiment, since in the subsequent power mode of operation the noise contribution is insignificant. One alternative is the development of additional channels in the starting apparatus, the sensors being disposed in nonstandard places. The reactor shaft might provide such a place in the case of the BN-350 reactor [i, 2]. The purpose of the additional channels was to monitor the power of the reactor during all manipulations involving the active zone. Special attention was paid to the creation of highly-stabilized current instruments, which have a number of advantages under the condition of low ~ -fields encountered in the physical initiation of the system. In developing a reactivity meter, allowance was also made for the various requirements of engineering psychology; the principal such requirement as regards "man-machine" systems is to present the information in such a way that the operator may correct any deviation from the norm quickly and easily. To this end a small number of basic instruments have been created such as to present the information in a simple, sometimes mnemonic, form. More and more use is now being made of command instruments, an obligatory condition of which is simplicity of representation of the operating mode monitored, usually in analog form. A correct solution of the problem is one in which the operator holds the process within specified limits by acting upon one executive device and using information from one command instrument.