[en] The development of rational methods of nuclear reactor state diagnostics is an important trend in a safaty control of NPP operation. The reactivity is the main indicator of nuclear reactor state. The reactivity determination is rather a complex problem, solved using analog and digital computers. The theoretical questions are discussed, related to mathematic model selection on the basis of which th device for reactivity determination so-called rectimeter is designed. A basic attention is paid to technical questions, related to the working-out and creation of the AIMR-8-2m an analog meter of instantaneous reactivity. The principle possibility of use of this device for determination of nuclear reactor global reactivity is shown. The expression for reactivity transition function in case of one-group approximation by delayed neutrons is obtained. The theoretical and practical questions on modeling of nuclear reactor kinetics. 10 refs.; 11 figs