Measurement of the subcriticality of power-stressed reactors with an analog reactimeter

Conclusions A technique has been developed for measuring the effects of reactivity in a subcritical reactor with an analog reactimeter. It is based on the compensation of the current applied to the reactimeter input from the neutron detector. The compensation of current produced by neutrons of the subcritical multiplying assembly formalizes the algorithm for reactivity calculation, making it an adequate model of a reactor with a source and making it possible to determine the subcriticality without prior entry into the critical state. In this case the measurements are made in the presence of neutron sources characteristic of power-stressed reactors. The regular devices of the control and safety system could be used to produce unsteady variation of the neutron flux.All of this permits the proposed method to be extended to zero-power reactors and to power-stressed reactors. Once the subcriticality has been measured an analog of the neutron source is introduced into the reactimeter. This instrument measures the effects of reactivity in the subcritical state without the reactor being previously put into the critical state, monitors the entry of the reactor into the critical state by checking the reactivity, and makes all the measurements usually made with analog reactimeters. If the intensity of the source does not change during measurements (5–10 min) when chambers sensitive to rays (e.g., KNK-56 chambers) are used as neutron-flux detectors, then the accuracy of -ray compensation does not affect the results of the measurements.Translated from Atomnaya Énergiya, Vol. 45, No. 5, pp. 375–376, November, 1978.     

Monitoring the reactivity of extremely subcritical reactors by means of reactivity meters,corrections being made to the analog of the source

Conclusions The advantage of the instrumental methods here described lies in the fact that no additional apparatus is required for determining subcriticality; it is sufficient simply to use an analog reactivity meter allowing the source component in the solving part to be varied. By using these methods we may determine the bias voltage (constituting the analog of a steady-state source in the reactor) in the subcritical state. If the intensity of the source does not alter very sharply over the period of measurement (5–10 min), the introduction of the source function into the instrument is an on-off operation, after which the reactivity meter is able to monitor any changes in K_eff continuously without first bringing the reactor into the critical state. This offers the possibility of making quite accurate measurements to ensure nuclear safety. In cases in which it is required to secure especially accurate and reliable results, these measurements may be repeated as from the critical state as part of a total monitoring operation. The safety of operations requiring passage into the critical state is also increased, since the passage into the critical state is monitored completely.Instrumental methods of measuring subcriticality are universal; they embrace not only critical assemblies but also any energy-stressed reactors in the subcritical state, and enable us to measure temperature effects, effects of poisoning, and so on, i. e., effects characteristic of energy-stressed reactors. In addition to this, these methods may be applied both in the presence and in the absence of a strong neutron background in the reactor.Translated from Atomnaya Énergiya, Vol. 41, No. 4, pp. 238–241, October, 1976.