The neutron spectra of research and power reactors are compared. The spectra were measured by the neutron-activation method and calculated using the KASKAD computer code. The a priori spectrum in the calculation was constructed as a superposition of physically validated spectra. A method of calibrating in-reactor detectors in nuclear power plants on the basis of the sensitivity to the 235 U fission rate in 1 g of uranium using the neutron fields of research reactors is proposed.The use of the neutron fields of research reactors for calibrating the detectors in the standard apparatus of high-power nuclear reactors has been described in [1]. As a rule, such detectors are calibrated according to the sensitivity to neutrons, which is insufficient for solving certain scientific-technical problems [2][3][4]. Specifically, to determine the power release in a reactor core it is necessary to calibrate in-reactor detectors, for example, direct-charge detectors [5], according to the sensitivity not only to neutrons but also to the fission rate of 235 U per 1 g U [6].The objective of the present work is to compare the 235 U fission rate in the neutron fields of research reactors and the RBMK-1000 power reactor and to investigate the dependence of this rate on the ratio of the thermal and epithermal neutrons in the spectra of the reactors studied. For this, the spectra of neutrons in the cores of seven research reactors and two RBMK-1000 channel reactors in the No. 2 unit of the Leningrad nuclear power plant were investigated.Neutron Field of the Kvant Critical Stand [2]. The 1410 mm in diameter and 500 mm high core in this stand consists of 175 fuel assemblies, placed in a water moderator. Ninety four compensation and absorbing rods are placed along the perimeter; a vertical IK-6 experimental channel with inner diameter 85 mm and wall thickness 3 mm is placed near the steel vessel of the stand. The neutron spectrum in the IK-6 channel was measured at the level of the center of the core of the critical stand at power 1 kW.Neutron Field of the F-1 Reactor [1, 3]. The spherical core of the F-1 graphite reator is 6 m in diameter and is surrounded by a 1 m thick graphite reflector. It is assembled from graphite blocks into which are inserted 35 mm in diameter and 100 mm high metal cylindrical blocks of natural uranium. The total uranium mass in the reactor is 48 tons and the graphite mass is 430 tons. The neutron spectrum was measured at core center in the horizontal central experimental channel at reactor power 24 kW.

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