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The electricity-generating channel (EGC) is the main element of a thermionic reactor-converter that determines the efficiency and reliability of the power installation as a whole. Historically, two EGC circuits have been developed in parallel: a multiple-element assembly, consisting of several series-connected converters in one channel, and a single-element EGC with extended electrodes. Low Joule heat losses due to the longitudinal current are the main advantage of the first circuit, which permits operation at a high-density local current, approaching very closely to the optimal point of the local current-voltage characteristic. In the second circuit the Joule heat losses and the voltage drop on the electrodes are high. As a result, the hottest part of the EGC, which is in the central region of the reactor core, operates at a low current density and the operating point is at a considerable distance from the optimal point on the local current-voltage characteristic. The specific power and the efficiency of a long EGC, therefore, are substantially lower than those of a multiple-element assembly.While losing in regard to electrical characteristics, however, the single-element EGC has incontrovertible advantages: the possibility of running a trial of it and the entire power installation by replacing the nuclear fuel with an electric heater placed in the central cavity of the cathode, loading the fuel at the last moment before startup of the installation, simple and reliable removal of the fission gases from the fuel, and better outgassing conditions. As a result a power installation with single-element EGCs is more reliable and safer.Other EGC systems were proposed in the 1970s, including some with a plane electrode geometry, in which the Joule heat losses were minimized. But the level of the technology made them unfeasible to implement. A wealth of technological experience has now been gained in making EGCs with a cylindrical electrode geometry, which allows the previously proposed systems to be taken up again and implemented in actual designs.In this study we make a comparative evaluation of the efficiency of three types of cylindrical EGCs are distinguished by their switching circuit: a single-element long circuit with uniformly distributed switching along the entire length of the reactor core. The comparison made with the specific core of the Topaz-2 reactor-converter, with temperature inhomogeneities that are typical of each type of EGC and are caused by uneven heat release and heat leaks along switching elements. A maximum cathode temperature of 1820 K and anode temperature of 1000 K were common parameters in the comparison of the three systems.The EGC output characteristics and the distributions of the cathode temperature, current density, and electrode potentials were determined by numerically solving a one-dimensional differential heat equation (1) where q(x) is the heat flux from the fuel to the cathode; I c is the current flowing along the cathode; R c is the electrical resistance per unit length of cathod...