In the 1980’s, the Research and Development I nstitute of Power Engineering (RDIPE) together with the Institute of Physics and Power Engineering (IPPE) has developed a number of reactor designs (with a capacity from 20 to 300 MW) for the Nuclear District Heat Plant. Fig.2 shows the schematic diagram of a reactor, and Table 1 incorporates the main parameters of the RKM-150 unit, for which the technical design and extensive calculational and experimental investigations were performed. Alongside with the concept consisting in sectioning the primary circuit into small volumes, the MM — based reactor safety is guaranteed by a set of other factors:

— Self-protecting features of the reactor: negative power, temperature and steam reactivity coefficient: the stability of the power generation field; — high heat capacity of reactor; — natural circulation of water in the primary circuit.

— The availability of safety systems: — reactor shutdown by introducing absorbers under the action of the force of gravity; — the micromodule replenishment with water during the vessel rupture from the pressurizer (passive) and then by pumps; the circulation of the secondary circuit with water from hydroaccumulators (passive) and then by pumps; heat removal from the MM by the evaporation of water from tanks (passive) via the water cooling tower (active) or to the network circulation loop (active).

— By additional technical solutions: moderate parameters (the heat fluxes of fuel elements, pressure, temperature); independent cooling system for absorbing rods; a good thermal contact between the MM vessel and graphite; pressure relation in circulating loops Pj >P2<P3 , etc.