The concept suggested consists in subdividing the primary circuit of a reactor in small volumes with minimal hydraulic connections between each other. This concept is realized by employing devices of special construction, the so-called micromodules (MM). A micromodular channel is a tubular pressure vessel (Fig. l) 100-200 mm in diameter, where a fuel assembly is positioned in its lower part and a heat exchanger (HE) (1-2 circuits) in the upper part. Using flow splitters, the natural circulation of coolant is maintained in the primary circuit of the MM. Each micromodule is provided with inlet and outlet pipelines for secondary circuit coolant and with a pipeline connecting the MM primary circuit with the pressurizer (P). Thus, from the viewpoint of the construction and thermal-hydraulic scheme, the MM is a miniature integral reactor. To provide a prescribed reactor power, the core is composed of the necessary number of micromodules which are inserted in the moderator (e. g., graphite, heavy water, etc.).

Qualitative analysis and quantitative estimations obtained in late 1970’s and early 1980’s revealed that based on the above mentioned concept, the creation of a reactor of enhanced safety is possible at the expense of the following factors:

— The subdivision of the primary circuit in a number of small independent volumes (up to 100 1), resulting in reduction of the scales and accident consequences due to the

depressurization in the primary circuit. In the event of the MM vessel rupture, the heat removal from fuel assemblies is accomblished due to water inflow from the pressurizer and then due to water supplied by feed pumps.

— The absence of circulation lines of large cross-sections in the primary circuit: in the case of the rupture of the small diameter (about 10 mm) pipeline connecting the MM with the pressurizer, the reliable cooling of fuel assemblies will be provided during a long-period of time at the cost of possible steam condensation in the-micromodule heat exchanger.

— The limited velocity and amount of coolant discharge while disrupting the primary circuit, makes the problem of the accident loca. lization within a plant easier.

— The compactness of the MM primar. y circuit creates ideal conditions for the natural circulation (NC) in the circuit in each flow regime.

— In the case of channel reactors, it is easier to solve the problem of organising independent (autonomic) cooling system for control rods in the MM-based reactors. This system can operate as an additional channel for heat removal in case of failure of major heat removal systems.

— In view of the fact that the MM power and correspondingly, its cross-sectional dimentions are dozens and hundreds times lower than the reactor power, there exists a real potentiality to conduct experiments on different operation conditions, accident conditions included, using electrically heated modules of the full scale hight.

The above given considerations stimulated the development of a reactor based on the proposed concept.