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Understating natural intercellular circulation in the core during fault situations and transitional regimes results in suboptimal designs of pool reactors. For RBT reactors at the Research Institute of NuclearReactors, it is shown that instead of using structure elements specially intended for organizing a natural circulation loop the natural intercellular circulation can be fully taken into account. In addition, these structural elements themselves carry additional failure-associated risks. The results of a computational analysis of the development of pre-accident situations associated with the disruption of forced circulation of coolant through an RBT core and equipment failure are presented.The advantages of the pool reactors RBT-6 and RBT-10/2 are high reliability, due to negative reactivity effects, and the impossibility of water draining out of the core [1]; the disadvantages are the absence of a hermetic protective shell and direct contact of first-loop coolant with atmospheric flow directed into ventilation. This is why special attention is being focused on the heat-engineering reliability of a reactor during normal operation and in cases where pre-accident situations develop.One possible emergency is degradation of heat removal from the core. The designs incorporate a special cooling system based on the principles of natural circulation through a separate loop. In the RBT-6 reactor, a bridge (bypass) with a valve, where a shortened natural-circulation loop forms when the valve opens, is created on sections of the inlet and outlet pipelines adjoining the pool. This problem was solved in RBT-10/2 by placing a natural-circulation valve on the active oxygen quencher located in the pool. In neither case is the natural intercellular circulation regarded as an independent safety element.The downside of adopting additional elements is that the number of possible failures increases. Specifically, there appeared a danger of the RBT-10/2 core being by-passed through a fully or slightly open natural-circulation valve. In RBT-6, it is not the valve that presents a danger but the bypass section of the pipeline connecting the inlet and outlet connections and creating thermal stresses because of the temperature difference in the first-loop.The RELAP5/Mod3.2 thermohydraulic code was used to perform a computational analysis of the development of prefault situations associated with the disruption of forced circulation for the most dangerous initial events: for RBT-6 -complete cessation of forced circulation in a short time and for RBT-10/2 -unauthorized opening of the natural-circulation valve during operation at power [2].Computational Model. Special features of the reactors studied in the present work are hydraulic equivalence of core cells, nonuniformity of power release over the cross section and height, extended surface of fuel elements and absence of gas gaps between the cladding and fuel matrix.In slow circulation regimes, for example, during heating and cooling, transitional and emergency regimes in facilities with...
Understating natural intercellular circulation in the core during fault situations and transitional regimes results in suboptimal designs of pool reactors. For RBT reactors at the Research Institute of NuclearReactors, it is shown that instead of using structure elements specially intended for organizing a natural circulation loop the natural intercellular circulation can be fully taken into account. In addition, these structural elements themselves carry additional failure-associated risks. The results of a computational analysis of the development of pre-accident situations associated with the disruption of forced circulation of coolant through an RBT core and equipment failure are presented.The advantages of the pool reactors RBT-6 and RBT-10/2 are high reliability, due to negative reactivity effects, and the impossibility of water draining out of the core [1]; the disadvantages are the absence of a hermetic protective shell and direct contact of first-loop coolant with atmospheric flow directed into ventilation. This is why special attention is being focused on the heat-engineering reliability of a reactor during normal operation and in cases where pre-accident situations develop.One possible emergency is degradation of heat removal from the core. The designs incorporate a special cooling system based on the principles of natural circulation through a separate loop. In the RBT-6 reactor, a bridge (bypass) with a valve, where a shortened natural-circulation loop forms when the valve opens, is created on sections of the inlet and outlet pipelines adjoining the pool. This problem was solved in RBT-10/2 by placing a natural-circulation valve on the active oxygen quencher located in the pool. In neither case is the natural intercellular circulation regarded as an independent safety element.The downside of adopting additional elements is that the number of possible failures increases. Specifically, there appeared a danger of the RBT-10/2 core being by-passed through a fully or slightly open natural-circulation valve. In RBT-6, it is not the valve that presents a danger but the bypass section of the pipeline connecting the inlet and outlet connections and creating thermal stresses because of the temperature difference in the first-loop.The RELAP5/Mod3.2 thermohydraulic code was used to perform a computational analysis of the development of prefault situations associated with the disruption of forced circulation for the most dangerous initial events: for RBT-6 -complete cessation of forced circulation in a short time and for RBT-10/2 -unauthorized opening of the natural-circulation valve during operation at power [2].Computational Model. Special features of the reactors studied in the present work are hydraulic equivalence of core cells, nonuniformity of power release over the cross section and height, extended surface of fuel elements and absence of gas gaps between the cladding and fuel matrix.In slow circulation regimes, for example, during heating and cooling, transitional and emergency regimes in facilities with...
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