The paper is devoted to studies on the influence of the sodium void reactivity effect (SVRE) on the safety and technical and economical characteristics of the BN-1200-type reactor. Different core options are considered for application to this reactor. These core options differ in design, dimensions, and, hence, SVRE value. It is shown by the analysis that the most flattened core with sodium plenum at the top assures reactor self-protection under beyond-design-basis accident conditions. Sodium plenum abandonment and core height increase causing an SVRE value increase deteriorate reactor self-protection, but at the same time, improve some technical and economical characteristics of the reactor. Self-protection means the possibility to avoid rapid core meltdown under conditions of the above-listed beyond-design accidents. The possibility of controlling beyond-design accidents (for instance, by restoring the power supply of the main pumps in a rather short time) is taken into account. Issues of choosing the optimal core design under these conditions are discussed.

Measures to decrease the sodium void effect of reactivity and the influence of this effect on the technicaleconomic performance and the safety of BN-1200 are analyzed. Three variants of the core structure differing by the structural implementation and dimensions are examined. It is shown that a flattened core with a sodium cavity, replacing the top end screen, gives self-protection of the reactor with respect to unanticipated accidents. The elimination of the sodium cavity and an increase of the core height result in degradation of the self-protection properties but at the same time improve the technical-economic properties of the reactor. The possibilities for optimizing the construction of the reactor from the standpoint of reaching a compromise between safety properties and the technical-economic characteristics are discussed.In the Soviet Union, after the Chernobyl accident, a requirement of zero integral void effect of reactivity was adopted for BN-800, which design was completed at the beginning of the 1990s. A new concept was developed to satisfy this requirement: a sodium cavity was to be organized above the core. A decrease of the density or removal of sodium from this cavity causes negative reactivity. It turned out that dimensions of the cavity that would ensure a negative or zero (integral) void effect of reactivity when sodium boils out of all fuel assemblies in the core can be found. Studies of accidents in BN-800 with the new core showed that the solution adopted is effective for increasing safety.A core with zero sodium void effect of reactivity is also proposed for advanced high-capacity sodium-cooled fast reactors which are now being designed. However, this makes it necessary to decrease the height of the core to 85 cm and, correspondingly, increase the radius, which creates difficulties in optimizing a high-power core, since the dimensions of the header plate and the rotating plug increase and the effectiveness of the control rods decreases. The sodium void effect of reactivity is important for unanticipated accidents with possible removal of a large quantity of sodium from the core.In this connection, discussions periodically arise about the desirability of a core with zero or close to zero sodium void effect of reactivity in advanced designs, especially since the normative documents on safety do not make the satisfaction of this condition mandatory. Rejection of this concept would make it possible to improve appreciably certain technicaleconomic characteristics of a reactor.Three models of the BN-1200 core, differing by the height of the core (85 and 100 cm), the diameter of the core, and composition of the top end screen (sodium cavity or breeding screen) and, correspondingly, the sodium void effect of

Along with mixed oxide fuel, the possibility of using in BN-1200 dense nitride fuel, making it possible to attain higher technical-economic performance, is also studied. However, safety analysis will determine the choice of fuel type. In this connection, it is important to perform a comparative analysis of the inherent safety properties for variants of the BN-1200 core with mixed uranium-plutonium and nitride fuel for the most serious unanticipated loss-of-power accident with failure of all emergency protection organs of the reactor simultaneously. A two-dimensional version of the COREMELT computer code was used in the calculations. The computational analysis showed that the inherent safety of BN-1200 is much greater with nitride than with mixed uranium-plutonium fuel.

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