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The development of BN-1200 is based on the greatest possible use of tested and scientifically validated and developed technical solutions implemented in , and the BN-800 design as well as new technical solutions that increase facility cost-effectiveness and safety. The BN-1200 design must permit the reactor to operate with different cores, including with denser fuel. The main fuel variant considered is oxide fuel and for the nearest term nitride fuel, for which the production technology involves the same steps as the oxide technology. The main approaches for choosing the parameters of the BN-1200 core as well as the results of computational studies are presented.An advanced sodium-cooled fast reactor satisfying the requirements for nuclear power in the 21st century is now being developed in our country. Initially, 1800 MW(e) capacity was being considered for this reactor [1]. However, for many reasons, specifically, to ensure transportability of the main equipment by rail, its capacity was lowered to 1200 MW. The development of BN-1200 is based on the maximum use of the solutions implemented in BN-350 and -600 and in the BN-800 design as well as new technical solutions which increase cost-effectiveness and safety.BN-1200 Core. The main variant considered is oxide fuel and for the nearest term nitride fuel, whose production technology is largely identical to that of oxide fuel. Variants with heterogeneous introduction of regions with depleted metallic uranium are also considered. Metallic fuel does not satisfy the BN-1200 thermodynamic indicators which have been adopted. For fuel elements to work reliably with such fuel, the maximum coolant temperature must be lowered by approximately 100°C.The high burnup adopted for the fuel (16-18% h.a.) is attained by using in the fuel-element cladding EP-450 type ferrite-martensite steel; for this, the maximum cladding temperature in the design is limited by 670°C. For such fuel-element cladding temperature, the coolant heating in the reactor is lowered (140°C). This makes it possible to lower the maximum sodium temperature at the entrance to the fuel assemblies; this temperature is associated with the nonuniformity of the coolant heating in the core and the fuel assemblies. In the future, a transition is planned to low-swelling ferrite-class high-temperature steel, dispersion hardened by oxides of rare-earth elements.An important characteristic of the core is its volume heat density, since it actually determines the core size and critical load. Initially, a high heat density (~500 MW/m 3 ) was chosen for fast reactors. This choice was made to minimize the

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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