Application of MCNP nonanalog techniques for calculations of reaction rate measurements at the BFS facilities

An analysis of measurements of criticality, central reaction rate ratios, reaction rate distributions and reactivity coefficients performed at the BFS critical assemblies is important both for obtaining information required to refine neutron data and assessing the accuracy of neutron-physical characteristics. The high core heterogeneity of critical assemblies dictates the need for using codes capable of reproducing an accurate description of the geometry and detailed representation of the energy dependence of neutron data. At the same time, serial verification tests of evaluated nuclear data libraries and processing and transport codes require significant computer time to obtain results of sufficient precision. In such cases, it is possible to achieve the required accuracy in measuring neutron-physical characteristics by using non-analogous calculation methods (variance reduction methods). The paper presents the algorithms for improving the efficiency of calculations based on non-analogous methods implemented in the MCNP code as exemplified by an analysis of experiments on measuring reaction rates in critical multiplying systems with a complex heterogeneous composition. The results of the analysis have shown that the combination of mesh-based weight window with energy splitting/roulette leads to a significant increase in the calculation efficiency and a reduction of computational time from a few days to a few hours at a statistical error in the spectral indexes of less than 2%.