Determination of the Neutron Fluxes Energy Spectrum of the WWR-SM Reactor of the INP AS RU

The article presents the results of analyzing the energy spectrum and spatial distribution of neutrons in the core of the WWR-SM (Water-Water Reactor, Serially Modernized) research reactor after switching to low enriched fuel (19.75 % 235U). The increase in the number of fuel assemblies (FAs) from 18 to 24 altered the neutron characteristics of the reactor. A combination of computational methods (IRT-2D and WIMS codes) and experimental data obtained from neutron activation analysis enabled a detailed study of flux distribution. Fast neutrons dominate in the central part of the core, while the proportion of thermal neutrons increases sig nificantly in the beryllium reflectors. Measurements showed that in vertical channels, the thermal neutron flux density is 2.3 times higher than that of fast neutrons. In horizontal experimental channels, values up to 1.8·1012 neutrons/(cm2·s) with a cadmium ratio of 28 were recorded, confirming their suitability for research. Analysis of the thermal power of FAs revealed its maximum values in the center of the core with a gradual decrease toward the periphery, correlating with the 235U burnup distribution. The obtained results have practi cal significance for optimizing fuel loading, planning refueling campaigns, and testing prospective fuel com positions (UO2+Al, U3Si2+Al). The study emphasizes the need for further verification of computational mod els and more detailed investigation of neutron spectra under various core configurations. The presented data contribute to enhancing the efficiency and safety of WWR-SM reactor operation while expanding its research potential in nuclear physics and materials science.