[en] Molecular aggregates are well known for their customizable optical properties. Vibronic coupling in monomers forming such aggregates offers rich opportunities for property tuning. We study generic molecular aggregate models of growing complexity (from a dimer up to a decamer) and report how vibronic coupling affects aggregate fluorescence intensity. The total aggregate fluorescence intensity is a measure sensitive to both vibronic coupling and Coulomb coupling between monomer transition densities. Using an exact diagonalization approach in the two-particle basis set, we show how the interplay between Coulomb and vibronic coupling affects aggregate fluorescence. Moreover, for H-aggregates we predict a periodic variation of the fluorescence intensity with aggregate size and show that vibronic interaction decreases the effect. (paper)

[en] A method for experimental determination of the relative power density distribution in a heterogeneous reactor based on measurements of fuel reactivity effects and importance of neutrons from a californium source is proposed. The method was perfected on two critical assembly configurations at the NARCISS facility of the Kurchatov Institute, which simulated a small-size heterogeneous nuclear reactor. The neutron importance measurements were performed on subcritical and critical assemblies. It is shown that, along with traditionally used activation methods, the developed method can be applied to experimental studies of special features of the power density distribution in critical assemblies and reactors.

[en] Results of critical experiments performed at five ASTRA facility configurations modeling the high-temperature helium-cooled graphite-moderated reactors are presented. Results of experiments on definition of space distribution of ²³⁵U fission reaction rate performed at four from these five configurations are presented more detail. Analysis of available information showed that all experiments on criticality at these five configurations are acceptable for use them as critical benchmark experiments. All experiments on definition of space distribution of ²³⁵U fission reaction rate are acceptable for use them as physical benchmark experiments. (authors)

[en] The application of experimental information on measured axial distributions of fission reaction rates for development of 3D numerical models of the ASTRA critical facility taking into account azimuthal asymmetry of the assembly simulating a HTGR with annular core is substantiated. Owing to the presence of the bottom reflector and the absence of the top reflector, the application of 2D models based on experimentally determined buckling is impossible for calculation of critical assemblies of the ASTRA facility; therefore, an alternative approach based on the application of the extrapolated assembly height is proposed. This approach is exemplified by the numerical analysis of experiments on measurement of efficiency of control rods mockups and protection system (CPS).

[en] The design of the ASTRA facility and critical assemblies that simulate physics features of modular high-temperature reactors (HTHR-Ms) with a graphite moderator and reflectors loaded with fuel particles having multilayer ceramic coatings is described in detail. Geometrical dimensions of the main elements and regions of the critical assemblies, composition of the materials used, and experimental results for various configurations of the critical assemblies are presented. A detailed computational benchmark model allowing for the structural and compositional features of the critical assembly configurations in question is developed on the basis of all the above data. The results are to be used for verification of the neutronics codes used for calculations of high-temperature helium-cooled reactors.

[en] The paper presents the results obtained from the computational/experimental studies of the spatial distribution of the ²³⁵U fission reaction rate in a critical assembly with an annular core and poison profiling elements inserted into the inner graphite reflector. The computational analysis was carried out with the codes intended for design computation of an HTHR-type reactor.

[en] The paper presents the results of experimental and computational study of flattening the spatial distribution of ²³⁵U fission reaction rate in a critical assembly ASTRA with the annular core and poison profiling elements placed in the inner graphite reflector near the boundary with core. Presented computational analysis of experimental data was performed with the set of codes used in HTGR design calculations. It was shown, that the power distribution flattening (²³⁵U fission reaction rate) in an annular core of a modular reactor such as GT-MHR could be achieved, in the certain degree, by the inserting the poison profiling elements in the inner reflector near the boundary with the core. The analysis of divergences of computational and experimental values of the ²³⁵U fission reaction rate shown that the maximal deviations lay in a range of 5-7 % excepting two points in the inner reflector for the 1A configuration, where value of deviation reaches 8 %. It should be noted that the error of the experiment does not exceed 5 %, so the calculational values only slightly exceed an experimental error. (authors)

[en] The paper presents computational analysis of some experimental results on spatial distribution of ²³⁵U fission reaction rates in a critical assembly with the annular core and different configurations of safety rods, placed into the inner reflector, made of graphite. Presented computational analysis of experimental data was performed with the set of codes used in HTGR design calculations. (authors)