[en] An automatic system was developed which is maximally adapted to the construction and special features of operation with the Gidra pulsed reactor. The system includes irradiation units for the measurement position, dual pneumotransport equipment with track and pneumatic automation elements, and a control installation. The reactor and the systems are an optimum complex of apparatus for doing activation analyses. Practically any methodology can be used in automatic or manual analyses. When a large number of uniform samples are being analyzed, the throughput is high

[en] Neutron-physical characteristics of the cores of high-temperature helium-cooled reactors (HTGR) with fuel elements of spherical configuration and single passage of fuel elements through the core have been considered. The possibility of simulating the HTGR core characteristics for fresh fuel elements and fuel burnup in the process of campaign using critical assemblies of the GROG facility is substantiated numerically. The calculations are made using the programs: WIMS-D (neutron-physical calculation of cell for the region of neutron energies), NEKTAR(calculation of resonance effects in the systems with double heterogeneity), PIT (determination of space-energy distribution of slow neutrons in systems of arbitrary geometry, with account for slow neutron scatteering on graphite) and NEPAL (multigroup finite-differential calculation of reactor in the P₁-approximation). On the basis of the obtained data analysis the conclusion is drawn that for practical realization of the experiments discussed the elements of the following types are required: fuel elements containing homogeneous mixture of moderator and fuel of 10% enrichment, fuel elements containing homogeneous mixture of moderator and fuel of natural composition; graphite elements and elements containing natural boron

[en] Results are presented of investigations performed at the GROG stand critical assemblies for determining the effect of technological cavity between the HTGR upper face reflector and pebble bed core on the reactor neutron-physical characteristics and for checking the calculation methods and programs. The experiments are performed with 6 assemblies of different cavity heights (0; 25 and 50 cm) and composition of absorbing elements. In the course of experiments critical parameters, reactivity effects as well as spectral indices, plutonium coefficient, the sup(238)U capture-to-sup(235)U fission medium-energy microscopic cross-section ratio and neutron gas temerature have been determined. Calculation of the assembly parameters is performed in the framework of the diffusion approximation using two approaches: cavity description by means of effective diffusion constants and applying appropriate conditions at the cavity boundary. The data obtained confirm a substantial effect of the technological cavity on the HTGR phys ical characteristics, the effect being determined to a statisfactory accuracy and with fast responce using the calculation techniques and propgrams in the framework of the diffusion approximation

[en] The presence of an electron clearance between the upper end of the reflector and the core of a high-temperature gas-cooled reactor with spherical fuel elements has a considerable influence on the neutron and physical characteristics of the reactor. In order to investigate this effect and test design methods and programs, the authors carried out a series of experiments on cores using a test rig type GROG. Models were erected of the reactor with their central axes horizontal. A diagram is shown of core with 50-cm erection clearance. The relationship between the reactivity of the assembly and the height of the chamber where the shape and composition of the remaining zones remains the same is shown. The presence of the chamber and its size have a slight effect on the neutron spectrum in the core. The calculated values of spectral characteristics lie within 6% of the experimental results

[en] This paper presents an investigation of the physical characteristics on critical assemblies during the development of new reactors. High-temperature gas-cooled reactors (HTGR), under development in the Soviet Union and elsewhere, have many new aspects. The I.V. Kurchatov Institute of Atomic Energy has built a critical stand GROG which is used to study the physics of HTGR, i.e., the physics that is common to all modifications and the specific physics that takes into account the features of different versions of HTGR, primarily the VG-400. In the initial stage, when the full-scale elements are absent, the physics of HTGR are studied on critical assemblies formed of model elements. Since products of the reaction of the fuel with neutrons are difficult to use in assemblies, problems pertaining to the burn-up of fuel are also considered on systems containing model elements

[en] On the basis of data available on the KAHTER critical assemblies there have been fulfilled calculations of basic neutron physical parameters of the reactor, the core of which represents a pebble bed inventory of AVR reactor type fuel elements. Effective neutron multiplication factors and spatial neutron flux distributions have been calculated by the two-dimensional GABI code. The 26-group nuclear data library APAMAKO-F was used in the epithermal range. Neutron thermolization was taken account of by the special PIT code. The efficiency of the regulating rods has been computered by the PNK code. The comparison of data available on the KAHTER critical assembly basic parameters with the calculations has proved their adequacy

[en] Using calculation-experimental way, the possibility has been considered to employ, in experiments with critical assemblies, instead of high-temperature gas-cooled reactor fuel elements with microparticles, model fuel elements with cores of homogeneous mixture of uranium dioxide and fluoroplastic-4 (polytetrafluoroethylene). Comparative analysis of neutron-physical characteristics of fuel elements with microparticles, dispersed in graphite matrix, and those of model fuel elements is carried out. Results are presented of experimental evaluations of macroscopic cross sections of the neutron interaction with fluoroplastic-4 and data testing the uranium dioxide-fluoroplastic-4 composition samples for radiation stability and moisture absorption. The conclusion is drawn that fluoroplastic matrix substitution for graphite matrix does not affect neutron-physical parameters of the fuel elements. The change in the thermal neutron fission cross section does not exceed 0.6%. Mechanical properties of the uranium dioxide-fluoroplastic-4 composition meet the requirements for fuel elements being simulated

[en] The GROG critical assembly designed for studying high-temperature gas-cooled reactor (HTGR) physics and in the first place, for investigating the neutron-physical characteristics of the VS-400 fuel elements is described. The GROG critical assembly represents a set of graphite blocks forming cubic laying with 450 cm side. Placing cylindrical elements with different compositions into the graphite laying channels provides various geometrical and physical parameters of the reactor core and reflector. After removing a part of the graphite blocks they can be replaced with the fragments of a reactor core under investigation. The assembly structure permits to place the monitors of the measurement system in any given order. On the basis of the calculated and experimental data analysis it is concluded that energy release field and spectral characteristics of even the such highly heter geneous system as HTGR with once-througo then-out spherical fuel elements are simulated well

No abstract available

[en] Aiming to assess the consequences of abandoning the employment of highly enriched nuclear fuel (HEU) in the reactor engineering, the opportunity has been studied to convert the 'Argus-90' reactor operating with the uranyl sulphate water solution fuel of 90% enrichment in uranium-235 to low-enriched fuel (LEU) of ∼20% enrichment. A unified technology for the preparation of a solution fuel of 20% and 90% enrichment in U-235 has been confirmed. The effect of low-enriched fuel on the core neutronics parameters has been studied as well as on the efficiency of operating controls of the reactor control and protection system and radiolytic parameters of the solution fuel. (author)