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The first phase of the work on checking the main assumptions of the concept for upgrading the core of the SM reactor has been completed. A full-scale reactor experiment has been performed for the purpose of creating in the reactor the conditions necessary for accelerated high-dose irradiation of materials, meeting the requirements for the fast-neutron flux density, water temperature, pressure, and composition, and making it possible to install large-diameter experimental channels and apparatus for regulating the temperature and neutron spectrum. The decrease of the fuel volume and excess reactivity are compensated by a 20% increase of the uranium content in a fuel element and by replacing the corrosion-resistant steel fuel-assembly jackets with zirconium-alloy jackets. The results of the calculations and experiments performed during the first phase have shown that the objective has been achieved -the reactor can be operated efficiently with the new arrangement of the core. The objective and problems of the second phase are formulated: increase of the neutron flux density in the experimental channels by a factor of 1.5 by increasing the power density in the core, using a fuel element with a low harmful absorption of neutrons, and equalizing the power-release distribution by using a consumable absorber.All water-cooled high-flux research reactors with neutron flux density Φ > 10 15 sec -1 ·cm -2 , except for FRMII (Germany), pass through a final stage of their life cycle. At the same time, areas of scientific interest are developing where there is no alternative to using neutrons as a research tool. These areas include nuclear and elementary particle physics, the study of the fine structure of materials by the methods of neutron scattering, accelerated search for new materials with a high radiation resistance, and obtaining radionuclide preparations with new user qualities. Reaching a new methodological level in these directions requires the development of a stationary neutron source with a forced neutron flux density. Increasing the neutron flux density and the fuel utilization efficiency of high-flux research reactors, which are the reactors in greatest demand and are the most productive reactors but require a large quantity of expensive fuel, are the most urgent and difficult problems. The fuel in the core of such reactors operates under stringent, close to limiting, conditions. Increasing the charac-

A comprehensive investigation of SM reactor fuel elements with 20% higher nuclear fuel load after reactor tests with elevated parameter values is performed. The radial swelling behavior of fuel elements and their kernels as a function of the fission products concentration, fission density, heat-flux density, and test temperature is presented. The swelling behavior of fuel particles in the transfer cross section of a fuel element kernel is examined. The dependences found make it possible to evaluate quantitatively the swelling of fuel elements for different values of the thermophysical parameters and choose safe values for the parameters taking account of the maximum swelling of the fuel elements and the technical possibilities.