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Mechanochemical and SHS methods are up-and-coming ways to produce finely dispersed boron carbide nano powder. With optimal process conditions the synthesized phases have ultra dispersed state with well-developed surfaces of the boundaries of grains and subgrains that have either a nano or microcrystalline structure, and that ensures its higher density after vibrocompaction treatment, which in its turn can result in a reduced burn-out rate and a slow-down absorption activity under the influence of neutron irradiation. The products of mechanochemical synthesis and SHS have specified composition and specific structural state and are related with fast solid-phase reactions. The presented research dealt with boron carbide powders that had been produced by mechanochemical or SHS methods, as well as by carbon char or amorphous boron reduction, or the reduction of boron carbide that had been produced by SPS sintering. The purpose of the research was to determine the most optimal SPS sintering modes and to investigate the structure and properties of the sintered boron carbide workpieces made from the powders produced by the above mentioned methods. Source materials for boron carbide synthesis by mechanochemical method or carbon reduction with subsequent crushing and grinding, as well as for SHS treatment were carbon char of PM-15 grade and amorphous boron of A grade taken in stoichiometric composition. SPS sintering of boron carbon powders produced as above mentioned took place at Spark Plasma Sintering (SPS) - Labox 650 plant in graphite dies of 15 mm in diameter in vacuum under 25… 50 MPa pressure. The study of В4С powder workpieces that had been produced by mechanical synthesis, SHS or carbon reduction or SPS sintering of carbon char and amorphous boron mixture, yielded the most efficient modes of SPS sintering for each powder under research. The highest relative density was observed with SPS sintering of В4С powders produced by mechanosynthesis or SHS.

Today there are numerous research works meant to improve nuclear fuel element performance in order to ensure reliable operation under increased burn-up conditions. In this context the pellet microstructure seems to be a very important parameter. An increase in the grain size diminishes the branching of boundaries and reduces the migration speed of gas-filled pores to the grain boundaries which are the routes of accelerated diffusion. The problem can be solved by introducing nano additives to uranium dioxide considering the influence of small addition agents upon the grain growth activation and microstructure evolution. The addition of nano particles of different powders should stimulate agglomeration process. This is one of modern tendencies in the development of new material technologies for fast reactors. In the research process the mechanical activation with simultaneous size reduction of gadolinium oxide and aluminum and gadolinium hydroxide powders (Gd2O3 №1, Gd2O3 №2, Al(OH)3 and Gd(OH)3) was done in planetary centrifugal mill "Pulverisette 5" made by Fritsch GmbH company (Germany). The technology of UO2 nuclear fuel manufacture has been developed in several variants including the agglomeration with pre-pressing or extruding, isostatic and hot pressing, rotary swaging, vibratory compacting, slip casting, etc. Today the main UO2 fuel element manufacturing technologies are cold pressing and agglomeration considering their simplicity and affordability. The conducted research permitted to ascertain some specific features of initial TiO2, Al(OH)3, Gd2O3, and Gd(OH)3 additives and determine their basic properties. The authors determined the most optimal modes of fine-grained additives production and proposed and optimal method of their introduction into UO2 mixtures. The paper also considers possible mechanisms of fine-grained additives influence on the fuel pellet production.