[en] There are described the results of the radial and axial redistribution of the fuel in the experimental fuel elements with the vibro-packed powder oxide fuel in the fast power reactors. Irradiation has been accomplished in reactors SM-2 and BR-5 at a linear power of 300-650 W/CM. The average density of the core has been changed within 8-9 G/CM: besides, the loading conditions, thermal stress and power operating time have been varied. It is established that with the increase of the linear power from 300 to 600 W/CM at an average loading rate of > = 80 W/(CMXH) the axial transfer of the fuel owing to leakage of the melt increases by 20%. At a linear power of about 500 W/CM, the mass transfer decreases down to 6%, with the increase of the core density up to 9 G/CM: it is shown that at the temperature in the centre of > 2600 deg C the axial transfer of the fuel via the steam phase may reach > = 5%. The thermal-physical characteristics of the vibro-packed oxide fuel which have been determined experimentally, as well as the mass transfer data, make it possible to draw a conclusion that the optimum density of the fuel amounts to 9-9.5 G/CM. (author)