The shaft furnace equipped with plasma-fuel burners for high temperature slagging treatment has been developed in State Corp.»RADON». A slag is considered as final wasteform. From this point of view a slag formation mechanism, radionuclides immobilization and basic slag properties have been studied. A chemical and phase compositions of institutional and nuclear power station waste slags were determined. Some details of slags structure were investigated by infra-red spectroscopy. A study of radionuclides distribution has shown that caesium and strontium are concentrated in the silicate slag constituent while iron and cobalt enter the metal fraction.