Properties and behavior of the platinum group metals in the glass resulting from the vitrification of simulated nuclear fuel reprocessing waste

Two types of platinum group metal particles were found in borosilicate nuclear waste glasses: needle-shaped RuO₂ particles and spherical PdRh_xTe_y alloys. They form a dense sediment of high electrical conductivity and relatively high viscosity at the bottom of the ceramic melting furnace. The sludge shows a non-Newtonian flow behavior. The viscosity and conductivity of the sludge depend not only on the platinum group metal content but also on the texture and morphology of the RuO₂ particles. RuO₂ forms long, needle-shaped crystals which are caused by alkalimolybdate salt melts that formed in the calcine layer. The salt melts oxidize the Ru present as small RuO₂ particles after calcination to higher oxidation states. Ruthenium (VI) compounds are formed, presumably, which are not stable with respect to RuO₂ under the melting conditions. RuO₂ precipitates and crystallizes into long, needle-like particles.