Properties and behavior of the platinum group metals in the glass resulting from the vitrification of simulated nuclear
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B. Luckscheiter Institut fur Nukleare Entsorgungstechnik, Kernforschungszentrum Karlsruhe GmbH, Postfach 3640, W-7500 Karlsruhe, Germany (Received 27 November 1990; accepted 1 July 1991)
Two types of platinum group metal particles were found in borosilicate nuclear waste glasses: needle-shaped RuO 2 particles and spherical PdRhxTey 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 RuO 2 particles. RuO 2 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 RuO 2 particles after calcination to higher oxidation states. Ruthenium (VI) compounds are formed, presumably, which are not stable with respect to RuO 2 under the melting conditions. RuO 2 precipitates and crystallizes into long, needle-like particles. I. INTRODUCTION
A vitrification process was developed at Kernforschungszentrum Karlsruhe, Institut fur Nukleare Entsorgungstechnik, Germany for the solidification of high-level liquid waste (HLLW) solutions from the nuclear fuel cycle as borosilicate glasses. In this process the HLLW (Table I) is fed into a Joule-heated ceramic melter together with a glass frit in the form of glass beads. The main process steps of drying, calcination, and melting proceed continuously at the surface of the glass melt. The melt is poured into canisters that will be sent to a geological repository. At KfK/INE two vitrification test runs, entitled W4 and W5, were performed in 1988 and in 1989, respectively, in which the HLLW simulate, containing platinum group metals (PM), was vitrified. These investigations were performed in order to test the PM behavior during the vitrification process. The PM in the HLLW (rhodium, ruthenium, and palladium) form precipitates in the glass melt and accumulate at the melter bottom. A dense sediment is formed with a higher electrical conductivity and a higher viscosity than the rest of the glass melt. The presence of this sediment has serious consequences on melter operation such as short circuits between the electrodes and plugging of the bottom drain of the melter. To prevent the formation of PM sludges, the effect of air bubbling into the melter was tested during test run W5. Results from the characterization of glass samples from the two vitrification test runs are presented which show the behavior of PM during the vitrification process. J. Mater. Res., Vol. 6, No. 12, Dec 1991 http://journals.cambridge.org
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In the first test run, W4, the HLLW simulate contained Ru, Rh, and Pd, whereas in the second test run, W5, Rh was left out to reduce costs. One of the objectives of this paper is to show that leaving Rh out of the HLLW simulate had no significant influence on the formation, size, and
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