HLW Glass Studies: Development of Crystal-Tolerant HLW Glasses

Abstract

iii SummaryTen glasses were prepared from high-level waste (HLW) AZ-101 simulant and additives. Eight of them were evaluated with a double crucible test for the effect of Cr, Ni, Fe, Al, Li, and RuO 2 on the accumulation rate of spinel crystals in the glass discharge riser of the HLW melter. The thicknesses of accumulated layers were incorporated into previously developed model of spinel settling (Matyáš et al. 2010a). In addition, the liquidus temperature (T L ) of glasses was measured and their crystallinity in mass % of spinel determined for heat-treatments at various temperatures for 24 h. The remaining two glass compositions enriched with Rh 2 O 3 and RuO 2 were investigated with scanning electron microscopy (SEM) and x-ray tomography for agglomeration of particles and impact of agglomerates on accumulation rate.The experimental study of spinel accumulation indicated that high concentrations of spinel-forming constituents in the glass can produce settling layers of a few cm thick in a few weeks. An excessive agglomeration of spinel in high-Ni-Fe glass (Ni1.5/Fe17.5) with agglomerates bigger than 500 µm resulted in the accumulation rate ~53.8 µm/h, which will produce ~2.6 cm thick layer in just 20 days of melter idling and can potentially plug the riser. Can noble metals decrease significantly this accumulation rate? There is an indication that they can. Additions of RuO 2 (in the form of ruthenium nitrosyl nitrate) to high-Ni glass (Ni1.5) effectively slowed down or stopped the spinel accumulation because of their effect on decreasing the average crystal size below 10 µm. Can the accumulation of thick but noncompacted spinel layer prevent the plugging of the riser? Maybe it can. Then, the additions of Al 2 O 3 and Li 2 O to glasses containing high concentration of spinel constituents might be the way. These components slowed down the growth of individual crystals but promoted the formation of a dendritic network of large needle-like spinel structures, which resulted in the non-compacted layers. There is a reasonable chance that spinel crystals locked in this configuration can be removed with glass during the pouring into canister.The previously developed model (Matyáš et al. 2010a) predicts well, R 2 = 0.981, the accumulation of crystals in the glasses with no or small-scale agglomeration, in which spinel settles as individual crystals and/or as clusters of a few crystals. But, in the case of excessive agglomeration, observed in the Ni1.5/Fe17.5 glass, the model under predicts the thicknesses of deposited layers. This under prediction was getting gradually worse with time as an increased number of larger agglomerates formed. Another factor that was not depicted by the model and that affected greatly the accumulation of spinel was the formation of 3D network of spinel needle-like structures in the Ni1.5/Al10 and Ni1.5/Li.38 glasses, which prevented a formation of a dense settled layer.The T L 's of tested glasses as determined with optical microscopy was between 950 to 1105°C. We have also measured the mass ...

Impact of non‐framework cation mixing on the structure and crystallization behavior of model high‐level waste glasses

“…Despite their significant presence in the nuclear waste melt, [13][14][15]21 and their known impact on the redox chemistry of iron, [22][23][24] the role of these ions in altering the crystallization behavior of spinels in borosilicate-based HLW glasses has been largely ignored or not reported in the literature. Therefore, the present study aims at understanding the impact of Li 2 O and CaO on the structure and crystallization behavior of model HLW glasses (based on the glass composition of the iron-rich waste present in the AZ-101 tank) 25 (Fischer Scientific,99%), and MnO (Fischer Scientific, 99%). The batches were melted in ambient atmosphere in 90% Pt-10% Rh crucibles (covered with a Pt lid) at 1450 • C for 1-2 h in an electric furnace (Carbolite BLF 1800, Carbolite-Gero Limited, Hope Valley, UK).…”

The effect of high-level waste glass composition on spinel liquidus temperature

“…[ [33]] data listed in Table 5. Both T,' and Ts values were obtained using T; and s; values listed in Tables 8 and 9 (simple oxides).…”

“…The liquidus temperature (Td of the primary crystalline phase is one of such properties, and upon vitrification spinel is the primary crystalline phase of most U.S. wastes from Pu production. Spinel crystals easily form during waste glass melting and exist in the melt at relatively high temperatures, at which they can interfere with melter operation [ [1]- [24]]. Several studies were performed to obtain the liquidus temperature (Td of spinel as a function of glass composition [ [1], [2], [4], [8], [10], [12], [22]].…”

Real‐time monitoring of crystal accumulation in the high‐level waste glass melters using an electrical conductivity method

“…The projected operation of these melters is limited by spinel crystals, which are the primary solid phase predicted to precipitate from high-level waste (HLW) glasses at Hanford. 1 The spinel crystals can grow larger than 200 lm during melter idling when the temperature in the riser of the melter is expected to drop as low as 850°C. 2,3 Limited mixing and low temperatures can lead to excessive crystal accumulation in the riser, and could potentially block the discharge of the molten glass into canisters.…”

“…2,3 Limited mixing and low temperatures can lead to excessive crystal accumulation in the riser, and could potentially block the discharge of the molten glass into canisters. [2][3][4] To resolve this issue, various options are being explored, including the development of crystal-tolerant glasses 1 and methods to monitor spinel crystal accumulation. One method under consideration is an electrical conductivity method which is based on the assumption that the spinel crystals have conductivities different from those of the glass, and therefore, their gradual accumulation can be assessed through the changes in measured conductivities during melter operation.…”