Investigation of Tc Migration Mechanism During Bulk Vitrification Process Using Re Surrogate

Abstract

SummaryAs a part of bulk vitrification (BV) performance enhancement tasks, laboratory scoping tests were performed in FY 2004FY -2005 to explore possible ways to reduce the amount of soluble Tc in the BV waste package. Theses scoping tests helped identify which mechanisms play an important role in the migration of Tc in the BV process (Hrma et al. 2005;Kim et al. 2005). Based on the results from these scoping tests, additional tests were identified that will improve the understanding of Tc migration and clearly identify the dominant mechanisms.The additional activities identified from previous studies were evaluated and prioritized for planning for Tasks 29 and 30 conducted in FY 2006. Task 29 focused on the improved understanding of Tc migration mechanisms, and Task 30 focused on identifying the potential process changes that might reduce Tc/Re migration into the castable refractory block (CRB). This report summarizes the results from the laboratory-and crucible-scale tests in the laboratory for improved understanding of the Tc migration mechanism using Re as a surrogate performed in Task 29. Other tests performed in Task 30, which specifically aimed at testing changes to the BV process that might reduce Tc/Re migration into the CRB, will be reported in a separate report. The FY 2004 work also showed that Re is an excellent surrogate for Tc. Therefore, Re was used for all the tests performed in this study because a broader set of non-radioactive Re tests could be conducted for less cost than radioactive Tc tests.The water and molten ionic salt (MIS) capillary experiments suggested that the capillary force is a dominant mechanism for MIS penetration in the CRB, which was also supported by the preliminary modeling results.Hot-stage microscopy observations of the feed-melting and liquid-formation processes showed that the behavior of a dry blended feed and a feed prepared from liquid simulant was significantly different. A dry blended feed seemed to form a higher fraction of liquid than a feed prepared from liquid simulant, which could cause an increase in MIS migration into CRB. This may suggest that the information obtained in the full-scale tests conducted with dry blended feed may need to be verified with drier prepared feed in future operations.Thin-section MIS/Re profile measurements with FS-38B CRB samples provided new information on how the MIS and Re penetrate as the melt progresses.• For the CRB above the melt line, Re penetrates into the CRB by a vapor disposition mechanism, i.e., Re deposits when the CRB is cold, becomes molten as the temperature increases, and then penetrates further into the CRB by capillary action. The extent of penetration and the concentration of Re above the melt line is small.• For the CRB at the melt line, Re penetrates into the CRB through both the vapor deposition at the early stages of processing and then liquid MIS formation and penetration at later processing stages. The CRB at the melt line close to the glass interface exhibited the highest soluble Re concentration.• For...

Reactions during melting of low-activity waste glasses and their effects on the retention of rhenium as a surrogate for technetium-99

“…15 suggest that there was no significant Re loss from the glass-forming melt between 700 and 1100°C within experimental uncertainty. From crucible tests on Re volatilization [21,35] and from small-scale melter tests on Re and 99m Tc-volatilization during idling (i.e., no feeding so no cold-cap on the melt surface) [12][13][14], it was found that Re/Tc volatilizes relatively fast from the glass melt. Matlack et al [13] fit Re and 99m Tc-volatilization data to a simple equation, c(t) = c 0 exp(−kt), where c is concentration, t is time, c 0 is concentration at t = 0, and k is rate constant.…”

Redox-dependent solubility of technetium in low activity waste glass

“…A carefully measured stoichiometric amount of K 2 CO 3 , dissolved in water, was added to the ammonium pertechnetate solution. The combined solution was evaporated to a low volume to expel ammonium carbonate and to precipitate crystalline KTcO 4 . The final 2 mL of supernatant liquid was discarded.…”

“…Technetium readily evaporates during melting of glass feeds (waste + additives) and out of the molten glass, leading to low retention in a final glass product. [4][5][6][7] To effectively manage technetium retention in the Hanford LAW glass, it is critical to understand if the solubility of technetium is a controlling factor. The speciation of technetium in glass has been previously studied, and the technetium species present in waste glass have been previously reported; [8][9][10][11] however, the solubilities of these species are unknown.…”

Formation of Technetium Salts in Hanford Low‐Activity Waste Glass

“…These waste streams will be mixed with glass formers and vitrified. Technetium is well‐known to evaporate out of the glass during the melting process . One of the objectives of this work was to measure the maximum loading of technetium in the glass.…”

“…Technetium is well-known to evaporate out of the glass during the melting process. [4][5][6][7] One of the objectives of this work was to measure the maximum loading of technetium in the glass. The glass will retain technetium up to some maximum concentration, and anything beyond that will tend to migrate to the melt surface and form a salt layer or volatilize into a gaseous form.…”