The FY 2003 risk assessment (RA) (Mann et al. 2003) of bulk vitrification (BV) waste packages used 0.3 wt% of the technetium (Tc) inventory as a leachable salt and found it sufficient to create a significant peak in the groundwater concentration in a 100-meter down-gradient well. Although this peak met regulatory limits, considering uncertainty in the actual Tc salt fraction, peak concentrations could exceed the maximum concentration limit (MCL) under some scenarios so reducing the leachable salt inventory is desirable.

No abstract

Executive SummaryThe U.S. Department of Energy is constructing a waste treatment plant at the Hanford Site in southeastern Washington State to vitrify the large amount of radioactive waste stored in underground tanks. Supplemental treatment technologies were studied to accelerate the cleanup of low-activity waste, and a decision was made to proceed with a pilot-scale test and demonstration facility to further evaluate bulk vitrification (BV).Experimental research, engineering-scale (ES) tests, and the science of glass melting provide ample evidence that a small fraction of Tc and Re were transferred out of the low-activity waste (LAW) glass feed and molten LAW glass and deposited on the surface and within the internal pore surfaces of the castable refractory block (CRB), both by low-temperature molten salt penetration and by hightemperature evaporation-condensation. In this task, laboratory experiments were undertaken to evaluate the capability of these two mechanisms to transport Tc/Re into the CRB during vitrification and to evaluate various means of CRB protection against the deposition of leachable radioactive Tc (and Re, its nonradioactive surrogate). All tests conducted in this task and described in this report used Re as a chemical surrogate for Tc.Both standard and newly designed experimental methods were applied to assess the extent of Tc/Re transport to the CRB, both unprotected (the baseline) and protected with a glaze or a tile. These methods include:• the measurement of Re concentration distribution in an unprotected CRB sample taken from an ES test • suspended refractory rod test for vapor deposition• simulant condensate penetration tests to determine the penetration of vapor condensates• gas permeability test and porosity measurement for vapor penetration• partially immersed rod test for molten salt penetration• manufacturing demonstration testing to determine the compatibility of tiles and CRB and the adherence of tile to CRB • refractory corrosion test for tiles.In many cases, experimental conditions were selected to enhance the transport mechanism so as to make the very small levels of Tc/Re normally transported easier to detect.The tests with unprotected (baseline) CRB showed that the molten LAW penetrates into CRB pores before it converts to glass, leaving deposits of sulfates and chlorides when the nitrate components decompose. Na 2 O from the LAW reacts with the CRB to create a durable glass phase that may contain limited quantities of insoluble Tc/Re. Limited data from a single CRB sample taken from an ES experiment indicates that, while a fraction of Tc/Re is present in the CRB in a readily leachable form, most of the Tc/Re deposited in the refractory is retained in the form of a durable glass phase.In addition to the direct penetration into the porous CRB, the molten salts from the LAW, mainly sulfates, chlorides, and nitrates, begin to evaporate from BV feeds at temperatures below 800°C and condense on iv solid surfaces at temperatures below 530°C. The condensed salt readily wets and pe...

No 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...