Bulk Vitrification Castable Refractory Block Protection Study

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...

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

“…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.…”

“…In a previous study (Hrma et al 2005), the CRB taken from ES-31B was ground into powder layer by layer in 1-mm intervals starting at the glass refractory interface and moving into the refractory block. The resulting refractory powder was washed and analyzed for soluble Re, and the remaining solids were dried and analyzed for Re.…”

“…One of the proposed mechanisms suggested by Hrma et al (2005) is that MIS penetrates into the CRB by capillary force through small open pores present in the CRB. To evaluate the proposed mechanism, a test setup was designed to demonstrate capillary action drawing MIS into the CRB.…”

“…It was assumed that MIS penetrates the CRB pores and forms a glassy phase that contains an insoluble Re based on the results of a previous study (Hrma et al 2005). It was suggested that the insoluble fraction of Re (assumed to be bound within a glassy layer inside the pore surface) may include ReO 2 that was not incorporated in a glass phase.…”

“…Tasks 14 and 15 helped identify which mechanisms play an important role in the migration of Tc in the BV process. Task 14 focused on the volatilization of Tc from the melting feed and molten glass (Kim et al 2005), and Task 15 focused on the migration of Tc into the CRB (Hrma 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.…”