Characterization of Technetium Speciation in Cast Stone

Abstract

Executive SummaryThis report describes the results from laboratory tests performed at Pacific Northwest National Laboratory (PNNL) for the U.S. Department of Energy (DOE) EM-31 Support Program (EMSP) subtask, "Production and Long-Term Performance of Low Temperature Waste Forms" to provide additional information on technetium (Tc) speciation characterization in the Cast Stone waste form. To support the use of Cast Stone as an alternative to vitrification for solidifying low-activity waste (LAW) and as the current baseline waste form for secondary waste streams at the Hanford Site, additional understanding of Tc speciation in Cast Stone is needed to predict the long-term Tc leachability from Cast Stone and to meet the regulatory disposal-facility performance requirements for the Integrated Disposal Facility (IDF). Characterizations of the Tc speciation within the Cast Stone after leaching under various conditions provide insights into how the Tc is retained and released. The data generated by the laboratory tests described in this report provide both empirical and more scientific information to increase our understanding of Tc speciation in Cast Stone and its release mechanism under relevant leaching processes for the purpose of filling data gaps and to support the long-term risk and performance assessments of Cast Stone in the IDF at the Hanford Site.Various formulations of Cast Stone that include secondary waste simulants and different LAW simulants have been characterized previously. For this report, smaller Cast Stone monoliths were prepared with high Tc concentrations spiked into the 7.8 M Na overall average LAW simulant using dry ingredients from two different sources (referred to as NW and SE). These small Cast Stone monoliths were used for this Tc speciation characterization. The following subtasks are included within the Tc speciation characterization task:1. Measure the reductive capacity of different types of dry ingredients, the 8 LAW simulants used in Westsik et al. (2013), and short-term cured Cast Stone before and after leaching with different leaching conditions. The reductive capacities of dry ingredients from NW and SE sources, various LAW simulants, and final Cast Stone before and after leaching tests were measured using two different methods, the Ce(IV) and Cr(VI) methods. One type of blast furnace slag (BFS) and the high-Ca fly ash were obtained from LaFarge North America Inc. in Pasco, Washington, and these materials are referred to as the NW slag and fly ash. The second BFS and the low-Ca fly ash are the same materials used in the Saltstone processing facility at the Savannah River Site in South Carolina and are referred to as the SE materials. Much more of the solids dissolve in the strong acid (pH ~1-2) environment created by the Ce(IV) method compared to circumneutral pH (~7.0) condition created by the Cr(VII) method. Among the dry ingredients, the reductive capacity measured by the Ce(IV) method was the highest for BFS (0.793 meq/g for NW slag and 0.800 meq/g for SE slag). The reductive ...

Technetium and Iodine Getters to Improve Cast Stone Performance

“…By comparison, only 0.015 and 0.167 meq are needed to reduce the Tc(VII) present in 100 mL for the C1 and C3 tests, respectively. The reduction capacity for BFS1 and BFS2, which was measured using the Ce(IV) methodology of Angus and Glasser (1985), was 0.793 and 0.800 meq/g [Um et al (2012[Um et al ( , 2013], and 0.884 and 0.725 meq/g [Langton et al (2013)], 2 respectively. Because only 1 g of getter material was used in the experiments conducted at a 1:100 getter-to-LAW simulant ratio, there was insufficient reduction capacity to reduce all of the Cr and Tc present in the LAW simulant.…”

“…The obvious implication of Oblath's (1984) and Langton's (1988) studies, and many more recent ones, is that the BFS has reduced the Cr(VI) and Tc(VI) species to their much more insoluble Cr(III) and Tc(IV) species. To quantify the reducing capacity of BFS, as well as the other two ingredients cement and fly ash, Roberts and Kaplan (2009), Kaplan et al (2008), Um et al (2012Um et al ( , 2013, and Langton and Almond (2013) measured the reductive capacity of the dry blend ingredients used in Cast Stone and Saltstone. They found that the BFS has a large reductive capacity (~790 to 820 μeq g -1 ) compared to Portland cement (198 μeq g -1 ) and fly ash (299 μeq g -1 ).…”

“…To corroborate that the significant decrease in leach properties of the redox-sensitive Tc element is in fact caused by reduction from the common oxidized form pertechnetate (TcO 4 -) to some lower valence state, most likely the relatively stable Tc(IV) species; direct solid-phase analyses of the Tc speciation have been attempted. Allen et al (1997), Shuh et al (2010), Harbour and Aloy (2007), Lukens et al (2005), and Um et al (2011aUm et al ( ,b, 2012Um et al ( , 2013 have directly determined the speciation of 99 Tc in Saltstone and Cast Stone waste forms made with waste simulant using synchrotron X-ray absorption spectroscopy. Results have been somewhat variable dependent on the size of the Saltstone/Cast Stone specimen subjected to the synchrotron x-rays and the treatment of the Cast Stone specimens prior to being characterized (length of curing time, whether the specimen was leached or exposed to air prior to characterization and the conditions of leaching or air exposure).…”

Secondary Waste Cementitious Waste Form Data Package for the Integrated Disposal Facility Performance Assessment

“…Some of these data were updated in the FY 2014 Special Analysis for the Saltstone Disposal Facility at the SRS (SRR CWDA 2014). Reductive capacity data from Um et al (2013) are included as well. In general, this data review is focused on the most recent LSW grout formulation work and Cast Stone data with secondary wastes and LAW simulants.…”

“…Reduction capacity measurements have been made on the same 26 Cast Stone waste forms made with LAW simulants that were studied by Westsik et al (2013). These results were reported in Um et al (2013), and are shown in Table 3.8.…”