Radionuclide Incorporation in Secondary Crystalline Minerals Resulting from Chemical Weathering of Selected Waste Glasses: Progress Report for Subtask 3d

Abstract

SummaryExperiments were conducted in fiscal year 1998 by Pacific Northwest National Laboratory' to evaluate potential incorporation of radionuclides in secondary mineral phases that form from weathering vitrified nuclear waste glasses. These experiments were conducted as part of the Immobilized LowActivity Waste-Performance Assessment (ILAW-PA) to generate data on radionuclide mobilization and transport in a near-field environment of disposed vitrified wastes.An initial experiment was conducted to identify the types of secondary minerals that form from two glass samples of differing compositions, LD6 and SRL202. Chemical weathering of LD6 glass at 90°C in contact with an aliquot of uncontaminated Hanford Site groundwater resulted in the formation of a crystalline zeolitic mineral, phillipsite. In contrast, similar chemical weathering of SRc202 glass at 90°C resulted in the formation of a microcrystalline smectitic mineral, nontronite.A second experiment was conducted at 90°C to assess the degree to which key radionuclides would be sequestered in the structure of secondary crystalline minerals; namely, phillipsite and nontronite. Chemical weathering of LD6 in contact with radionuclide-spiked Hanford Site groundwater indicated that substantial &actions of the total activities were retained in the phillipsite structure.Similar chemical weathering of SRL202 at 90°C, also in contact with radionuclide-spiked Hanford Site groundwater, showed that significant fractions of the total activities were retained in the nontronite structure.These results have important implications regarding the radionuclide mobilization aspects of the ILAW-PA. Additional studies are required to c o n f i i the results and to develop an improved understanding of mechanisms of sequestration and attenuated release of radionuclides to help refine certain aspects of their mobilization. ~ ~~Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.

Radionuclide Incorporation in Secondary Crystalline Minerals Resulting from Chemical Weathering of Selected Waste Glasses: Progress Report: Task kd.5b

“…The degree of sequestration of 99 Tc in experiments using 1M NaOH solutions could not be assessed due to matrix interference during scintillation counting. The degree of radionuclide sequestration observed for these waste glasses was comparable to the previous data obtained for other weathered glasses (LD6 and LAWA33) containing zeolitic minerals as the crystalline weathering products (Mattigod et al 1998(Mattigod et al , 2002a. These data indicate that one of the critical factors in radionuclide sequestration is the types and quantities of secondary minerals that form from weathering glasses.…”

“…Our previous studies with SRL202, LD6, and LAWA33 glasses showed that the weathered glasses exhibited different degrees of sequestration of radionuclides that were spiked into contacting solutions (Mattigod et al 1998(Mattigod et al , 2002a. These studies also showed that the secondary crystalline minerals that formed from weathering glasses sequestered portions of the radionuclides.…”

“…Under such alkaline conditions, several complex geochemical reactions are known to occur in glass waste forms, neighboring engineered structures, and surrounding sediments that include dissolution of several carbonate and silicate minerals, precipitation of secondary and tertiary mineral phases, radionuclide adsorption onto minerals (primary, secondary, and tertiary), and sequestration of radionuclides into secondary and tertiary mineral phases. Several studies of waste and volcanic glasses and grouts have shown that formation of zeolitic and smectitic secondary crystalline minerals as a product of chemical weathering is a very common phenomena (Collella et al 1978;Holler and Wirsching 1978;Van Iseghem et al 1985;Van Iseghem and Grambow 1988;Haaker et al 1985;Lutze et al 1988;Caurel et al 1988;McGrail et al 1997aMcGrail et al , 1997bMcGrail et al , 1998Ebert and Tam 1997;Luo et al 1997;Fortner et al 1997, Mattigod et al, 19982002a). Extensive reviews by Barrer (1982) and Newman and Brown (1987) of literature on isomorphous substitution in zeolites and smectites suggest that radionuclides in waste forms could, upon chemical weathering, be sequestered in these secondary crystalline mineral structures.…”

Statement of Work (SOW) for FY 2001 to FY 2006 for the Hanford Low Activity Tank Waste Performance Assessment Program

“…and U Mobilitv. Past l l A W studies have shown that zeolites form as secondary phases following the glass waste form dissolution (Mann et al 1996 andMattigod et al 1998). Zeolites have the ability to sequester many different ions within their crystalline structure.…”

“…Zeolites have the ability to sequester many different ions within their crystalline structure. Preliminary tests (Mattigod et al 1998) have shown that these four potentially mobile contaminants are partially incorporated into the structures of the weathering products of 5-86 RPP-6702, Rev. 0 Formerly HNF-SD-WM-PAP-062 Rev.…”