SummaryAnalytical results are presented on the chemical composition and other physical properties of a glass, given the identification BNFL-A-S98, made at Pacific Northwest National Laboratory' that is representative of the low-activity waste glass composition proposed by BNFL, Inc? for immobilization of envelope A double-shell tank wastes at the Hanford Site. This glass was prepared for use in a testing program to be conducted at Pacific Northwest National Laboratory and at Argonne National Laboratory for the purpose of characterizing its long-term corrosion behavior. Detailed examination of the glass microstructure using transmission electron microscopy showed structural features indicative of amorphous phase separation. A remelt was performed on a smaller batch (100 g) to ensure rapid cooling. The glass microstructure was reexamined and showed no evidence of phase separation.Selected long-term (some to 860 d) product consistency tests were terminated, and the leachates were analyzed on tests with three other representative low-activity waste glass formulations (L8-1, L8-3, and L8-7). The results showed no evidence of corrosion rate acceleration at three times the duration of tests where another well-studied glass, LD6-5412, had been completely altered under identical test conditions. These tests (and others not discussed in this report) provide clear evidence that low-activity waste glasses with at least 20 mass% Na20 can be made that have excellent long-term corrosion resistance. However, glass composition has a large impact on long-term behavior and so careful experiments with several different techniques are essential to ensuring that a particular glass will have good long-term corrosion resistance.For example, accelerated corrosion tests were performed on the BNFL-A-S98 glass using the pressurized unsaturated flow (PUF) technique at flow rates of 0.5, 1.0, and 2.0 mL/d at 99.5"C. After only 100 to 200 h, effluent pH and electrical conductivity increased sharply, indicating a rapid acceleration in the corrosion rate of the glass. An examination of the reacted glass showed that the grains were covered with a crystalline zeolite precipitate, sodium aluminosilicate hydrate, and a thick amorphous gel layer. Crystallization of the zeolite is thought to be the cause of the observed corrosion rate acceleration because this phase acts as sink for removal of Si and A1 from the aqueous phase, which increases the chemical potential difference between the glass and solution. Measurements of alteration rinds provided an estimate of the glass corrosion rate at between 2 to 5 g/m*-d. This measurement was found to be consistent with corrosion rates calculated from chemical analysis of effluent samples and is near the expected forward rate of reaction for this glass at the test temperature and the estimated pH of pore waters in the PUF tests.A unique combination of PUF equipment and the ultracentrifugation apparatus (UFA) was developed for the quantitative assessment of changes in hydraulic properties (water retention and ...
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.
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