Leach rates glass

Bulk glass leach rates based on various isotopes were calculated by the equation ... 

One disadvantage of borosilicate glass is the low solubility of sulphates, molybdates, chromates, and halogenides, which may cause separation of metastable phases (Fig. 1) at relatively low contents of these components (1-3 wt%, dependent on glass composition Camara et al. 1980 Kawamoto et al. 1981 Stefanovsky 1989 Stefanovsky Lifanov 1989). At higher concentrations, yellow phase formation may occur (Morris Chidley 1976 Stefanovsky Lifanov 1988 Lutze 1988). The yellow phase, consisting of alkali and alkaline earth molybdates, sulphates, chromates, and halogenides, concentrates Cs and Sr radionuclides, and its presence increases leach rates of these radionuclides. 

Most leach rate measurements of both matrix elements and radionuclides were performed at 90 °C using MCC-1 or PCT tests. According to these tests, leach rates range from 10 1 to 10g m 2 d (Lutze 1988). For example, the mass and elemental leach rates (in g-m 2-d ) for the PNL 76-68 glass containing 33 wt% waste oxides were determined at mass - 0.42, Ca - 0.068, Cs - 1.03, Mo - 1.40, Na - 1.32, Sr - 0.075, B - 1.12, and Si - 0.73. These values are typical for borosilicate waste glass as measured by the MCC-1 procedure (90 °C, 28 d). Leach rates of Fe-group elements and ACTs under the same test conditions are considerably lower (10-3 and 10 4g-m 2-d , respectively). 

Most of the detailed examinations were performed for the Synroc-C variety (Ringwood et al. 1988). Table 2 compares typical elemental leach rates (7-d MCC-1 test, 90 °C) from Synroc-C containing 10 wt% HLW calcine with those from borosilicate and aluminophosphate glasses, documenting that Synroc-C exhibits the lowest leach rates. 

Malow, G., Lutze, W. Ewing, R. C. 1984. Alteration effects and leach rates of basaltic glasses implications for the long-term stability of nuclear waste form borosilicate glasses. Journal of Non-Crystalline Solids, 67, 305-321. 

Whitehead, N. E., Seward, D. Veselsky, J. 1993. Mobility of trace elements and leaching rates of rhyolitic glass shards from some New Zealand tephra deposits. Applied Geochemistry, 8, 235-244. 

Waste Form. The leach rates of radionuclides by groundwater are slow both for vitrified HLW and for SUF. A glass lifetime of at least 3000 years is expected, assuming unlimited supply of water (24). If the limited solubility of glass in groundwater is considered as well as the low water flow the lifetime would be several orders of magnitude larger. 

Figures 3a and 3b show logarithmic plots of the leach rate in g/cm2-day based on various isotopes as a function of time. You can see that the leach rates decrease rapidly at the beginning of the test and level off after about ten days. This behavior is consistent with previous studies on simulated waste glasses. -...

Figure 3a. Leach rate as a function of time—upper-middle glass section...

Figure 4, Leach rate as a function of time and glass section for Cs and (--------bottom section (- -), upper-middle section (-top section.

The leach rates seen here can be compared to those from a similar test on simulated waste glass of the same composition. The leach rates for cesium and strontium from the fully radioactive Rlass were the same as the leach rates of the simulated glass. Although both tests were done on bulk glass samples, they differed in configuration. The fully radioactive samples were disks cm (height) x 3.5 cm (diameter)], and the... 

The release rate was determined for 10 radioisotopes from fully radioactive waste glasses in deionized water for a period of 1.75 years. For cesium and strontium, good agreement exists between the leach rates for simulated and fully radioactive glass of the same composition. 

Two possible causes for the increase in leach rates at long times for Zr and Cs are increased glass dissolution rates or cross-contamination of the leachate samples. At this point... 

In terms of Equation (2) the leach rate for Ce corresponds to a very low value of a and values of b which are very dependent on flow rate. This is consistent with the idea that Ce is so strongly associated with the glass matrix that it is transported into solution almost exclusively by dissolution of the glass. 

Generally, the leaching rates of puddle glass determined by the one-pass dynamic leaching method decrease with time in a similar fashion to those determined on synthetic glasses using various static methods. 

Gross dissolution of the waste can be measured from the loss of weight of the sample or by determination of the appearance in the leaching medium of the major matrix constituent e.g. glass). However, the selective leaching of important fission product elements has been observed in this work to be significantly different from that of the bulk waste matrix. The units normally used to describe leach rates, g/cm day, appear to imply... 

Based on dissolved ions only, the titanate waste showed an overall leach rate of x 10 5 g/cm day and a rate of 5 3 x 10 7 g/cm day for the fission waste oxides only. The results indicate that the leaching which is occurring is associated with the silicate phases in the ceramic, i.e., the Si02 formed from the silicon and the zeolite. The glass samples showed overall leach rates of 6-15 x 10 5 g/cm day and fission waste oxide leach rates of 1.8-2.7 x 10 g/cm day, where the higher rates in both cases were observed in the phosphate-containing glass. 

The leaching characteristics of the ceramic waste were compared with some vitrified waste forms under identical experimental conditions. The results are complex due to different leaching rates for various elements and the attack on the glass surface which results in the formation of a surface film which periodically sloughs off, however, some general comparison can he made. 

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