Field experiments have run for over 14 years to evaluate the behaviour of the same high-sodium content radioactive waste borosilicate glass buried in a loamy soil (glass K-26) and in an open testing area (glass Bs-10). Processing of field data for glass Bs-10 tested in an open area has resulted in a dissolution rate r = 0.42 µm/y and caesium diffusion coefficient D ≍ 1.8 10−20 m2/s at testing temperatures up to 19 oC. Both ion-exchange and hydrolysis control the corrosion of this glass. Processing of field data for K-26 glass revealed an insignificant role of glass dissolution. The caesium diffusion coefficient was estimated as D ≍ (3.4-5.1) 10−21 m2/s. Due to the relatively low storage temperatures (4.5 oC) used the leaching behaviour of glass K-26 is believed to be controlled by ion exchange processes. This mechanism is likely to remain dominant until the decay of 137Cs in the glass is below exemption levels.
The behaviour of waste glass was investigated under open site disposal
conditions. This glass was produced by vitrification of intermediate level
radioactive waste from nuclear power plants. Two types of borosilicate
glasses were obtained for two different reactor wastes, WWER and RBM.K.
Leaching and alteration mechanisms are discussed as well as the data
processing technique used for these long term tests. The decay of
radionuclides was accounted for in order to obtain correct results. The
leaching factors obtained can be used for the assesment of radionuclide
retention. Discontinuous leaching of Cs-137 has been observed during more
than 8 years testing time. The fluctuating leaching rate depends on glass
composition. The average leaching rate remains within (0.4 – 4) μg/sq.
sm·day.Alteration of waste glass includes the formation of surface layers and
cracks on the glass surface. SEM analysis of glass was used to show these
surface layers. The thickness of the layers was determined to be within 2–6
μm. The structure of these layers depends on glass composition and the
interfacing environment.
Performance assessment calculations for low-activity waste glass to be disposed at the Hanford site depend on simulations of long-term glass corrosion behavior and contaminant transport that are being performed via reactive chemical transport modeling. Confidence in the underlying physical and chemical processes that are being approximated by the computer model could be significantly enhanced through carefully-controlled field testing, which includes studies of buried ancient glasses. Field tests with simulated low-activity waste glasses have been initiated on the Hanford site and at the Ballidon site in the United Kingdom. In addition, a joint PNNL – SIA RADON research project has been initiated to analyze a unique data set collected over 12+ years during a Russian in-situ testing program with actual low-activity waste glass. The glasses buried at Hanford are scaled down cylinders (45 kg mass) and include a representative glass composition for Hanford low-activity waste and a glass designed to be highly reactive. Each glass was doped with chemical analog tracers (Re, Se, and Mo). Design of the lysimeter test facility and sampling locations were done with aid of predictive calculations performed with the STORM code. These same glasses were also buried at Ballidon but in the form of small glass coupons. The coupons will be retrieved over a period of several years for detailed analysis along with core samples of the surrounding limestone, which will be analyzed for contaminant transport profiles. Analysis of porewater from underneath the Russian burial site ha s been compared with modeling calculations. Good agreement between the model and the field data has been obtained so far using estimated parameters for the glass corrosion. Independent laboratory tests are in progress to parameterize the STORM model for quantitative comparisons.
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