Gas phase migration of carbon-14 through barrier materials applicable for use in a high-level nuclear waste repository located in tuff

Linda Rose Bauer, Purdue University

Abstract

A study of the movement of $\sp{14}$CO$\sb2$ through proposed barrier media has been conducted. Diffusion coefficients for crushed tuff, bentonite and a 90:10% by wt. mixture of crushed tuff and bentonite were measured for two diffusion lengths. The ability of $\sp{14}$CO$\sb2$ to penetrate a microsilica-containing portland cement mortar proposed for repository use was also examined. The specimens were subjected to uniaxially-applied compressive loads prior to the diffusion tests to simulate the onset of environmentally-induced microcracks. Based on these experiments, the apparent diffusion coefficients for the soil-based media were as follows: crushed tuff, 1.73 $\pm$ 0.26 $\times$ 10$\sp{-2}$ cm$\sp2$/s; bentonite, 1.56 $\pm$ 0.61 $\times$ 10$\sp{-2}$ cm$\sp2$/s; and the crushed tuff/bentonite mixture, 1.77 $\pm$ 0.13 $\times$ 10$\sp{-2}$ cm$\sp2$/s. These values correspond roughly to breakthrough times of 5 and 10-15 min respectively for the 6.5- and 23-cm thicknesses studied. No significant differences in the diffusion coefficients measured at different diffusion lengths were detected. Percent recovery tests conducted on the 6.5-cm tuff columns resulted in a recovery level of 98 $\pm$ 5%. This result was interpreted as evidence that the $\sp{14}$CO$\sb2$ was not adsorbed on the ruff or the other materials with which it came in contact. The percent recovery tests also verified the integrity of the seals relied on to prevent the loss of $\sp{14}$CO$\sb2$ from the test chambers. With respect to the mortar studies, the penetration of $\sp{14}$CO$\sb2$ through 1-cm thick microfractured discs was, in all cases, limited to the upper 1 mm of the cement surfaces. Serial sampling of the diffusion cells and pH testing of the cement surfaces indicated that the $\sp{14}$CO$\sb2$ was consumed by the reaction of CO$\sb2$ with free Ca(OH)$\sb2$ in the cement to form calcite (CaCO$\sb3$). The formation of the calcite apparently acted as a chemical and/or physical barrier to $\sp{14}$CO$\sb2$ migration. Only when a disc was completely fractured could significant penetration of $\sp{14}$CO$\sb2$ be observed. The results suggest that the presence of a tuff- or bentonite-based backfill would not significantly affect $\sp{14}$CO$\sb2$ release rates from a repository. Conversely, $\sp{14}$CO$\sb2$ diffusion through simulated cement seals can apparently only occur after severe physical damage has been induced. These results may have implications for the ability of a repository to comply with the applicable regulatory release limits for C-14.

Degree

Ph.D.

Advisors

Landolt, Purdue University.

Subject Area

Environmental science

COinS