SUMMARYAssessing long-term performance of Category 3 waste cement grouts for radionuclide encasement requires knowledge of the radionuclide-cement interactions and mechanisms of retention (i.e. sorption or precipitation). This understanding will enable accurate prediction of radionuclide fate when the waste forms come in contact with groundwater. A set of diffusion experiments using carbonated and non-carbonated concrete-soil half cells was conducted under unsaturated conditions (4% and 7% by wt moisture content). Spiked concrete half-cell specimens were prepared with and without colloidal metallic iron addition and were carbonated using supercritical carbon dioxide. Spikes of I and Re were added to achieve measurable diffusion profile in the soil part of the half-cell. In addition, properties of concrete materials likely to influence radionuclide migration such as carbonation were evaluated in an effort to correlate these properties with the release of iodine and rhenium.The behavior of rhenium and iodine release was comparable within a given half-cell test. The release of iodine and rhenium at 4% moisture content initiated with an initial concentration spike in the sediment immediately contacting the concrete monolith. The concentration profile rapidly decreased to zero within the first 5 cm of the sediment profile. Normalized concentration profiles for iodine and rhenium in sediments at 7% moisture content were asymmetrical increasing over the first 1 to 4 cm from the interface, reaching maximum concentrations 2 -5 cm from the interface, and decreasing asymptotically thereafter. The apparent release profile for sediments at 7% moisture content may be the result of i) limited diffusion within the concrete monolith which depletes the concentration source, or ii) compaction of the sediment half-cell resulting in separation from the concrete monolith, both of which effectively "cuts off" the source. Because of the method by which the sediment was sampled, it is unknown whether or not the later occurred. However, future efforts to quantify of the concentration profile within the concrete monoliths will aid in further understanding the release profiles. Comparing the release profiles at 4% and 7% moisture content suggests the rate of diffusion increases with increasing moisture content. However, because the source term was "cut-off" from the migrating contaminant at the higher moisture content the rate of diffusion was not sustained throughout the duration of testing.Diffusivity values for iodide in soil ranged from 8.66 x 10 -8 to 1.65 x 10 -7 cm 2 /s, and from 9.70 x 10 -8 to 5.75 x 10 -8 cm 2 /s for rhenium at 4% moisture content. At 7% moisture content diffusivity values for iodide ranged from 9.793 x 10 -7 to 1.52 x 10 -6 cm 2 /s, and from 8.43 x 10 -7 to 1.23 x 10 -6 cm 2 /s for rhenium. This indicates that unsaturated conditions a 3% increase in moisture content affords a one to two order of magnitude increase in diffusivity. The concrete diffusivity of iodide and rhenium ranged from 7.95 x 10 -14 to 1.20 x ...
Assessing long-term performance of Category 3 waste cement grouts for radionuclide encasement requires knowledge of the radionuclide-cement interactions and mechanisms of retention (i.e., sorption or precipitation). This understanding will enable accurate prediction of radionuclide fate when the waste forms come in contact with groundwater. A set of experiments was initiated during Fiscal Year (FY) 2006 to study the diffusion of rhenium (Re) and iodine (I) from spiked soil into concrete. These half-cells experiments were conducted with concrete half cells that were prepared with and without metallic iron and carbonation and the soil half-cells poised at moisture contents of 4%, 7%, and 15% by mass. Initial concentrations of I and Re were approximately the same in all experiments (between 12.5 and 15 mg/g of soil). The data showed that the greatest concentration of I and Re were measured within the first 10 mm of the concrete monolith (~10-20 mg/g) with a gradual decrease in concentrations of both elements over the remaining 20 to 30 mm of concrete monolith. Higher moisture contents and carbonation enabled greater depths of diffusion into the concrete monoliths. At 4% moisture content, the behavior of I and Re were markedly different. The concentration of I was 40 to 60 mg/g within the first 5 mm of the concrete half-cell in contact with sediment, whereas the diffusion of Re was limited to < 5 mg/g within the same depth of concrete. Such disparity in diffusivity of I and Re into concrete from spiked low-moisture content (4%) soil half cell suggested potential vapor-phase diffusion of I. Vaporization of I from relatively dry soil (4% moisture content) in the half cell was confirmed by extracting I-stained polyvinyl chloride sections of the soil half cell with isopropanol. These extracts contained 250 to 800 ppb I, but no Re was detected. Dynamic leach tests (American National Standards Institute ANSI-16.1) were conducted on a set of Re/I and technetium (Tc)/I spiked concrete specimens containing 4%, 8%, and 12% by mass-40 to +60 mesh iron powder or reactive nanoscale zero valent iron particles (RNIP) (Toda America, Inc., Schaumburg, IL). The objective of these tests was to examine whether these iron additives would influence leaching of redox-sensitive contaminant species. The results indicate that inclusion of RNIP relative to metallic iron particles significantly increases the leachability of Re and I from waste-form specimens. The leaching behavior of I and Re in the presence of RNIP was at a significantly increased rate throughout the duration of the test. The leaching indices indicate that in the presence of 4% and 8% zero valent iron (ZVI), Tc had higher leachability than Re. However, there was no difference in leachability of Tc and Re when the waste form contained 12% ZVI. The RNIP formulation consists of elemental iron, magnetite, water, and a water-soluble polymer to maintain a stabilized iron slurry. It is hypothesized that the polymer may form a surface coating on the iron particles that reduces their r...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.