Concrete Property and Radionuclide Migration Tests

Abstract

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...

Becquerelite mineral phase: crystal structure and thermodynamic and mechanical stability by using periodic DFT

“…[27][28][29][30] This quantication is important in many applications as the immobilization of the SNF, in the treatment of uranium mine tailings, and the evaluation of the long-term performance of low level radioactive waste (LLRW) cement grouts for radionuclide encasement. [31][32][33][34] Moroni and Glasser 27 studied the reactions between calcium and silicon oxides, representing the principal components of cement, or calcium silicate hydrogel (CSH) with schoepite in aqueous suspensions at 85 C to evaluate the solubility of uranium at highly alkaline conditions and to test the immobilization potential. Several solubility-limiting phases as weeksite and becquerelite were identied.…”

“…The formation of crystalline phases led to the decrease of uranium solubility. Similarly, in diffusion experiments carried out in order to evaluate the performance of LLWR cement grouts, [31][32][33][34] several uranium phases were identied as soddyite, becquerelite, uranophane, and autunite. Again, the lack of reliable thermochemical data for these phases was underlined.…”

“…Again, the lack of reliable thermochemical data for these phases was underlined. 31 Uranium disequilibrium data in conjunction with petrographic analyses indicated that becquerelite phase can persist for times of the order of hundreds of thousands of years and that it is highly resistant to dissolution in uranium and calcium bearing groundwaters. 35 The long-term stability of this mineral phase in the presence of calcium supports the experiments that indicate that natural becquerelite has lower solubility than synthetic becquerelite.…”