Carbon-14 geochemistry at the Savannah River Site
Carbon-14 is among the key radionuclides driving risk at the E-Area Low-Level Waste Disposal Facility on the Savannah River Site (SRS). Much of this calculated risk is believed to be the result of having to make conservative assumptions in risk calculations because of the lack of site-specific data. The original geochemical data package (Kaplan 2006) recommended that performance assessments and composite analyses for the SRS assume that {sup 14}C did not sorbed to sediments or cementitious materials, i.e., that C-14 K{sub d} value (solid:liquid concentration ratio) be set to 0 mL/g (Kaplan 2006). This recommendation was based primarily on the fact that no site-specific experimental work was available and the assumption that the interaction of anionic {sup 14}C as CO{sub 2}{sup 2-}) with similarly charged sediments or cementitious materials would be minimal. When used in reactive transport equations, the 0 mL/g Kd value results in {sup 14}C not interacting with the solid phase and moving quickly through the porous media at the same rate as water. The objective of this study was to quantify and understand how aqueous {sup 14}C, as dissolved carbonate, sorbs to and desorbs from SRS sediments and cementitious materials. Laboratory studies measuring the sorption of {sup 14}C, added as a carbonate, showed unequivocally that {sup 14}C-carbonate K{sub d} values were not equal to 0 mL/g for any of the solid phases tested, but they required several months to come to steady state. After six months of contact, the apparent K{sub d} values for a clayey sediment was 3,000 mL/g, for a sandy sediment was 10 mL/g, for a 36-year-old concrete was 30,000 mL/g, and for a reducing grout was 40 mL/g. Furthermore, it was demonstrated that (ad)sorption rates were appreciably faster than desorption rates, indicating that a kinetic sorption model, as opposed to the steady-state K{sub d} model, may be a more accurate description of the {sup 14}C-carbonate sorption process. A second study demonstrated that the {sup 14}C-carbonate sorbed very strongly onto the various materials and could not be desorbed by anion exchanged with high concentrations of carbonate or nitrate. High phosphate concentrations were able to desorb {sup 14}C-carbonate from the 36-year-old concrete sample, but not the clayey sediment sample. Together these geochemistry studies support the use of non-zero K{sub d} values in risk calculations on the SRS. For performance assessment (PA) calculations, {sup 14}C would be moving with the groundwater, remaining in contact with sediment for days, not months. Therefore for purposes of SRS risk calculations, it is appropriate to select sorption values after a couple days of contact, departing from the traditional definition that states K{sub d} values reflect the system under steady state conditions. Such an “apparent K{sub d} value,” would be expected to provide a better (and more conservative) estimate of what to expect under SRS PA conditions. Based on these results, recommended apparent K{sub d} values for use in the PA are 1 mL/g for sandy sediments and 30 mL/g for clayey sediments.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- USDOE (United States)
- DOE Contract Number:
- DE-AC09-08SR22470
- OSTI ID:
- 1079895
- Report Number(s):
- SRNS-STI-2008-00445; TRN: US1300147
- Country of Publication:
- United States
- Language:
- English
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