Water samples previously acquired from superheated (>140 C) zones within hydrological test boreholes of the Drift Scale Test (DST) show relatively high fluoride concentrations (5-66 ppm) and low pH (3.1-3.5) values. In these high temperature regions of the rock, water is present superheated vapor only--liquid water for sampling purposes is obtained during the sampling process by cooling. Based on data collected to date, it is evident that the source of the fluoride and low pH is from introduced man-made materials (Teflon{trademark} and/or Viton{trademark} fluoroelastomer) used in the test. The test materials may contribute fluoride either by degassing hydrogen fluoride (HF) …
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Lawrence Livermore National Laboratory (LLNL), Livermore, CA
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Description
Water samples previously acquired from superheated (>140 C) zones within hydrological test boreholes of the Drift Scale Test (DST) show relatively high fluoride concentrations (5-66 ppm) and low pH (3.1-3.5) values. In these high temperature regions of the rock, water is present superheated vapor only--liquid water for sampling purposes is obtained during the sampling process by cooling. Based on data collected to date, it is evident that the source of the fluoride and low pH is from introduced man-made materials (Teflon{trademark} and/or Viton{trademark} fluoroelastomer) used in the test. The test materials may contribute fluoride either by degassing hydrogen fluoride (HF) directly to produce trace concentrations of HF gas ({approx}0.1 ppm) in the high temperature steam, or by leaching fluoride in the sampling tubes after condensation of the superheated steam. HF gas is known to be released from Viton{trademark} at high temperatures (Dupont Dow Elastomers L.L.C., Elkton, MD, personal communication) and the sample water compositions indicate near stoichiometric balance of hydrogen ion and fluoride ion, indicating dissolution of HF gas into the aqueous phase. These conclusions are based on a series of water samples collected to determine if the source of the fluoride is from the degradation of materials originally installed to facilitate measurements. Analyses of these water samples show that the source of the fluoride is the introduced materials, that is the Viton{trademark} packers used to isolate test zones and/or Teflon{trademark} tubing used to draw water and steam from the test zones. In particular, water samples collected from borehole (BH) 72 high temperatures ({approx} 170 C) prior to introduction of any Viton{trademark} or Teflon{trademark} show pH Values (4.8 to 5.5) and fluoride concentrations well below 1 ppm over a period of six months. These characteristics are typical of condensing DST steam that contains only some dissolved carbon dioxide generated by water-mineral-gas reactions in the rock. With the introduction of the Viton{trademark} packer materials and Teflon{trademark} sampling tube in BH72, the water samples show pH values drop to 3.8, while fluoride rises to 2.4 ppm within three days. After nine days, the pH values reach as low as 3.4 and fluoride concentrations rise as high as 7.5 ppm in the collected samples. The background information describing the fluoride issue and a summary of the water collection activities along with the analytical results are provided below. The results of the field test confirm the hypothesis that the source of the fluoride in specific samples from the DST is the introduced test materials (i.e. Viton{trademark} and/or Teflon{trademark}). This is positive from the perspective of repository performance, particularly waste package and drip shield degradation behavior, as deleterious introduced materials would be avoided in an operating repository. Ongoing laboratory testing to be Completed in January 2002, and additional testing in BH72 and BH55 will address further details, such as the specific material introducing the fluorine and the material breakdown process.
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DeLoach, L & Jones, RL.Effects of Introduced Materials in the Drift Scale Test,
report,
January 11, 2002;
Livermore, California.
(https://digital.library.unt.edu/ark:/67531/metadc1411177/:
accessed June 11, 2024),
University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu;
crediting UNT Libraries Government Documents Department.
