The partitioning of uranium and neptunium onto hydrothermally altered concrete
Cementitious materials that are used to construct the ground support for high-level repositories have a high probability of interacting with radionuclide-bearing fluids derived from failed waste packages. Cementitious materials provide a highly alkaline environment; pore fluids in concrete can have pH {gt} 10 for thousands to hundreds of thousands of years. Studies have shown that fresh concrete and cement phases strongly retard or immobilize certain actinides. Consequently, cementitious materials may serve as a barrier to the release of the radionuclides to the far field. However, the effect of thermal alteration of these materials, which may occur in high-level repositories, on their interaction with radionuclides has not been addressed. In contrast to retardation, colloidal silica-enriched particles that are abundant in the pore fluids of cementitious materials may facilitate radionuclide migration through the near-field into the adjacent geological environment. Due to the uncertainties of these two opposite effects, it is important to investigate the interaction of actinides with cementitious materials under varying conditions. It is expected that cementitious materials in high-level waste repositories will be subjected to and altered by hot dry and/or humid conditions forhundreds to thousands of years by the time they interact with radionuclide-bearing fluids. After alteration, the chemical and mineralogical properties of these materials will be significantly different from that of the as-placed or fresh concrete. To assess the effect that this alteration would have on radionuclide interactions, samples of hardened concrete (untreated concrete) were hydrothermally heated at 200 C for 8 months (treated concrete). The concrete used in the experiments consisted of portland cement with an aggregate of dolomitic limestone. X-ray diffraction analysis has shown that portlandite and amorphous calcium silicate hydrate gels were converted to the crystalline calcium silicate hydrate minerals tobermorite, xonotlite, and scawtite, and clay minerals by the hydrothermal treatment. Calcite, dolomite, and quartz in the aggregate were unchanged by the treatment. This paper presents the results of batch experiments to obtain partition coefficients for U(VI) and Np(V) on untreated and treated concrete in 0.01 M NaCl and 0.01 M NaHCO{sub 3} solutions as functions of the concentration of the radionuclides, pH and time.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE Office of Environment, Safety and Health (EH) (US)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 750354
- Report Number(s):
- UCRL-JC-136032; YN0100000; TRN: US0000659
- Resource Relation:
- Journal Volume: 88; Journal Issue: 9-11; Conference: MIGRATION 1999, 7th International Conference in the Chemistry and Migration Behavior of Actinides and Fission Products in the Geosphere, Lake Tahoe, NV (US), 09/26/1999--10/01/1999; Other Information: PBD: 14 Oct 1999
- Country of Publication:
- United States
- Language:
- English
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