The Hydrothermal Stability of Cement Sealing Materials in the Potential Yucca Mountain High Level Nuclear Waste Reposito
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THE HYDROTHERMAL STABILITY OF CEMENT SEALING MATERIALS IN THE POTENTIAL YUCCA MOUNTAIN HIGH LEVEL NUCLEAR WASTE REPOSITORY. J. L. Krumhansl*, T. E. Hinkebein*, and J. Myers** * Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM., 87185 ** IT Corporation, 5301 Central Ave., NE, Suite 700, Albuquerque, NM, 87108 ABSTRACT Cementitious materials, together with other materials, are being considered to seal a potential repository at Yucca Mountain. A concern with cementitious materials is the chemical and mineralogic changes that may occur as these materials age while in contact with local ground waters. A combined theoretical and experimental approach was taken to determine the ability to theoretically predict mineralogic changes. The cementitious material selected for study has a relatively low Ca:Si ratio approaching that of the mineral tobermorite. Samples were treated hydrothermally at 200°C with water similar to that obtained from the J-13 well on the Nevada Test Site. Post-test solutions were analyzed for pH as well as dissolved K, Na, Ca, Al, and Si. Solid phases formed during these experiments were characterized by scanning electron microscopy and X-ray diffraction. These findings were compared with predictions made by the geochemical modeling code EQ3NR/EQ6. It was generally found that there was good agreement between predicted and experimental results.
INTRODUCTION Planning for the safe isolation of nuclear wastes at Yucca Mountain involves assessing the chemical stability of construction materials such as grout for far longer than can be addressed by direct observation or historical records. To meet this need one must resort to indirect methods. Two typical methods employed are (a) "accelerated tests" carried out at elevated temperatures, and (b) computational models that predict final stable phase assemblages assuming that cement-groundwater systems reach thermodynamic equilibrium. Because both assessment methods are developmental, it is important to determine the extent to which they give consistent results. This was done by comparing the results of the geochemical solubility/speciation/reaction path code EQ3NR/EQ6 (EQ3/6; Version 3245, Release 1090 [1]) with autoclave experiments allowing for the interaction of cement + water ± tuff. The comparison was carried out at 200°C, where cement-solution interactions proceed rapidly while still approximating conditions that would be relevant to the repository at Yucca Mountain. This work performed under the auspices of the U.S. Department of Energy, Office of Civilian Radioactive Waste Management, Yucca Mountain Site Characterization Project, under Contract DE-AC04-76DP00789.
Mat. Res. Soc. Symp. Proc. Vol. 245. ©1992 Materials Research Society
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EXPERIMENTAL METHODS AND MATERIALS To minimize the possibility of unfavorable tuff-cement interactions at Yucca Mountain, high-silica cements are favored over those with compositions having normative portlandite. Two materials were tested: a grout containing only the cement paste and a concrete with chips
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