State-of-the-Art Evaluation of Repository Sealing Materials and Techniques.

Under the auspices of the OECD/NEA Stripa Project, a state-of-the-art evaluation of repository sealing materials and techniques has been undertaken with particular application to crystalline rock. The objectives of the evaluation were (1) to review progress to date in the development of repository sealing materials and emplacement techniques; (2) to identify priority materials for sealing narrow aperture fractures in crystalline rock, including issues of technical concern to the long-term effectiveness of such materials; and (3) to provide a framework for advancing an in situ fracture-sealing test program that can be designed and implemented at the Stripa Mine.The classes of sealing materials that were considered included cementitious materials, natural clay materials, chemical grouts, fracture-filling synthetic minerals, ceramics, and metals. Of these, cementitious materials and natural clay materials were recommended as high-priority materials for further study because they can be designed to meet desired repository performance characteristics, there is considerable history of successful use in similar engineering applications, and there is indirect evidence that they will continue to perform as expected for long periods of time. Techniques for the emplacement of these sealing materials in rock fractures include pressure injection, dynamic (or vibratory) injection, and electrophoresis.

Tuff-Cement or Concrete Interactions in the Repository Environment

Hydrothermal interactions of tuff or tuff minerals with cement or concrete were investigated at 200 0C under a confining pressure of 8 or 30 MPa for I to 20 weeks. These chemical interactions produced crystalline hydrous calcium silicates such as Al-substituted tobermorite, xonotlite and gyrolite. Tobermorite was the most common interaction product of cement and tuff or tuff minerals because of pozzolanic reaction. The extent of cement hydration and the quantity of cement in the cement mix affected the abundance of tobermorite as expected. The reaction of concrete with tuff resulted in the formation of smectite in addition to tobermorite due to the generation of slight acidic conditions in this reaction mixture under the present hydrothermal conditions. The formation of tobermorite by the interaction of tuff with cement or concrete has positive implications to the physical (bonding) and chemical (sorption) properties in the repository sealing system because tobermorite (Al-substituted) acts both as a binding agent and as a cation exchanger.

Geochemical Performance Evaluation and Characterization of a Potential Cementitious Repository Sealing Material for Application in the Topopah Spring Tuff Nnwsi Investigations

Preliminary geochemical evaluations of some portland cement based materials have been made in Nevada Nuclear Waste Storage Investigations (NNWSI), for possible nuclear waste repository sealing applications in welded tuff focused in the Yucca Mountain area. Portland cement based sealing materials have been evaluated in the NNWSI for possible sealing applications in a nuclear waste repository in the Topopah Spring tuff member. Cementitious sealing materials developed for long-term stability should be as nearly as possible in thermodynamic equilibrium with the host rock, or any disequilibrium should not have negative impact upon the integrity of the host rock. A primary step in achieving this equilibrium condition is to minimize the chemical potential between the sealant and the host rock. Two different approaches were evaluated to achieve this compatibility. The one approach utilized indigenous materials for the formulation of the concrete and the other utilized reactive admixtures to adjust the bulk chemical composition of the concrete formulation to approximate the local rock bulk chemistry. Testing of both formulations at conditions that represented the maximum credible temperature and pressure conditions of a repository were completed and show that the use of an indigenous tuff in the formulation without adjusting the matrix chemistry caused alterations which might compromise the performance of the concrete. In contrast, the chemically adjusted cementitious formulation exhibited minimal alteration in the J-13 groundwater of the designed test.