LBNL’s natural systems research under the UFD campaign is focused on the use of scientific principles, experimentation, mathematical modeling, and field observations/measurements to develop a fundamental understanding of the behavior of nuclear waste disposal in an argillaceous rock environment. A key reason for considering argillaceous rock formations for nuclear waste disposal is the low permeability characteristic of these materials. Permeabilities arising from the primary porosity of these rock types are low because of the extremely fine-grained constituents and the very small pores associated with the fine-grained material. Permeability values are less than 10 microdarcies ( 10-17 m2 or about 10-10 m/s), and are often two to four orders of magnitude lower. While fractures can occur, argillaceous rock demonstrates the tendency to self-seal fractures, which reduces the effects of fractures on bulk permeability. This occurs as a result of swelling from the increased water potential in fractures, the generation of fracture infilling materials and mineral precipitation, and plastic deformation of the rock. The low permeability of clay and shale rock are well-known in the hydrogeology community where these rock types represent aquitards that severely limit groundwater movement, and in petroleum geology, where they act as caprocks limiting the rise of buoyant petroleum fluids. Other favorable characteristics of clay/shale rock are the strong sorptive behavior for many radionuclides and in saturated systems, the low flow rates leads to typically reducing conditions because of the lack of oxygen transport from the surface. Clay and shale rock also act to chemically buffer the effects of materials introduced through repository construction, operation, and emplaced materials.
Repository construction and operations can impact the character of the near-field argillaceous rock. The opening of new fractures within a narrow zone around underground drifts in argillaceous rock is commonly observed, along with desaturation of the near-field rock as a result of pre-closure ventilation and the introduction of oxygen. One of the critical questions being addressed in the current UFD campaign is the effects of tunneling and pre-closure operations. In the post-closure period, interactions of the rock with the repository backfill and other emplacement materials as well as waste heat need to be thoroughly investigated and understood. These questions are being addressed through two research topics, coupled process modeling and fracture network modeling. For further information, follow the links below:
The UFD natural systems activities also address multi-phase flow behavior in low-permeability argillaceous rock, the use of natural analogues, and the characteristics and distribution of argillaceous formations in the United States. Further information is provided on this in the following link: