Numerical Modeling of Isothermal and Nonisothermal Flow in Unsaturated Fractured Rock: A Review

In recent years, considerable efforts have been made to study the feasibility of geologic disposal of high-level nuclear wastes in deep unsaturated zones in desert environments. The tuff formations at and near the Nevada Test Site, which are under consideration for this purpose, are comprised of fractured-porous material, with hydrologic properties quite different from those encountered in most previous unsaturated flow studies dealing with soils. Another difference from conventional unsaturated flow is that in the vicinity of the waste packages, flow is driven by high temperatures (exceeding 100°C) and large temperature gradients. The approximations developed in soil science for weaklynonisothermal flow are not applicable to this situation, and a multiphase description of flow is required, similar to approaches used in modeling of geothermal reservoirs and thermally enhanced oil recovery. The conventional approach to unsaturated flow is applicable, however, to a variety of problems relating to natural (undisturbed) and far-field flow conditions. This paper reviews recent work on numerical modeling of unsaturated flow undertaken in the context of nuclear waste isolation studies. Concepts and applications of broader interest are summarized, including the role of fractures in partially saturated flow, the response of a fractured medium to infiltration events, and a simplified description of flow based on an effective continuum approximation. It is pointed out that the heat released from the waste packages gives rise to multi-phase flow with heat pipe effects, which may have a dramatic impact on thermal and hydrologic conditions. A number of important issues are identified which have not been adequately explored. These include the possibility that liquid water may flow along the rough walls of fractures, the bulk of which is drained. Pre-existing or induced fracture coatings may have significant hydrologic effects. Large-scale moisture movement may be important to describe natural (nearly isothermal) hydrologic conditions as well as waste-induced gas phase convection far beyond the thermally disturbed zone. The importance of model validation and calibration with laboratory and field measurements of unsaturated flow in fractured rock is emphasized.

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