ABSTRACT
The U.S. Department of Energy is constructing and operating the Waste Isolation Pilot Plant (WIPP), a research and development facility near Carlsbad, New Mexico, to determine whether or not defense-generated high-level radioactive waste can be stored safely in bedded salt. The goal of the WIPP modeling program is to develop the capability to predict room responses from one site to another without a priori knowledge of the actual room responses. Data from one of the early WIPP excavations, called the South Drift, have already been used to form an initial evaluation of computational models for predicting room closures as a result of salt creep [Morgan et al., 1985]. In that study, a significant unresolved discrepancy existed between predicted and measured room closures. It was suggested that future studies address alternate forms of the constitutive law. In this paper, an alternate form of the creep model for salt is used that is founded upon the deformation- mechanism map for the micromechanical deformation processes. This model embodies both steady-state and transient creep. Also, quasi-static plasticity is incorporated into the complete constitutive model for salt. The conclusion is drawn that the combination of the mechanistic creep model, plasticity, and flow potential can approximate the late time South Drift deformation. Further improvement of the model fit of plasticity in the future is expected to further improve the simulation.
INTRODUCTION
For the design of future repositories of high-level radioactive waste underground in bedded salt deposits, it is essential to have a detailed understanding of thermal/structural interactions of the underground openings. This understanding must encompass both temperatures and times exceeding current conventional mine experience and must be commensurate with the need to assure the public of the long-term safety of any bedded salt repository developed in the future. To accomplish these ends, numerical calculations are the only method that can support the eventual assurance of the long-term adequacy of the design. These numerical techniques use finite element methods, theoretical models of constitutive behavior, and laboratory material properties to simulate the closure of underground openings in a representative bedded salt stratigraphy. Validation of these computational techniques is necessary to guarantee reliable results, and validation is possible only through comparison to in situ test data. Such data are now being obtained in the underground facility of the Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico. The WIPP experimental program is for the express purpose of providing the research and development for future defense high-level waste disposal [Matalucci et al., 1982]. The earliest underground openings at the WIPP facility were for the underground confirmation of the site characterization as initially determined from deep boreholes and geophysical studies. However, in addition, one of these openings, the South Drift (2.4 m high x 7.6 m wide), has provided the earliest geomechanical closure data for a long (~1000 m), isolated drift. Such an isolated drift is ideally compatible with the current two-dimensional numerical codes developed for analysis of bedded salt thermal/structural response. The predominant mode of drift closure has to this point been believed to be creep deformation.
