Geomechanical/Geochemical Modeling Studies Conducted Within the International DECOVALEX Project J. T. Birkholzer 1 , J. Rutqvist 1 , E.L. Sonnenthal 1 , D. Barr 2 , M. Chijimatsu 3 , O. Kolditz 4 , Q. Liu 5 , Y. Oda 6 , W. Wang 4 , M. Xie 4 , C. Zhang 5 Lawrence Berkeley National Laboratory, Berkeley, USA Office of Repository Development, Department of Energy, Las Vegas, USA Hazama Cooperation, Tokyo, Japan University of Tubingen, Tubingen, Germany Chinese Academy of Sciences, Wuhan, China Japan Atomic Energy Agency, Tokai, Japan Abstract – The DECOVALEX project is an international cooperative project initiated by SKI, the Swedish Nuclear Power Inspectorate, with participation of about 10 international organizations. The general goal of this project is to encourage multidisciplinary interactive and cooperative research on modeling coupled thermo-hydro-mechanical-chemical (THMC) processes in geologic formations in support of the performance assessment for underground storage of radioactive waste. One of the research tasks, initiated in 2004 by the U.S. Department of Energy (DOE), addresses the long-term impact of geomechanical and geochemical processes on the flow conditions near waste emplacement tunnels. Within this task, four international research teams conduct predictive analysis of the coupled processes in two generic repositories, using multiple approaches and different computer codes. Below, we give an overview of the research task and report its current status. I. INTRODUCTION An international cooperative project entitled DECOVALEX (an acronym for DEvelopment of COupled models and their VALidation against EXperiments) was established in 1992 by a number of national regulatory authorities and waste management organizations involved in nuclear waste disposal, to cooperate in developing and testing models capable of simulating coupled processes. Three multi-year project stages have been completed in the past decade, mainly focusing on coupled thermo-hydro-mechanical (THM) processes [1, 2]. Currently, a fourth multi-year project stage of DECOVALEX is under way, referred to as DECOVALEX-THMC. The new project stage expands the traditional geomechanical scope of the previous DECOVALEX stages by incorporating thermo-hydro- chemical (THC) processes important for repository performance. In this paper, we report on DOE’s Task D (one of five research tasks in DECOVALEX-THMC), which aims at evaluating the long-term impact of geomechanical and geochemical processes on hydrologic properties and flow conditions near waste emplacement tunnels (drifts). Geomechanical and geochemical processes may lead to changes in hydrological properties that are important for repository performance because the flow processes in the vicinity of emplacement tunnels will be altered from their initial state. These changes can be permanent (irreversible), in which case they persist after the thermal conditions have returned to ambient; i.e., they will affect the entire regulatory compliance period. Geochemical processes also affect the water and gas chemistry close to the waste packages, which are relevant for waste package corrosion, buffer stability, and radionuclide transport. To better understand these processes and their impact on performance, the international research teams participating in Task D were asked to conduct predictive analysis of the long-term coupled processes in generic repositories with simplified conditions and geometry. Participating research teams model the THM and THC processes in the fractured rock close to a representative emplacement tunnel as a function of time, predict long- term changes in hydrological properties, and evaluate the impact on near-field flow processes. Two generic repositories situated in different host rock types and featuring different emplacement conditions are analyzed for comparison. Four research teams from China, Germany, Japan, and USA have conducted research activities regarding the geomechanical and geochemical research areas in Task D. As shown in Table 1, the research teams use different codes with different model characteristics. Since all teams model the same task configuration, research results from the participating teams can be directly compared. In the next sections, we briefly review the basic repository concepts studied in Task D and describe the