Disposal R&D in the Used Fuel Disposition Campaign: A Discussion of Opportunities for Active International Collaboration

For DOE's Used Fuel Disposition Campaign (UFDC), international collaboration is a beneficial and cost-effective strategy for advancing disposal science with regards to multiple disposal options and different geologic environments. While the United States disposal program focused solely on Yucca Mountain tuff as host rock over the past decades, several international programs have made significant progress in the characterization and performance evaluation of other geologic repository options, most of which are very different from the Yucca Mountain site in design and host rock characteristics. Because Yucca Mountain was so unique (e.g., no backfill, unsaturated densely fractured tuff), areas of direct collaboration with international disposal programs were quite limited during that time. The decision by the U.S. Department of Energy to no longer pursue the disposal of high-level radioactive waste and spent fuel at Yucca Mountain has shifted UFDC's interest to disposal options and geologic environments similar to those being investigated by disposal programs in other nations. Much can be gained by close collaboration with these programs, including access to valuable experience and data collected over recent decades. Such collaboration can help to efficiently achieve UFDC's long-term goals of conducting 'experiments to fill data needs and confirm advanced modeling approaches' (by 2015) and of having a 'robust modeling and experimental basis for evaluation of multiple disposal system options' (by 2020). This report discusses selected opportunities of active international collaboration, with focus on both Natural Barrier System (NBS) and Engineered Barrier System (EBS) aspects and those opportunities that provide access to field data (and respective interpretation/modeling) or allow participation in ongoing field experiments. This discussion serves as a basis for the DOE/NE-53 and UFDC planning process for FY12 and beyond

Natural convection in tunnels at Yucca Mountain and impact on drift seepage

The decay heat from radioactive waste that is to be disposed in the once proposed geologic repository at Yucca Mountain (YM) will significantly influence the moisture conditions in the fractured rock near emplacement tunnels (drifts). Additionally, large-scale convective cells will form in the open-air drifts and will serve as an important mechanism for the transport of vaporized pore water from the fractured rock in the drift center to the drift end. Such convective processes would also impact drift seepage, as evaporation could reduce the build up of liquid water at the tunnel wall. Characterizing and understanding these liquid water and vapor transport processes is critical for evaluating the performance of the repository, in terms of water-induced canister corrosion and subsequent radionuclide containment. To study such processes, we previously developed and applied an enhanced version of TOUGH2 that solves for natural convection in the drift. We then used the results from this previous study as a time-dependent boundary condition in a high-resolution seepage model, allowing for a computationally efficient means for simulating these processes. The results from the seepage model show that cases with strong natural convection effects are expected to improve the performance of the repository, since smaller relative humidity values, with reduced local seepage, form a more desirable waste package environment

The Effects of Unheated Sections on Mositure Transport in the Emplacement Drift

A thermal-hydrologic natural-ventilation model is configured for simulating temperature, humidity, and condensate distributions in the coupled domains of the in-drift airspace and the near-field rockmass. Meaningful results are obtained from the model for a practical application in which the beneficial effects of unheated drift sections are analyzed. Sensitivity to the axial dispersion coefficient is also studied with the model

Large-scale impact of CO2 storage in deep saline aquifers: A sensitivity study on pressure response in stratified systems

Large volumes of CO{sub 2} captured from carbon emitters (such as coal-fired power plants) may be stored in deep saline aquifers as a means of mitigating climate change. Storing these additional fluids may cause pressure changes and displacement of native brines, affecting subsurface volumes that can be significantly larger than the CO{sub 2} plume itself. This study aimed at determining the three-dimensional region of influence during/after injection of CO{sub 2} and evaluating the possible implications for shallow groundwater resources, with particular focus on the effects of interlayer communication through low-permeability seals. To address these issues quantitatively, we conducted numerical simulations that provide a basic understanding of the large-scale flow and pressure conditions in response to industrial-scale CO{sub 2} injection into a laterally open saline aquifer. The model domain included an idealized multilayered groundwater system, with a sequence of aquifers and aquitards (sealing units) extending from the deep saline storage formation to the uppermost freshwater aquifer. Both the local CO{sub 2}-brine flow around the single injection site and the single-phase water flow (with salinity changes) in the region away from the CO{sub 2} plume were simulated. Our simulation results indicate considerable pressure buildup in the storage formation more than 100 km away from the injection zone, whereas the lateral distance migration of brine is rather small. In the vertical direction, the pressure perturbation from CO{sub 2} storage may reach shallow groundwater resources only if the deep storage formation communicates with the shallow aquifers through sealing units of relatively high permeabilities (higher than 10 x 18 m{sup 2}). Vertical brine migration through a sequence of layers into shallow groundwater bodies is extremely unlikely. Overall, large-scale pressure changes appear to be of more concern to groundwater resources than changes in water quality caused by the migration of displaced saline water

Motivation, description, and summary status of geomechanical andgeochemical modeling studies in Task D of the InternationalDECOVALEX-THMC Project

The DECOVALEX project is an international cooperativeproject initiated by SKI, the Swedish Nuclear Power Inspectorate, withparticipation of about 10 international organizations. The general goalof this project is to encourage multidisciplinary interactive andcooperative research on modelling coupledthermo-hydro-mechanical-chemical (THMC) processes in geologic formationsin support of the performance assessment for underground storage ofradioactive waste. One of the research tasks, initiated in 2004 by theU.S. Department of Energy (DOE), addresses the long-term impact ofgeomechanical and geochemical processes on the flow conditions near wasteemplacement tunnels. Within this task, four international research teamsconduct predictive analysis of the coupled processes in two genericrepositories, using multiple approaches and different computer codes.Below, we give an overview of the research task and report its currentstatus

A Modeling Study Evaluating the Thermal-Hydrological Conditions In and Near Waste Emplacement Tunnels At Yucca Mountain

In heated tunnels such as those designated for emplacement of radioactive waste at the proposed geologic repository at Yucca Mountain, temperature gradients cause natural convection processes that may significantly influence the moisture conditions in the tunnels and in the surrounding fractured rock. Large-scale convection cells in the heated tunnels would provide an effective mechanism for turbulent mixing and axial transport of vapor generated from evaporation of pore water in the nearby formation. As a result, vapor would be transported from the elevated-temperature sections of the tunnels into cool end sections (where no waste is emplaced), would condense there, and subsequently drain into underlying rock units. To study these processes, we have developed a new simulation method that couples existing tools for simulating thermal-hydrological (TH) conditions in the fractured formation with a module that approximates turbulent natural convection in heated emplacement drifts. The new method simultaneously handles (1) the flow and energy transport processes in the fractured rock, (2) the flow and energy transport processes in the cavity, and (3) the heat and mass exchange at the rock-cavity interface. An application is presented studying the future TH conditions within and near a representative waste emplacement tunnel at Yucca Mountain. Particular focus is on the potential for condensation along the emplacement section, a possible result of heat output differences between individual waste packages

Comparative simulation study of coupled THM processes nearback-filled and open-drift nuclear waste repositories in Task D of theInternational DECOVALEX Project

As part of the ongoing international DECOVALEX project, fourresearch teams used five different models to simulate coupled thermal,hydrological, and mechanical (THM) processes near underground wasteemplacement drifts. The simulations were conducted for two genericrepository types, one with open and the other with back-filled repositorydrifts, under higher and lower post-closure temperature, respectively. Inthe completed first model inception phase of the project, a goodagreement was achieved between the research teams in calculating THMresponses for both repository types, although some disagreement inhydrological responses are currently being resolved. Good agreement inthe basic thermal-mechanical responses was also achieved for bothrepository types, even though some teams used relatively simplifiedthermal-elastic heat-conduction models that neglect complex near-fieldthermal-hydrological processes. The good agreement between the complexand simplified process models indicates that the basic thermal-mechanicalresponses can be predicted with a relatively high confidencelevel

Geomechanical/ Geochemical Modeling Studies onducted Within the International DECOVALEX Project

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

Results From an International Simulation Study on Couples Thermal, Hydrological, and Mechanical (THM) Processes Near Geological Nuclear Waste Repositories

As part of the ongoing international DECOVALEX project, four research teams used five different models to simulate coupled thermal, hydrological, and mechanical (THM) processes near waste emplacement drifts of geological nuclear waste repositories. The simulations were conducted for two generic repository types, one with open and the other with back-filled repository drifts, under higher and lower postclosure temperatures, respectively. In the completed first model inception phase of the project, a good agreement was achieved between the research teams in calculating THM responses for both repository types, although some disagreement in hydrological responses is currently being resolved. In particular, good agreement in the basic thermal-mechanical responses was achieved for both repository types, even though some teams used relatively simplified thermal-elastic heat-conduction models that neglected complex near-field thermal-hydrological processes. The good agreement between the complex and simplified process models indicates that the basic thermal-mechanical responses can be predicted with a relatively high confidence level