Investigating the Temperature Limits of Bentonite Backfilled Repositories: Coupled THMC Modeling, Lab Mockup Testing and Field Experiments

2020 ◽  
Author(s):  
Jens Birkholzer ◽  
Liange Zheng ◽  
Jonny Rutqvist ◽  
Sharon Borglin ◽  
Chun Chang ◽  
...  
Keyword(s):  
Nuclear Waste ◽  
Field Experiments ◽  
Elevated Temperatures ◽  
Spent Nuclear Fuel ◽  
The United States ◽  
Transport Processes ◽  
Maximum Temperature ◽  
Nuclear Waste Repository ◽  
Swelling Properties ◽  
Clay Formation

<p>Compacted bentonite is commonly considered for use as backfill material in emplacement tunnels of nuclear waste repositories because of its low permeability, high swelling pressure, and retardation capacity of radionuclide. To assess whether this material can maintain its favorable features when undergoing heating from the waste package and hydration from the host rock, we need a thorough understanding of the thermal, hydrological, mechanical, and chemical evolution under disposal conditions. Laboratory and field tests integrated with THMC modeling have provided an effective way to deepen such understanding; however, most of this work has been conducted for maximum temperatures around 100°C. In contrast, some international disposal programs have recently started investigations to understand whether local temperatures in the bentonite of up to 200°C could be tolerated with no significant changes in safety relevant properties. For example, the United States disposal program is evaluating the feasibility of geological disposal of large spent nuclear fuel canisters that are currently in dry storage. Direct disposal of these canisters is attractive for economical and safety reasons, but faces the challenge of exposing the bentonite to significant temperatures increases. As a result, strong thermal gradients may induce complex moisture transport processes and bentonite-rock interactions while cementation and perhaps also illitization effects may occur, all of which could  strongly affect the bentonite properties.</p><p>Here, we present initial investigations of bentonite behavior exposed to strongly elevated temperatures. We first show results from coupled thermal, hydrological, mechanical and chemical (THMC) simulations of a generic nuclear waste repository in a clay formation with a bentonite-based buffer exposed to a maximum temperature of 200°C. Modeling results illustrate possible performance impacts, such as the time frame and condition of the early unsaturated phase during bentonite hydration, the porosity and permeability after the bentonite becomes fully saturated, and changing in swelling properties. We then discuss preliminary data from a bench-scale laboratory mockup experiment which was designed to represents the strong THMC gradients occurring in a “hot” repository, and we briefly touch on a full-scale field experiment to be conducted soon in the Grimsel Test Site underground research laboratory in Switzerland (referred to as HotBENT, with bentonite exposure from up to 200<sup>o</sup>C). </p>

scholarly journals Can we safely go to 200 °C? An integrated approach to assessing impacts to the engineered barrier system in a high-temperature repository

2021 ◽  
Vol 1 ◽  
pp. 83-84
Author(s):  
Jens T. Birkholzer ◽  
Liange Zheng ◽  
Jonny Rutqvist
Keyword(s):  
Elevated Temperatures ◽  
Current Knowledge ◽  
Test Site ◽  
Transport Processes ◽  
Rock Properties ◽  
Maximum Temperature ◽  
Nuclear Waste Repository ◽  
Grimsel Test Site ◽  
Engineered Barrier ◽  
Clay Formation

Abstract. This presentation gives on overview of the complex thermo-hydro-mechanical-chemical (THMC) processes occurring during the disposal of heat-producing high-level radioactive waste in geologic repositories. A specific focus is on the role of compacted bentonite, which is commonly used as an engineered backfill material for emplacement tunnels because of its low permeability, high swelling pressure, and radionuclide retention capacity. Laboratory and field tests integrated with THMC modeling have provided an effective way to deepen our understanding of temperature-related perturbations in the engineered barrier system; however, most of this work has been conducted for maximum temperatures around 100 ∘C. In contrast, some international disposal programs have recently started investigations to understand whether local temperatures in the bentonite of up to 200 ∘C could be tolerated with no significant changes to safety relevant properties. Raising the maximum temperature is attractive for economical and safety reasons but faces the challenge of exposing the bentonite to significant temperature increases. Strong thermal gradients may induce complex moisture transport processes while geochemical processes, such as cementation and perhaps also illitization effects may occur, all of which could strongly affect the bentonite and near-field rock properties. Here, we present initial investigations of repository behavior exposed to strongly elevated temperatures. We will start discussing our current knowledge base for temperature effects in repositories exposed to a maximum temperature of 100 ∘C, based on data and related modeling analysis from a large heater experiment conducted for over 18 years in the Grimsel Test Site in Switzerland. We then show results from coupled THMC simulations of a nuclear waste repository in a clay formation exposed to a maximum temperature of 200 ∘C. We also explore preliminary data from a bench-scale laboratory mock-up experiment, which was designed to represent the strong THMC gradients occurring in a “hot” repository, and we finally touch on a full-scale field heater test to be conducted soon in the Grimsel Test Site underground research laboratory in Switzerland (referred to as HotBENT).

Inferring saline tracer transport characteristics from time-lapse GPR experiments

2021 ◽  
Author(s):  
Peter-Lasse Giertzuch ◽  
Alexis Shakas ◽  
Bernard Brixel ◽  
Joseph Doetsch ◽  
Mohammadreza Jalali ◽  
...  
Keyword(s):  
Electrical Conductivity ◽  
Velocity Field ◽  
Nuclear Waste ◽  
Time Lapse ◽  
Transport Processes ◽  
Nuclear Waste Repository ◽  
Tracer Transport ◽  
Tracer Injection ◽  
Flow And Transport ◽  
Conductivity Sensor

<p>Monitoring and characterization of flow and transport processes in the subsurface has been a key focus of hydrogeological research for several decades. Such processes can be relevant for numerous applications, such as hydrocarbon and geothermal reservoir characterization and monitoring, risk assessment of soil contaminants, or nuclear waste disposal strategies.</p><p>Monitoring of flow and transport processes in the subsurface is often challenging, as they are usually not directly observable. Here, we present an approach to monitor saline tracer migration through a weakly fractured crystalline rock mass by means of Ground Penetrating Radar (GPR), and we evaluate the data quantitatively in terms of a flow velocity field and localized difference GPR breakthrough curves (DRBTC).</p><p>Two comparable and repeated tracer injection experiments were performed within saturated rock on the decameter scale. Time-lapse single-hole reflection data were acquired from two different boreholes during these experiments using unshielded and omnidirectional borehole antennas. The individual surveys were analyzed by difference imaging techniques, which allowed ultimately for tracer breakthrough monitoring at different locations in the subsurface. By combining the two complimentary GPR data sets, the 3D tracer velocity field could be reconstructed.</p><p>Our DRBTCs agree well with measured BTCs of the saline tracer at different electrical conductivity monitoring positions. Additionally, we were able to calculate a DRBTC for a location not previously monitored with borehole sensors. The reconstructed velocity field is in good agreement with previous studies on dye tracer data at the same research locations. Furthermore, we were able to resolve separate flow paths towards different monitoring locations, which could not be inferred from the electrical conductivity sensor data alone. The GPR data thus helped to disentangle the complex flow field through the fractured rock.</p><p>Out technique can be adapted to other use cases such as 3D monitoring of fluid migration (and thus permeability enhancement) during hydraulic stimulation and tracing fluid contaminants – e.g. for nuclear waste repository monitoring.</p>

Introduction

2020 ◽  
pp. 1-19
Author(s):  
Rosemary A. Joyce
Keyword(s):  
Nuclear Waste ◽  
Common Sense ◽  
Planning Process ◽  
Pilot Project ◽  
The United States ◽  
Nuclear Waste Repository ◽  
Alternative Futures ◽  
Us Government ◽  
Marker Design ◽  
Waste Repositories

Providing an introduction to the planning process for the Waste Isolation Pilot Project and the marker design that continues to be the basis of nuclear waste repository proposals in the United States, including for Yucca Mountain, this chapter lays the groundwork for consideration of the contradictions between opinions produced through expert consultation and the expertise of archaeologists. US government efforts described enlisted a variety of “experts” to propose alternative futures, identify models for communication over long spans of time, and assess the likely durability of proposed designs for a marker over nuclear waste repositories. To understand these expert reports, this chapter introduces the concept of an anthropology of common sense as a way to understand how government experts understood the archaeological sites that they offered as models.

Developing a Concept for a National Used Fuel Interim Storage Facility in the United States

2013 ◽  
Author(s):  
Donald Wayne Lewis
Keyword(s):  
United States ◽  
Nuclear Fuel ◽  
Nuclear Waste ◽  
Spent Nuclear Fuel ◽  
The United States ◽  
Yucca Mountain ◽  
Storage Facility ◽  
Spent Fuel ◽  
Geological Repository ◽  
Interim Storage

In the United States (U.S.) the nuclear waste issue has plagued the nuclear industry for decades. Originally, spent fuel was to be reprocessed but with the threat of nuclear proliferation, spent fuel reprocessing has been eliminated, at least for now. In 1983, the Nuclear Waste Policy Act of 1982 [1] was established, authorizing development of one or more spent fuel and high-level nuclear waste geological repositories and a consolidated national storage facility, called a “Monitored Retrievable Storage” facility, that could store the spent nuclear fuel until it could be placed into the geological repository. Plans were under way to build a geological repository, Yucca Mountain, but with the decision by President Obama to terminate the development of Yucca Mountain, a consolidated national storage facility that can store spent fuel for an interim period until a new repository is established has become very important. Since reactor sites have not been able to wait for the government to come up with a storage or disposal location, spent fuel remains in wet or dry storage at each nuclear plant. The purpose of this paper is to present a concept developed to address the DOE’s goals stated above. This concept was developed over the past few months by collaboration between the DOE and industry experts that have experience in designing spent nuclear fuel facilities. The paper examines the current spent fuel storage conditions at shutdown reactor sites, operating reactor sites, and the type of storage systems (transportable versus non-transportable, welded or bolted). The concept lays out the basis for a pilot storage facility to house spent fuel from shutdown reactor sites and then how the pilot facility can be enlarged to a larger full scale consolidated interim storage facility.

scholarly journals Claystone formations in Germany: what we (don't) know about them and how we can change this

2021 ◽  
Vol 1 ◽  
pp. 47-48
Author(s):  
Bernhard Schuck ◽  
Tilo Kneuker
Keyword(s):  
Phase I ◽  
Nuclear Waste ◽  
Site Selection ◽  
Numerical Models ◽  
Selection Procedure ◽  
Opalinus Clay ◽  
Nuclear Waste Repository ◽  
Fracture Density ◽  
Clay Formation

Abstract. Deep geological formations are considered for safe long-term disposal of high-level radioactive waste. Such a repository would be requested to prevent radionuclides from entering the biosphere for a period of 1 million years (StandAG, 2017). Consequently, a holistic characterization including lithological, mineralogical, geochemical, hydrological, structural and geomechanical properties of any potential repository-hosting rock formation is required. Nine claystone formations have been identified as “sub-areas” within the German site-selection procedure (BGE, 2020). The area covered by these formations comprises about half of the total area considered as being qualified for further exploration. However, despite its relevance to act as a geological barrier for, e.g. hydrocarbons or radionuclides, the characterization of clay-rich formations at depths exceeding 300 m in Germany has attained substantially less attention than economically more relevant units hosted by, e.g. sandstones or rock salt, which have been intensively explored. The BGR project BASTION aims at contributing to characterizing these claystone formations and emphasizes properties relevant to host a repository for nuclear waste. Investigations comprise (micro)structural/petrographic, mineralogical, geochemical, geophysical, hydraulic and thermomechanical analyses. In project phase I (2013–2019), claystones deposited in Northern Germany during the Lower Cretaceous were studied. These rocks belong to the fourth largest sub-area hosting claystones. Two of the main foci were to explore variations in lithology, mineralogy and geochemistry, and to identify deformation mechanisms (natural and artificial) by microstructural analyses. Although rocks appeared to be quite homogeneous on the 10–100 m scale, the results revealed distinct structural and sedimentary heterogeneities on the meter scale affecting fracture density. Another sub-area located in Southern Germany hosts the Opalinus Clay Formation (OPA). This up to 150 m thick claystone formation was deposited during the Middle Jurassic (Franz and Nitsch, 2009). Owing to its self-sealing capacity and ability to retain fluids, it is supposed to host the nuclear waste repository of Switzerland (Bossart et al., 2017). The OPA is quite well understood in terms of its lithology and (bio)stratigraphy, and there have been mineralogical, hydrological and petrophysical analyses, mostly documented in university theses a few decades old and sometimes difficult to access. However, it is questionable to what extent these investigations reflect the situation at depths relevant for the site-selection procedure. Well-documented data on the OPA and its properties relevant for nuclear waste disposal are available via the Swiss site-selection procedure (Bossart et al., 2017). However, as there remain substantial questions regarding the nature of the German portions of the OPA (e.g. spatial distribution of lithology, mineralogy, microstructures) at depths greater than a few decameters, it is unclear to what degree insights obtained in the Swiss site-selection procedure also account for Germany. Therefore, phase II of BASTION, which began in 2020, aims to use the multidisciplinary approach developed during phase I to characterize properties of the OPA relevant for the save long-term disposal of nuclear waste by identifying and quantifying structural and rheological heterogeneities. This will constitute important input for numerical models in any long-term safety assessment.

scholarly journals Heating-induced creep and potential creep rupture of clay liners for nuclear waste repository

2020 ◽  
Vol 205 ◽  
pp. 10002
Author(s):  
Karam A. Jaradat ◽  
Sherif L. Abdelaziz
Keyword(s):  
High Temperatures ◽  
Nuclear Waste ◽  
Elevated Temperatures ◽  
Creep Rupture ◽  
Nuclear Waste Repository ◽  
Clay Liners ◽  
Model Soil ◽  
Dem Simulations ◽  
Waste Repository ◽  
Rate Process Theory

The aim of this study is to assess the potential of encountering a heating-induced creep rapture of clay liners in nuclear waste repository. Groundwater and soil contaminations may occur if the elevated temperatures, expected in the vicinity of nuclear waste repository, trigger creep rapture of the clay liners. In this study, we utilize simulations based on the discrete element method (DEM) to understand the conditions under which heating-induced creep rupture can take place. In lieu of the conventional local/non-local damping mechanism usually utilized in DEM simulations to dissipate energy, the DEM simulations presented in this study incorporate the rate process theory as a damping mechanism to model soil creep. The results of a base anisotropic model at 70 °C show a dramatic increase in the creep rate at high temperatures showing creep rupture. Such undesired behavior can be mitigated by engineering clay liner materials to sustain and resist the expected high temperatures expected around nuclear waste repository.

The Basalt/Water System: Considerations for a Nuclear Waste Repository

1983 ◽  
Vol 26 ◽  
Author(s):  
D.L. Lane ◽  
M.J. Apted ◽  
C.C. Allen ◽  
J. Myers
Keyword(s):  
Nuclear Waste ◽  
Elevated Temperatures ◽  
Open Systems ◽  
Water System ◽  
Reaction Rates ◽  
Potassium Feldspar ◽  
Total Carbon ◽  
Nuclear Waste Repository ◽  
Solution Ph ◽  
Weakly Alkaline

ABSTRACTHigh-level nuclear Waste emplaced in a repository in basalt will lead to elevated temperatures and chemical reactions between the basalt and repository groundwater. The resultant changes in groundwater chemistry and the formation of secondary minerals will affect radionuclide release rates from the repository. In this study, Grande Ronde Basalt and synthetic groundwater were reacted at temperatures of 100°, 150°, and 300°C at a pressure of 30 MPa. Dickson-type sampling autoclaves were used to follow solution composition changes with time. Solution PH remained weakly alkaline, while steady state concentrations of F-, Cl-, SO4-2, and total Carbon were similar to or lower than initial values. Alteration assemblages at 300°C included silica, zeolites, potassium feldspar, and iron smectite. These assemblages are metastable, and prediction of alteration in the basalt and packing material depends on “accelerated” tests results and must be supplemented by study of metastable assemblages in natural analog systems and by tests in open systems with varying flow rates. Experimental data are consistent with rapid adjustment of Eh to reducing values; however, more work on reaction rates and buffering capacity is in progress.

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