The 7th US/German Workshop on Salt Repository Research, Design, and Operation was held in Washington, DC on September 7-9, 2016. Over fifty participants representing governmental agencies, internationally recognized salt research groups, universities, and private companies helped advance the technical basis for salt disposal of radioactive waste. Representatives from several United States federal agencies were able to attend, including the Department of Energy's Office of Environmental Management and Office of Nuclear Energy, the Environmental Protection Agency, the Nuclear Regulatory Commission, and the Nuclear Waste Technical Review Board. A similar representation from the German ministries showcased the covenant established in a Memorandum of Understanding executed between the United States and Germany in 2011. The US/German workshops' results and activities also contribute significantly to the Nuclear Energy Agency Salt Club repository research agenda.
Salt formations represent a promising host for disposal of nuclear waste in the United States and Germany. Together, these countries provided fully developed safety cases for bedded salt and domal salt, respectively. Today, Germany and the United States find themselves in similar positions with respect to salt formations serving as repositories for heat-generating nuclear waste. German research centers are evaluating bedded and pillow salt formations to contrast with their previous safety case made for the Gorleben dome. Sandia National Laboratories is collaborating on this effort as an Associate Partner, and this report summarizes that teamwork. Sandia and German research groups have a long-standing cooperative approach to repository science, engineering, operations, safety assessment, testing, modeling and other elements comprising the basis for salt disposal. Germany and the United States hold annual bilateral workshops, which cover a spectrum of issues surrounding the viability of salt formations. Notably, recent efforts include development of a database for features, events, and processes applying broadly and generically to bedded and domal salt. Another international teaming activity evaluates salt constitutive models, including hundreds of new experiments conducted on bedded salt from the Waste Isolation Pilot Plant. These extensive collaborations continue to build the scientific basis for salt disposal. Repository deliberations in the United States are revisiting bedded and domal salt for housing a nuclear waste repository. By agreeing to collaborate with German peers, our nation stands to benefit by assurance of scientific position, exchange of operational concepts, and approach to elements of the safety case, all reflecting cost and time efficiency.
A summary of recommendations for near-term intermediate-scale testing pertaining to a salt repository is provided in this report. Each proposal was asked to implement a phased progression, initiating with test plan production in FY 2017 and early-stage testing, if possible. Beyond 2017, testing is anticipated to progress to an underground setting and involve intermediate-scale field activities. Each test concept was presented at the June 6th 2016 meeting in Las Vegas NV and a team of DOE-NE, DOE-EM, and National Laboratory staff discussed the rnerits of each proposal. Discussions among managers and researchers in the weeks following the meeting led to selection of a path forward for phased testing that includes a series of small diarneter borehole tests designed to illuminate thermomechanical processes and potential vapor and brine transport. These tests are intended to be implemented at the WIPP facility and involve collaboration between SNL, LANL, and LBL. This document summarizes the test concepts generated by the te s of researchers and decisions made subsequent to the June 6th meeting.
Salt formations hold promise for eternal removal of nuclear waste from our biosphere. Germany and the United States have ample salt formations for this purpose, ranging from flat-bedded formations to geologically mature dome structures. As both nations revisit nuclear waste disposal options, the choice between bedded, domal, or intermediate pillow formations is once again a contemporary issue. For decades, favorable attributes of salt as a disposal medium have been extoled and evaluated, carefully and thoroughly. Yet, a sense of discovery continues as science and engineering interrogate naturally heterogeneous systems. Salt formations are impermeable to fluids. Excavation-induced fractures heal as seal systems are placed or natural closure progresses toward equilibrium. Engineering required for nuclear waste disposal gains from mining and storage industries, as humans have been mining salt for millennia. This great intellectual warehouse has been honed and distilled, but not perfected, for all nuances of nuclear waste disposal. Nonetheless, nations are able and have already produced suitable license applications for radioactive waste disposal in salt. A remaining conundrum is site location. Salt formations provide isolation, and geotechnical barriers reestablish impermeability after waste is placed in the geology. Between excavation and closure, physical, mechanical, thermal, chemical, and hydrological processes ensue. Positive attributes for isolation in salt have many commonalities independent of the geologic setting. In some cases, specific details of the environment will affect the disposal concept and thereby define interaction of features, events and processes, while simultaneously influencing scenario development. Here we identify and discuss high-level differences and similarities of bedded and domal salt formations. Positive geologic and engineering attributes for disposal purposes are more common among salt formations than are significant differences. Developing models, testing material, characterizing processes, and analyzing performance all have overlapping application regardless of the salt formation of interest.
Observational petrofabrics, thermal, mechanical, and hydrological measurements were made on reconsolidated salt samples extracted from the field site in which a study called Backfilling and Sealing of Underground Repositories for Radioactive Waste in Salt was conducted. Similar characterization was completed more than a decade ago, so this work furthers previous measurements after sustained consolidation in situ. Porosity determined by traditional point-counting on polished sections and helium porosimeter methods ranged from 20-25% with consolidation governed by brittle processes, as evidence of fluid-aided, grain-boundary processes was rarely observed. Thermal conductivity in the range of 2.3 W/(m∙K) is consistent for granular halite in this porosity range. Gas flow measurements yielded permeability of the order of 5e-13m2. Pressure-sensitive compressive strengths at 0.5, 1.0, and 2.0 MPa confining pressure were 8, 9, and 14 MPa, respectively, with apparent elastic moduli increase with deformation.
The 6th US/German Workshop on Salt Repository Research, Design, and Operation was held in Dresden. Germany on September 7-9, 2015. Over seventy participants helped advance the technical basis for salt disposal of radioactive waste. The number of collaborative efforts continues to grow and to produce useful documentation, as well as to define the state of the art for research areas. These Proceedings are divided into Chapters, and a list of authors is included in the Acknowledgement Section. Also in this document are the Technical Agenda, List of Participants, Biographical Information, Abstracts, and Presentations. Proceedings of all workshops and other pertinent information are posted on websites hosted by Sandia National Laboratories and the Nuclear Energy Agency Salt Club. The US/German workshops provide continuity for long-term research, summarize and publish status of mature areas, and develop appropriate research by consensus in a workshop environment. As before, major areas and findings are highlighted, which constitute topical Chapters in these Proceedings. In total, the scientific breadth is substantial and while not all subject matter is elaborated into chapter format, all presentations and abstracts are published in this document. In the following Proceedings, six selected topics are developed in detail.
Salt as a geologic medium has several attributes favorable to long-term isolation of waste placed in mined openings. Salt formations are largely impermeable and induced fractures heal as stress returns to equilibrium. Permanent isolation also depends upon the ability to construct geotechnical barriers that achieve nearly the same high-performance characteristics attributed to the native salt formation. Salt repository seal concepts often include elements of reconstituted granular salt. As a specific case in point, the Waste Isolation Pilot Plant recently received regulatory approval to change the disposal panel closure design from an engineered barrier constructed of a salt-based concrete to one that employs simple run-of-mine salt and temporary bulkheads for isolation from ventilation. The Waste Isolation Pilot Plant is a radioactive waste disposal repository for defense-related transuranic elements mined from the Permian evaporite salt beds in southeast New Mexico. Its approved shaft seal design incorporates barrier components comprising salt-based concrete, bentonite, and substantial depths of crushed salt compacted to enhance reconsolidation. This paper will focus on crushed salt behavior when applied as drift closures to isolate disposal rooms during operations. Scientific aspects of salt reconsolidation have been studied extensively. The technical basis for geotechnical barrier performance has been strengthened by recent experimental findings and analogue comparisons. The panel closure change was accompanied by recognition that granular salt will return to a physical state similar to the halite surrounding it. Use of run-of-mine salt ensures physical and chemical compatibility with the repository environment and simplifies ongoing disposal operations. Our current knowledge and expected outcome of research can be assimilated with lessons learned to put forward designs and operational concepts for the next generation of salt repositories. Mined salt repositories have the potential to isolate permanently vast inventories of radioactive and hazardous wastes.
Sandia is participating in the third phase of a United States (US)-German Joint Project that compares constitutive models and simulation procedures on the basis of model calculations of the thermomechanical behavior and healing of rock salt (Salzer et al. 2015). The first goal of the project is to evaluate the ability of numerical modeling tools to correctly describe the relevant deformation phenomena in rock salt under various influences. Among the numerical modeling tools required to address this are constitutive models that are used in computer simulations for the description of the thermal, mechanical, and hydraulic behavior of the host rock under various influences and for the long-term prediction of this behavior. Achieving this goal will lead to increased confidence in the results of numerical simulations related to the secure disposal of radioactive wastes in rock salt. Results of the Joint Project may ultimately be used to make various assertions regarding stability analysis of an underground repository in salt during the operating phase as well as long-term integrity of the geological barrier in the post-operating phase A primary evaluation of constitutive model capabilities comes by way of predicting large-scale field tests. The Joint Project partners decided to model Waste Isolation Pilot Plant (WIPP) Rooms B & D which are full-scale rooms having the same dimensions. Room D deformed under natural, ambient conditions while Room B was thermally driven by an array of waste-simulating heaters (Munson et al. 1988; 1990). Existing laboratory test data for WIPP salt were carefully scrutinized and the partners decided that additional testing would be needed to help evaluate advanced features of the constitutive models. The German partners performed over 140 laboratory tests on WIPP salt at no charge to the US Department of Energy (DOE).
With an abundance of scientific information in hand, what are the remaining geomechanics issues for a salt repository for heat-generating nuclear waste disposal? The context of this question pertains to the development of a license application, rather than an exploration of the entire breadth of salt research. The technical foundation supporting a licensed salt repository has been developed in the United States and Germany since the 1960s. Although the level of effort has been inconsistent and discontinuous over the years, site characterization activities, laboratory testing, field-scale experiments, and advanced computational capability provide information and tools required for a license application, should any nation make that policy decision. Ample scientific bases exist to develop a safety case in the event a site is identified and governing regulations promulgated. Some of the key remaining geomechanics issues pertain to application of advanced computational tools to the repository class of problems, refinement of constitutive models and their validation, reduction of uncertainty in a few areas, operational elements, and less tractable requirements that may arise from regulators and stakeholders. This realm of issues as they pertain to salt repositories is being addressed in various research, development and demonstration activities in the United States and Germany, including extensive collaborations. Many research areas such as constitutive models and performance of geotechnical barriers have industry applications beyond repositories. And, while esoteric salt-specific phenomenology and micromechanical processes remain of interest, they will not be reviewed here. The importance of addressing geomechanics issues and their associated prioritization are a matter of discussion, though the discriminating criterion for considerations in this paper is a demonstrable tie to the salt repository safety case.
An excellent scientific understanding of salt reconsolidation mechanisms has been established from experimental results and observational microscopy. Thermal, mechanical, and fluid transport properties of reconsolidating granular salt are fundamental to the design, analysis, and performance assessment of potential salt repositories for heat-generating nuclear waste. Application of acquired knowledge to construction techniques could potentially achieve high-performance seal properties upon construction or during the repository operational period, which lessens reliance on modeling to argue for evolving engineering characteristics and attainment of sealing functions at some future time. The robust database could be augmented by select reconsolidation experiments with admixtures and analogue studies with appropriate documentation of microprocesses.