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Review of Literature and Assessment of Factors Relevant to Performance of Grouted Systems for Radioactive Waste Disposal

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Review of Literature and Assessment of Factors Relevant to Performance of Grouted Systems for Radioactive Waste Disposal CNWRA 2009-001 REVIEW OF LITERATURE AND ASSESSMENT OF FACTORS RELEVANT TO PERFORMANCE OF GROUTED SYSTEMS FOR RADIOACTIVE WASTE DISPOSAL Prepared for U.S. Nuclear Regulatory Commission Contract NRC NRC–02–07–006 Prepared by R.T. Pabalan1 F.P. Glasser2 D.A. Pickett1 G.R. Walter1 S. Biswas1 M.R. Juckett1 L.M. Sabido1 J.L. Myers1 1Center for Nuclear Waste Regulatory Analyses 6220 Culebra Road San Antonio, Texas 78228-0510 2University of Aberdeen Aberdeen, Scotland, United Kingdom Center for Nuclear Waste Regulatory Analyses San Antonio, Texas April 2009 ABSTRACT For many decades, radioactive wastes related to the reprocessing of nuclear fuel have been stored in underground storage tanks at former nuclear materials production sites at Aiken, South Carolina (Savannah River Site); Hanford, Washington; and Idaho Falls, Idaho (Idaho National Laboratory). The U.S. Department of Energy (DOE) plans to remove highly radioactive or key radionuclides from the tanks to the maximum extent practical and close the emptied tanks by filling the tanks, pipework, and concrete vaults with grout. This cement-based material is expected to provide structural support, encapsulate and stabilize the residual tank waste and tank heel, act as a physical barrier to inhibit the flow of groundwater through the waste, and serve as a barrier to plant roots or to inadvertent intrusion by burrowing animals or humans drilling or excavating at the site. DOE performance assessments demonstrating that disposal actions at the sites will meet performance objectives have included quantitatively evaluating the influence of cement-based barriers in mitigating radionuclide release to the environment. In these performance assessments, assumptions are made regarding the physical integrity (e.g., hydraulic conductivity) and chemical condition (e.g., pH and Eh affecting radionuclide solubility and sorption) of the cement-based engineered barrier and the changes to these properties that occur with time. Although much research has been done on the use of cement-based materials for immobilizing and stabilizing toxic metals and radioactive wastes, the ability of these materials to maintain the low permeability and other properties necessary to retain radionuclides for the long time periods—up to 10,000 years—required for nuclear waste disposal is uncertain. This report was prepared for the U.S. Nuclear Regulatory Commission (NRC) to provide information NRC staff can use in their consultations with DOE. This information will support independent evaluation of the DOE performance assessments related to tank closures and disposal of radioactive wastes in near-surface disposal facilities. This report briefly reviews the nature of Portland cement and the hydration and setting of cement-based materials. Degradation mechanisms, broadly classified into chemical and physical processes, and the role of the service environment in the degradation of cement-based materials are discussed. The report reviews published modeling approaches, ranging from simple diffusion or empirical equations to more complex coupled reactive–transport models, for predicting the chemical degradation of cement-based materials. Conceptual and mathematical models that can be used to evaluate the potential influence of fast pathways and bypassing pathways on radionuclide releases from grouted tanks are discussed. Further, the report summarizes and evaluates models and data available for estimating radionuclide release from cement-based waste forms. Finally, factors, as well as features, events, and processes, that are potentially important to evaluating the performance of grouted tanks and concrete vaults at DOE sites are assessed. Additional information is provided in two appendices. Data derived from published literature related to the permeability and diffusion properties of cement-based materials are presented in Appendix A. In Appendix B, available data are reviewed and new models for solubility limits are developed for selected radioelements in environments expected in cement-based disposal facilities at the Savannah River Site and Idaho National Laboratory. iii CONTENTS Section Page ABSTRACT.................................................................................................................................iii FIGURES ................................................................................................................................... ix TABLES ....................................................................................................................................xiii EXECUTIVE SUMMARY .......................................................................................................... xv ACKNOWLEDGMENTS ..........................................................................................................xvii 1 INTRODUCTION..........................................................................................................1-1 1.1 Background Information...................................................................................1-1 1.2 Objective of Report...........................................................................................1-3 2 NATURE OF PORTLAND CEMENT............................................................................2-1 2.1 Production........................................................................................................2-1 2.2 Product Variability............................................................................................2-1 2.3 Cement Products..............................................................................................2-1 2.4 Supplementary Cement Materials ....................................................................2-1 2.5 Organic Matter in Cement ................................................................................2-2 3 HYDRATION OF CEMENT..........................................................................................3-1 3.1 General Considerations....................................................................................3-1 3.2 Cement Hydration Products .............................................................................3-3 4 DEGRADATION OF CEMENT-BASED ENGINEERED BARRIERS ...........................4-1 4.1 Chemical Degradation Mechanisms.................................................................4-1 4.1.1 Carbonation......................................................................................4-1 4.1.2 Sulfate Attack....................................................................................4-5 4.1.3 Leaching...........................................................................................4-7 4.1.4 Corrosion........................................................................................4-11 4.1.5 Alkali–Silica Reaction.....................................................................4-16 4.2 Physical Degradation Mechanisms ................................................................4-16 4.2.1 Shrinkage........................................................................................4-16 4.2.2 Thermal Cracking ...........................................................................4-20 4.2.3 Freezing and Thawing....................................................................4-21 4.3 Durability and the Composite Nature of Concrete..........................................4-21 4.4 Role of the Service Environment....................................................................4-22 4.5 Standards and Standard Methods for Testing Performance in Service Environments ....................................................................................4-25 4.6 Limitations of Tests on Performance of Cement-Based Materials .................4-26 4.7 Framework for Predicting the Long-Term Behavior of Cement-Based Engineered Barriers.......................................................................................4-27 5 APPROACHES TO MODELING THE DEGRADATION OF CEMENT-BASED MATERIALS ...................................................................................5-1 5.1 Carbonation......................................................................................................5-2 5.2 Sulfate Attack...................................................................................................5-9 5.2.1 Empirical Correlation........................................................................5-9 5.2.2 Reactive–Transport Models............................................................5-10 v CONTENTS (continued) Section Page 5.3 Leaching.........................................................................................................5-22 5.3.1 Chemical Models of Leaching.........................................................5-22 5.3.2 Reactive–Transport Leaching Models............................................5-30 5.4 Summary ........................................................................................................5-45 6 CONCEPTUAL AND MATHEMATICAL MODELS OF POTENTIAL FAST AND BYPASSING PATHWAYS ASSOCIATED WITH IN-SITU TANK CLOSURES............6-1 6.1 Identification and Classification of Potential Fast and Bypassing Pathways....6-1 6.1.1 Cracks in Grout and Concrete Structures.........................................6-6 6.1.2 Conduits............................................................................................6-9 6.2 Movement of Fluids Through Potential Fast and Bypassing Pathways..........6-11 6.2.1 Fluid Flow in Cracks .......................................................................6-13
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