Geological Repository Systems for Safe Disposal of Spent Nuclear Fuels and Radioactive Waste

Michael J Apted, Joonhong Ahn

• Publisher: Woodhead Publishing
• Year: 2010
• eBook ISBN: 9781845699789
• Topic: Technology & Engineering
• Subtopic: Environmental Management
• Index: Technology & Engineering

Geological disposal has been internationally adopted as the most effective approach to assure the long-term, safe disposition of the used nuclear fuels and radioactive waste materials produced from nuclear power generation, nuclear weapons programs, medical, treatments, and industrial applications. Geological repository systems take advantage of natural geological barriers augmented with engineered barrier systems to isolate these radioactive materials from the environment and from future populations.Geological repository systems for safe disposal of spent nuclear fuels and radioactive waste critically reviews the state-of-the-art technologies, scientific methods, regulatory developments, and social engagement approaches directly related to the implementation of geological repository systems.Part one introduces geological disposal, including multiple-barrier geological repositories, as well as reviewing the impact of nuclear fuel recycling practices and underground research laboratory activities on the development of disposal concepts. Part two reviews geological repository siting in different host rocks, including long-term stability analysis and radionuclide transport modelling. Reviews of the range of engineered barrier systems, including waste immobilisation technologies, container materials, low pH concretes, clay-based buffer and backfill materials, and barrier performance are presented in Part three. Part four examines total system performance assessment and safety analyses for deep geological and near-surface disposal, with coverage of uncertainty analysis, use of expert judgement for decision making, and development and use of knowledge management systems. Finally, Part five covers regulatory and social approaches for the establishment of geological disposal programs, from the development of radiation standards and risk-informed, performance-based regulations, to environmental monitoring and social engagement in the siting and operation of repositories.With its distinguished international team of contributors, Geological repository systems for safe disposal of spent nuclear fuels and radioactive waste is a standard reference for all nuclear waste management and geological repository professionals and researchers. - Critically reviews the state-of-the-art technologies, scientific methods, regulatory developments, and social engagement approaches related to the implementation of geological repository systems - Chapters introduce geological disposal and review the development of disposal concepts - Examines long-term stability analysis, the range of engineered barrier systems and barrier performance

Part I

Part I Introduction to geological disposal of spent nuclear fuels and radioactive waste

1

Multiple-barrier geological repository design and operation strategies for safe disposal of radioactive materials

M. Apted, INTERA Inc., USA

J. Ahn, University California, Berkeley, USA

Abstract:

This introductory chapter provides basic insights and guiding principles for establishing and evaluating the long-term safe isolation of radioactive wastes in geological repositories. The intended audience is new technical researchers and reviewers, interested in understanding how their specific expertise is integrated into a multi-discipline safety assessment. The focus is on deep geological disposal, appropriate for the disposal of spent nuclear fuel (SNF), reprocessed high-level waste (HLW), and long-lived, intermediate level waste (LL/ILW). Many of the principles discussed here, however, equally apply to near-surface disposal of lower activity wastes. Two basic types of processes affecting the long-term safe containment and isolation of radioactive waste in deep geological repositories are examined; (1) delay-and-decay processes and (2) concentration-attenuation processes. The robustness of different types of isolation processes, based on their effectiveness and reliability, are discussed. A ‘top-down’ safety assessment of an integrated, multiple-barrier repository system is vital in order to identify and prioritize safety-important barriers and processes, and to use such safety-importance insights to guide an efficient and effective research, development, and design program.

Key words

containment

geological repository

high-level waste (HLW)

mass transfer

multiple barriers

performance assessment

radioactive waste

repository design

safety assessment

safety importance

solubility

sorption

spent nuclear fuel (SNF)

transport

1.1 Introduction

Geological disposal has been the recommended approach for the permanent disposal of radioactive wastes since the seminal US National Academy of Sciences/National Research Council’s 1957 report The Disposal of Radioactive Wastes on Land (NAS, 1957). In the NAS report, disposal of liquid high-level wastes from weapons production was recommended. In this regard, the concept of the geologic disposal at the beginning was quite different from present ones, which are based on solid waste forms. The bases for the contemporary concept and principles of geologic disposal and the method for safety assessment were considered to be established in the 1970s, (e.g. see NEA, 1977, 1991; KBS, 1983).

As of today, over 30 nations with nuclear power plants (NPPs) are engaged at various stages in exploring the feasibility, conducting site selection, characterization and design programs, and/or licensing of geological repositories (Witherspoon and Bodvarsson, 2006). There are also programs exploring the possibility of international repositories for a volunteer consortium of nations (e.g. IAEA, 2004). While interesting options involving advanced fuel cycle transmutation, reprocessing, and even extended storage are also being evaluated in some countries (e.g. NEA, 2002), it has become a well-accepted consensus that all of these options eventually require geological disposal as part of the overall solution for safe management and permanent disposal of radioactive wastes (NAS, 2001; Chapman and McCombie, 2003).

While confidence has been well established in the engineering community, it is still recognized as a contentious regulatory, social, and scientific issue. Discussions include scientific points, such as long-term safety confirmation, so-called unknown ‘unknowns’ issue (MacFarlane and Ewing, 2006), as well as social points, such as equity between generations and equity between the repository-host community and the rest of the country (NAS, 2003). On the regulatory side, especially in the United States, there have been active discussions involving regulatory agencies, academia, and law makers in order to reach consensus for safety standards for long-term safety of geologic disposal (NAS, 1990, 1995). These discussions have affected, and have been affected by, the site selection processes and development of a repository concept in each country. Often, discussions have included various options of nuclear fuel cycles, hoping that some type of fuel cycles could reduce difficulties significantly. As mentioned in the previous paragraph, however, after three decades of discussion, we reached an understanding that geologic disposal of radioactive wastes is necessary for any type of fuel cycle.

The purpose of this introductory chapter is to provide some basic insights and guiding principles for establishing and evaluating the long-term safe isolation of radioactive wastes in geological repositories. The focus is on deep geological disposal, appropriate for the disposal of spent nuclear fuel (SNF), reprocessed high-level waste (HLW), and long-lived intermediate level waste (LL/ILW), as advocated by the International Atomic Energy Agency (IAEA, 2009). Many of the principles discussed here, however, equally apply to near-surface, trench-type disposal of lower activity wastes, as discussed in other chapters of this book.

The intended audience for this chapter are workers just beginning in the field of geological disposal of radioactive wastes, experts from traditional engineering and scientific disciplines who may be called upon to review multi-disciplinary geological repository programs, and any interested reader with college-level mathematical and technical training. For more experienced workers, there are numerous sources on advanced research, design, and development studies that are being conducted internationally; indeed, the other chapters in this volume provide an excellent window into many of these important, on-going research, development, and deployment (RD&D) areas. By contrast, this chapter’s objective is to provide a short primer on the basis for repository concepts and a simplified context by which to understand better the relevance and safety importance of current RD&D studies.

1.2 Multiple-barrier geological repository for radioactive materials

To understand basic strategies for assuring long-term, safe disposal of radioactive waste, it is first necessary to introduce the concept of a system of multiple barriers. Figure 1.1 shows a representative deep-geological repository design with multiple barriers that include (moving from the inside to the outside):

1.1 Representative illustration of a multiple-barrier deep geological repository system.

• a waste form (also called a ‘waste matrix’),

• a combination of metal canisters (also called ‘container,’ ‘package’, or ‘overpack’),

• an encompassing buffer and backfill, and

• the host rock (geosphere) of the repository site.

The main objective of this multiple-barrier system is to secure as long a time as possible for radionuclides contained in the waste form to reach the human environment, so that radiological impacts of disposed wastes would be reduced to an acceptable level. Placing high-level nuclear waste to stable rock formation at > 300 m depth by itself provides important radiation-shield, anti-intrusion, and psychological barriers that greatly promote public safety and security compared to temporary surface storage.

When radioactive waste is emplaced within such a conceptual repository design, as shown in Fig. 1.1, two basic periods of analysis regarding the long-term isolation of that nuclear waste can be identified:

• containment period, during which groundwater is initially prevented from contacting the waste form by the canister, and after failure of the canister due to corrosion;

• low-release period, during which groundwater can contact the waste form, allowing the release and transport of dissolved radionuclides through the multiple barriers of the repository eventually to reach the accessible surface environment.

Repository concepts are therefore developed around a combination of both engineered and natural barriers that can favorably affect the containment and low-release behavior of a repository. The number, type, and assigned safety functions of these various multiple barriers varies among concepts, depending on factors such as the type of waste form, radionuclide inventory in the waste form, the type of host rock, the geological, hydrological, and geochemical settings, the required regulatory time scale for safety analysis, etc. In order to develop insights into specific repository concepts, it is useful to identify and understand basic disposal strategies and how they are successful.

1.3 Basic disposal strategies for radioactive materials

Two basic strategies, or principles, for assuring the long-term safe disposal of nuclear waste by a geological repository can be broadly identified:

• containment (sometimes called ‘delay and decay’) and

• constraining concentration (sometimes called ‘dilute and disperse’).

There are, as discussed below, many different way...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Woodhead Publishing Series in Energy
• Preface
• Part I: Part I Introduction to geological disposal of spent nuclear fuels and radioactive waste
• Part II: Geological repository systems: characterisation, site surveying and construction
• Part III: Engineered barrier systems for geological repositories: containment materials and technology
• Part IV: Performance assessment, expert judgement and knowledge management for geological repository systems
• Part V: Radiation protection, regulatory methodologies environmental monitoring and social engagement for geological repository systems
• Index