Geologic Disposal of High-Level Radioactive Waste

Roland Pusch, Raymond N Yong, Masashi Nakano

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9780815367666
• eBook ISBN: 9781351256780
• Topic: Biological Sciences
• Subtopic: Environmental Science
• Index: Biological Sciences

Geologic Disposal of High-Level Radioactive Waste examines the fundamental knowledge and conditions to be considered and applied by planners and other professionals when establishing national repository concepts, and constructing repositories for the long-term isolation of highly radioactive waste from surrounding crystalline rock. It emphasizes the important roles of structural geology, hydrogeology, hydrochemistry, and construction techniques. It specifically examines the disposal of steel canisters with spent reactor fuel in mined repositories (MR) at medium-depth, and in very deep boreholes (VDH).

While disposal in mined repositories has been widely tested, the option of placing high-level radioactive waste in deep boreholes has been considered in the US, UK, and elsewhere in Europe, but has not yet been tested on a broad scale. This book examines the possibility of safe disposal for very long periods, proposing that the high salt content and density of groundwater at large depths are such that potentially contaminated water would not rise high enough to affect the more shallow biosphere.

Features:

• Presents the best practices for disposal of spent fuel from nuclear reactors.
• Assesses waste isolation capacities in short- and long-term perspectives, and the associated risks.
• Describes site selection principles and the economics of construction of different types of repositories.
• Includes an appendix which provides the latest international recommendations and guidelines concerning the disposal of highly radioactive waste.

1	Managing High-Level Radioactive Wastes

1.1 GENERATION OF HLW IN NUCLEAR FUEL CYCLE

One of the biggest sets of challenges and responsibilities confronting operators of nuclear power generating plants is management of the high level of radioactivity of used and discarded elements associated with the operation of the power plants. The level of radioactivity generated by these elements renders them in a class generally identified by regulatory bodies as high-level radioactive wastes (HLWs). Figure 1.1 gives a schematic view of the two main contributors to this set of problems in the nuclear fuel cycle.

Radioactive wastes will be produced at all stages of the nuclear fuel cycle and also in the front-end processes. The low-level radioactive wastes (LLW, using the IAEA classification scheme) shown in the diagram come from various generating sources in the nuclear fuel cycle—mainly as byproducts obtained in the course of operation of the reactor and the other facilities in the nuclear fuel cycle. Final disposal of the LLW will be in secure landfills, secure containers, or in solidified form for placement in the landfill (Pusch et al., 2017). The management and disposal of the high-level radioactive wastes is a problem of considerable concern for most operators and regulatory agencies of nuclear power plants, since HLW mainly comes from spent nuclear fuel. Final disposal of the spent nuclear fuel is the main issue that is addressed in this book.

1.2 RADIONUCLIDES AND RADIOACTIVITY IN HIGH-LEVEL RADIOACTIVE WASTES

1.2.1 HIGHLY RADIOACTIVE NUCLIDES

High-level radioactive wastes (HLWs) include both spent nuclear fuel and various highly radioactive materials such as transuranic (TRU) radionuclides obtained in the reprocessing of spent nuclear fuel. Transuranic wastes, which are solid radioactive elements obtained from the irradiation of uranium and thorium in the reactors, contain isotopes that have atomic numbers higher than uranium. They include such isotopes as ²³⁹Pu (plutonium), ²⁴³Am (americium), and ²³⁷Np (neptunium), with half-lives of 24.1 × 10³, 7.38 × 10³, and 2.14 × 10⁶ years, respectively.

While the major constituent in spent fuel is ²³⁸U, the different kinds of reactors used—for example, pressurized water reactors (PWR), boiling water reactors (BWR), and liquid-metal fast-breeder reactors (LMFBR)—will produce different proportions of constituents in the spent fuel. In general, one would have about 95% ²³⁸U and about 1% ²³⁵U, with the remaining 4% being plutonium, activation and fission products, and transuranics. The long half-lives of highly radioactive isotopes and the high thermal heat generated by the spent nuclear fuel make final containment-disposal of the fuel schemes critically important—particularly in respect to long-term (thousands and thousands of years’) safety and protection of biotic receptors and the environment.

FIGURE 1.1 The nuclear fuel cycle showing sources generating high-level radioactive wastes.

The activation products are obtained in the fission process when neutrons interact with different elements present, mainly as impurities, in the nuclear fuel and are essentially products of neutron capture. The activation products include ¹⁴C and ³⁶Cl, and metals such as Co, Nb, and Ni which have become radioactive. The half-lives of ⁵⁴Co and ⁶⁰Co are 0.193 seconds and 5.3 years respectively, while the half-lives of ⁹⁴Nb, ⁹⁵Nb, ⁵⁶Ni, and ⁵⁹Ni are 2 × 10⁴ years, 35 days, 6 days and 8 × 10⁴ years respectively.

The fission products, which are obtained in the fission process as nuclei formed by the fission of higher-mass elements, consist of numerous kinds of radioisotopes. They include ³H with β⁻ radiation and a half-life of 12.3 years; ⁸⁵Kr with β⁻ and γ radiation and a half-life of 10.7 years; ¹³⁵Cs with β⁻ radiation and a half-life of 2.3 E6 years; and ¹²⁹I with β⁻ and γ⁻ radiation and a half-life of 1.6 E7 years.

Actinides are chemical elements with atomic numbers that range from 89 (actinium, Ac) to 103 (lawrencium, Lr). Absorption by ²³⁸U of some of the neutrons produced when ²³⁵U is fissioned in the reactor produces plutonium and various kinds of actinides. Transuranics are transuranium elements with atomic numbers greater than 92 (uranium)—except for Np and Pu, they are generally considered as synthetic elements since they are produced in the course of reactions in reactors or particle accelerators.

A representative number of radioactive nuclides and their half-lives are shown in Table 1.1.

TABLE 1.1
Representative Radioactive Nuclides Shown in Order of Their Half-Lives in Their Respective Groups

Nuclides	Decay	Half Life
	Activation and Fission Products
Cobalt-62	β^−, γ	1.51	m
Niobium-95	β^−, γ	3.5	d
Iodine-131	β^−, γ	8.04	d
Cerium-141	β^−, γ	32.5	d
Strontium-89	β^−, γ	50....

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Authors
• Chapter 1 Managing High-Level Radioactive Wastes
• Chapter 2 Host Rock of High-Level Radioactive Wastes Repositories
• Chapter 3 Engineered Barriers in Repositories
• Chapter 4 Practical Issues in Disposal of High-Level Radioactive Waste in Crystalline Rock
• Chapter 5 Maturation of Clay Barriers—Guiding Experiments and Modeling
• Chapter 6 Assessment and Choice of MR and VDH Concepts—Practical Issues
• Index