1
Introduction

1.1 Background of Nuclear Waste Problem

By definitions a waste is a material for which no further use is foreseen. For legal and regulatory purposes a radioactive (nuclear) waste is that waste which contains or is contaminated with radionuclides at concentrations or activities greater than clearance levels as established by the regulatory body. It is always recognized that this definition is purely for regulatory purposes, and that material with activity concentrations equal to or less than clearance levels is still radioactive from a physical viewpoint, although the associated radiological hazards are considered negligible [International Atomic Energy Agency (IAEA), 2003a]. Over recent years large amounts of radioactive waste have been generated during the production and application of radioactive materials both for peaceful and military purposes. The knowledge of the hazard associated with exposure to these wastes led to the adaptation of waste management strategies that relies on the concepts of containment and confinement. In radioactive waste repository, confinement may be provided by the wasteform and its container, whereas containment may be provided by the surrounding host rock (IAEA, 2013). The selection of the wasteform type and disposal option is determined based on the hazard imposed by the wastes. Although containment and confinement concepts have proven efficiency in isolating nuclear waste, there were some cases dating back to the early 1950s where radioactive wastes were disposed of unsolidified in unlined trenches. These practices led to radioactivity leaks in many sites, such as in Hanford, Washington, USA. The evaluation of the remediation costs and the hazard imposed from these practices on human health and the environment resulted in recognition of the need to have more rigorous confinement and containment strategies. This led to the development of new waste management systems which utilize volume reduction techniques and solidification/stabilization technologies to produce stable wasteforms and implement the multi-barrier disposal concept to ensure safe disposal of these wastes.

Currently safe management of nuclear wastes is a subject that is receiving considerable attention from public and different governmental, regional and international bodies. This recognition has not only stemmed from the huge volume of the cumulative wastes and the diversity of their chemical, biological and radiological hazards but also because the public relates their acceptance for new nuclear power programmes to their confidence in the waste management practice (Abdel Rahman, 2012). In the following sections, the facilities that generate nuclear wastes will be briefly introduced, different waste classification schemes and waste management activities will be presented and matrix material for nuclear waste immobilization will be highlighted.

1.2 Nuclear Industry Facilities

The nuclear fuel cycle ( NFC) and radioisotope production and application facilities are considered the main generators for nuclear wastes. The NFC includes all operations associated with the production of nuclear energy, namely mining and milling, processing and enrichment of uranium or thorium; manufacture of nuclear fuel; operation of nuclear reactors (including research reactors); reprocessing of nuclear fuel; any related research and development activities and all related waste management activities (including decommissioning). During the lifecycle activities of these facilities, different amounts of wastes with varying characteristics are produced. Within the operational and decommissioning phases only nuclear wastes are generated whereas other phases produce non-nuclear wastes, for example soils from excavation, building materials and so on. Nuclear wastes produced within the operational phase are usually characterized by their limited amounts; on the other hand, a much larger volume of waste is generated during the decommissioning phase (IAEA, 2007). This section will introduce operational processes that take place in different nuclear facilities and lead to generation of radioactive wastes, whereas the wastes generated during the decommissioning phase of these facilities will be discussed in Chapter 6.

1.2.1 NFC Facilities

The NFC refers to activities associated with the production of electricity using nuclear reactors (IAEA, 2003a). They are classified based on the existence of recycling option into two categories, namely open and closed NFCs, as illustrated in Figure 1.1 (Ojovan and Lee, 2005). Facilities that operate from nuclear ore extraction to fuel loading into a nuclear reactor are known as front-end NFC facilities; these include mines, mills, fuel enrichment and fuel fabrication facilities. After using the fuel in the reactor, the facilities that deal with used (spent) fuel and radioactive waste are referred to as back-end NFC facilities; they include fuel storage and/or fuel reprocessing plants. The operation of each facility is associated with the generation of different types of nuclear wastes. It is worth mentioning that nuclear materials generally can pose chemical, radiological and flammability hazards. Accordingly, there is a need to specify these hazards and implement certain safety measures to counter these hazards. Table 1.1 lists the safety aspects associated with the hazard of nuclear wastes at NFC facilities (IAEA, 2005a).

Figure 1.1 Open and closed NFCs.

Reproduced with permission from Ojovan and Lee, 2005. © 2005, Elsevier

Table 1.1 Hazard identification at different NFC facilities

Reproduced with permission from IAEA, 2005a. © 2005, IAEA.

X, hazard may be of concern; XX, hazard of concern.

1.2.1.1 Mining and Milling Facilities

Mining uranium ore is the first step in any NFC, where uranium is extracted from a mine and then concentrated in a mill. The uranium mill is usually located near the mine to reduce shipping charges. The concentration processes involved include crushing, grinding, leaching, precipitation, solvent extraction and ion exchange (Benedict et al., 1981). The concentrate is composed of uranyl nitrate solution, [UO2(NO3)2], and solid ammonium diuranate, [(NH4)2U2O7], which is known as yellow cake. The operation of these facilities generates large amounts of solid wastes in the form of natural materials, that is displaced soil, and radioactive contaminated tailings. The radioactivity content in tailings is above the background level; usually they are returned to the pit from where the uranium ore was originally extracted and the site rehabilitated for further use (see Section 6.4). In some cases this operation is not economically feasible, so the tailings are stored then transported to a long-term stable structure and the site is rehabilitated for further use (Alexander and McKinley, 2007). Also, large volumes of effluent are generated during the operation of mines and mills; historically these effluents were held in storage ponds and eventually evaporated to solids (Benedict et al., 1981). Currently the treatment of these effluents and their control is becoming a concern because of the strengthened regulatory requirements. The main problems that arise when dealing with these effluents are due to their large volumes and the nature of contaminants where both radioactive and non-radioactive toxicants exist (IAEA, 2004).

1.2.1.2 Uranium Refining Facilities

Refining uranium concentrate is performed by purifying the concentrate, where chemical impurities are removed, followed by conversion of purified concentrate into a suitable chemical form. The purification is conducted by dissolving the concentrate in nitric acid and then applying solvent extraction to remove impurities. Purified concentrate is then converted to uranium trioxide (UO3) or uranium dioxide ( UO2), depending on the type of reactor. To produce UO3, either thermal denitration (TDN) or ammonium diuranate (ADU) could be used, where ammonium uranyl carbonate (AUC) is used to obtain UO2. TDN is a one-step process from which fine UO3 powder is produced. With ADU and AUC, the purified uranium is subjected to precipitation, filtration and calcinations/calcinations with hydrogen; Figure 1.2 illustrates these processes. The wastes arising from refining processes are mainly generated during the purification step. They include liquid effluent sludge, insoluble and filter aid, and drums (IAEA, 1999a).

Figure 1.2 Flowchart for the production of uranium trioxide and uranium dioxide....