Titanate ceramics for high-level nuclear waste immobilization

Peaceful civilian uses of radioactive materials, such as nuclear power and the application of medical radioisotopes, generate radioactive by-products (wastes), some of which have to be isolated from the biosphere for long periods of time (10,000-1,000,000 years). Many countries have agreed that deep geologic disposal (Fig. 11.1) is the most viable long-term solution for high-level waste (HLW). However, at present, there is only one geologic HLW repository under construction (in Finland) and substantial amounts of HLW are stored in temporary facilities. Consequently, the management of HLW engenders a great deal of scientific, engineering, political and social interest. Definitions of HLW vary, but include spent nuclear fuel (SNF) and the residues from reprocessing of SNF. Several countries (e.g. Canada, Finland, Spain, Sweden, United States) have adopted a once-through nuclear fuel cycle with eventual direct disposal of SNF, whereas others (e.g. Belgium, China, France, India, Japan, Russia, United Kingdom) employ multiple cycles of fuel usage with intermediate reprocessing steps. Reprocessing generates small volumes (relative to SNF) of highly radioactive, heat-producing waste. It is generally envisaged that SNF and/or HLW-bearing ''waste forms'' will be placed in geologic repositories, with a design similar to the one shown in Fig. 11.1. Borosilicate glass was the first waste form proposed for the immobilization of HLW. It is currently the waste form of choice in many countries that reprocess their commercial SNF, primarily due to the simplicity of production and because of a well-established glass industry. The use of glass has been questioned, however, especially regarding its long-term thermodynamic stability and its aqueous durability, particularly at elevated temperature, under the conditions of a geologic repository, where the potential for higher dissolution rates can result in the release of radionuclides into the environment. Consequently, ceramic materials were proposed as alternative waste forms for HLW, and these are regarded as attractive materials for the long-term isolation of HLW in the geologic environment. In this book chapter, we will review different types of ceramic waste forms, focusing on titanate ceramics, their design and fabrication, and their aqueous durability, and discuss radiation-damage effects in individual phases and how these can be modeled. In addition, we will review how the design and development of these ceramics have been influenced by important results obtained from studies of so-called ''natural-analogue materials''.