C. M. WAI, University of Idaho, USA

Abstract: Minimizing liquid waste generation in the nuclear fuel cycle is of great importance to the future of nuclear energy. Separation techniques utilizing green solvents, supercritical fluid carbon dioxide and ionic liquids, for dissolution and extraction of uranium dioxide and fission products relevant to nuclear waste management are described in this chapter. An industrial demonstration of the supercritical fluid technology for recovering enriched uranium from the incinerator ash produced by the light water reactor fuel fabrication process by Areva NP in Richland, Washington is a good example of the new trend for treating nuclear wastes. Prospects and advantages of these emerging green techniques for nuclear fuel reprocessing and radioactive waste treatment are discussed.

Key words: green extraction techniques, supercritical fluid, ionic liquid, nuclear waste, spent fuel.

14.1 Introduction

With the threat of global warming, developing environmentally sustainable energy sources to replace traditional fossil fuels is of ultimate importance for the survival of our civilization. Nuclear power is free of carbon emission. Currently, nuclear energy contributes to about 20% of the electricity gener­ated in the USA compared to 80% of that in France. One public concern regarding expanding use of nuclear energy for power generation in many countries including the USA is the economic and environmental issues associated with managing the wastes produced by nuclear power genera­tion. Traditional methods of treating nuclear wastes and reprocessing spent fuel require aqueous acids and organic solvents for dissolution and separa­tion of radioactive elements with the unavoidable consequence of generat­ing large volumes of liquid wastes. Minimizing waste generation in the nuclear fuel cycle is obviously of great importance to the future of nuclear energy. Searching new technologies for managing nuclear wastes in an environmentally sustainable way has become an active research area in recent years. This chapter presents two emerging techniques utilizing envi­ronmentally sustainable solvents, namely supercritical fluid carbon dioxide and room temperature ionic liquids, for separation of actinides, lanthanides and fission products relevant to nuclear waste management and spent fuel reprocessing.

Supercritical fluid carbon dioxide (sc-CO2) and ionic liquids (ILs) are considered green solvents for chemical reactions and separations (Phelps et al. 1996, Welton 1999). Research in sc-CO2 dissolution and extraction of metal species started in the early 1990s (Laintz et al. 1991, 1992). Even in the early stage of the technology development, it was realized that one potential application of this new extraction technique would be in the area of nuclear waste treatment, because supercritical fluid extraction does not require conventional liquid solvents. Two decades later, supercritical fluid extraction technology has emerged as an acceptable green technique for treating nuclear wastes. One example is a recent announcement by AREVA NP to construct a supercritical fluid extraction plant for recovering enriched uranium from the incinerator ash waste produced by the light water nuclear reactor fuel fabrication process (Smith and Thomas 2008). The AREVA NP project currently in progress in Richland, Washington represents the first industrial effort of adopting a non-traditional green technology for nuclear waste management. Some information regarding the AREVA’s supercritical fluid extraction technology will be described later in Section 14.4.

Ionic liquids have unique properties including non-flammable nature, near zero vapor pressure and high solubilities for a variety of compounds (Welton 1999, Dietz and Dzielawa 2001). These properties make ILs attrac­tive for replacing volatile organic solvents traditionally used in various liquid-liquid extractions. In addition, ILs like sc-CO2 show good radiation stability (Visser and Rogers 2003, Dietz and Dzielawa 2001, Mekki et al. 2006) an attractive property for their utilization as media for processing radioactive materials. Both sc-CO2 and IL-based separation techniques are emerging as potential alternatives for replacing the conventional aqueous acids and organic solvent-based techniques for managing nuclear wastes.