E. M. N. CHIRWA, University of Pretoria, South Africa

Abstract: Due to the impact of the rapidly growing demand for energy worldwide, as well as concerns over global warming, there has been a resurgence of interest in nuclear energy in the developed world. However, further deployment of this otherwise cleaner source of energy in other lesser-developed regions is hindered by concerns over accumulation of radioactive waste from nuclear reactor operation and fuel processing. This chapter discusses emerging biological technologies for treating radioactive waste, focusing on biological reduction and recovery processes that may in future improve the viability of this energy source. The processes discussed include biological reduction of uranium (VI), biosorption of fission products and isotopic biofractionation processes. These technologies offer a possibly cost — effective and environmentally friendly alternative to physical-chemical processes currently used for treating radioactive waste in the nuclear — power industry.

Key words: nuclear waste minimization, uranium (VI) reduction, Cr(VI) reduction, radioisotope bioseparation, cationic species biosorption, fission product recovery.

15.1 Introduction

Due to the impact of the rapidly growing demand for energy, as well as concerns over global warming, there has been a resurgence of interest in nuclear energy in the developed world. Nuclear energy is one of the few economically viable base-load electricity generation technologies, which avoids the production of about eight percent of the present level of CO2 emissions in the energy sector (Mourogov et al, 2002). Currently, there are about 438 nuclear power plants in operation in 31 countries around the world providing about 14 percent of the world’s primary energy needs (IAEA, 2009). The world’s nuclear generating capacity currently stands at about 372 GWe, with the United States of America and France as the major producers, 27 and 17 percent, respectively (Fig. 15.1).

Electricity consumption in developing countries, such as South Africa, has been steadily increasing since the 1980s and, currently, the electricity

0 5 10 15 20 25 30 35

15.1 Global nuclear power generating capacity (%) per country (IAEA, 2009).

consumption/capita is estimated at about 5039.7 kWh. It is predicted that by the year 2025 electricity demand will exceed supply (Musango et al., 2009). To accommodate future expansion in the domestic and industrial electricity consumption, there is need to develop safer, more efficient, and environmentally friendly technologies to replace the current fossil-fuel based power generation technologies.

In the developed world, most nuclear power plants in operation today have reached or are nearing their design life. Most of these power plants were constructed in the 1960s and 1970s. These need to be replaced by new, environmentally sustainable power generation technologies with improved safety features. An example of these new generation reactor systems is the Pebble Bed Reactor (PBR) technology, a Generation IV reactor technol­ogy that utilizes graphite as the neutron moderator. In this latter system, the reactor core is cooled by an inert gas such as helium instead of water (Koster et al., 2003). Because the reactor can be allowed to operate at higher temperatures than the conventional water cooled reactors, the efficiency of the system is greatly enhanced. The drawback is that impurities in the con­tainment material (graphite) are difficult to treat due to the inert nature of the graphite. This results in the accumulation of large volumes of low radia­tion level waste beyond the capacity of designated waste storage areas.

Potential radioactive pollution to the environment does not only concern nuclear power plants. Other activities such as radioisotope manufacturing
and biomedical research also release large amounts of potentially harmful radioisotopes. Most of the radioactive pollutants from the latter activities are organic in nature and are amenable to biological degradation (Cerniglia et al, 1984; Bouwer and Zehnder, 1993). However, due to the toxic nature of the waste stream, an additional effort is required to isolate specialized bacteria that are resistant to the toxic effects of the released compounds and that are capable of breaking the complex structures of the organic compounds (Tikilili and Chirwa, 2009).

The following section provides a concise review of the waste compounds originating from nuclear power generation and other radioisotope releasing activities and how these could be treated for beneficial use. Biochemical processes that have yielded positive results are presented as part of the review and their impact on the future of power generation is evaluated.