Environmental impact of the nuclear fuel cycle: Fate of actinides
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tinides and the nuclear fuel cycle When uranium is irradiated in a nuclear reactor, the transuranium elements, mainly Pu, as well as the minor actinides, Np, Am, and Cm, become part of the physics and chemistry of the nuclear fuel cycle, as shown in Figure 1. Irradiated thorium forms isotopes of uranium, mainly 233U, as well as transuranium elements, illustrated in Figure 2. Typically, a nuclear reactor will generate ~20 metric tonnes (mt) of used nuclear fuel per year. Worldwide, the approximately 430 nuclear reactors have generated a global inventory of about 270,000 metric tonnes of heavy metal (mtHM), which increases by ~10,000 mtHM/year. In the United States, the used fuel inventory is ~62,000 mtHM, and projected inventories to the end-of-life for presently operating nuclear power plants is at least twice this amount. For typical “burn-ups” (40 MWd/kg of U), approximately 1% of the uranium is converted to transuranium elements, mainly Pu. The global inventory of Pu is presently ~1,900 mt, growing by 70 to 90 metric tonnes per year, mostly retained in the used nuclear fuel.4 For the minor actinides, 237Np, 241Am + 243Am, and 244Cm, global production rates are on the order of 3 to 6, 2 to 2.7, and 0.35 to 0.50 mt per year, respectively. These actinides have a profound impact on strategies for nuclear waste management, as their long half-lives (e.g., 239Pu = 24,100 years; 237Np, 2.1 million years) account for most of the long-term radioactivity, as illustrated in Figure 3.5 These actinides are also fissile, and, in fact, approximately one-third of the energy
generated by a typical light water reactor comes from the fission of 239 Pu that forms by neutron capture and decay reactions on 238U: 238
U + 1no → 239U ® 239Np ® 239Pu
(1)
239
Pu + 1no → fission fragments + 2–3 neutrons (1–2 MeV) + energy (200 MeV/fission).
(2)
The Pu and U can be reclaimed from the used fuel by chemical processing, either to be used to (1) generate energy from a mixed oxide fuel (MOX) of U and Pu or to (2) produce nuclear weapons. Removing the transuranium elements from the used fuel can also reduce the time considered to be important for geologic disposal (although long-lived fission products, such as 135Cs, 129I, and 99Tc still require long-term containment) and change the thermal history of a repository (alpha-decay of actinides is a long-term heat source in a geologic repository). Other strategies, such as an open fuel cycle, reduce the proliferation risk by direct disposal of the used nuclear fuel without processing,6 but this requires long-term containment and an understanding of the behavior of actinides in the environment.
Actinide materials and reprocessing The chemical processing of used nuclear fuel dates to the early days of the Manhattan Project. The first materials used
Rodney C. Ewing, Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, USA; [email protected] Wolfgang Runde, Los Alamos National Laboratory, NM 87545, USA; [email protected] Thomas E. Albrecht-Schmitt, Department of Civil Engineering
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