Nuclear waste minor actinides

The elucidation of actinide chemistry in solution is important for understanding actinide separation and for predicting actinide transport in the environment, particularly with respect to the safety of nuclear waste disposal.72,73 The uranyl CO + ion, for example, has received considerable interest because of its importance for environmental issues and its role as a computational benchmark system for higher actinides. Direct structural information on the coordination of uranyl in aqueous solution has been obtained mainly by extended X-ray absorption fine structure (EXAFS) measurements,74-76 whereas X-ray scattering studies of uranium and actinide solutions are more rare.77 Various ab initio studies of uranyl and related molecules, with a polarizable continuum model to mimic the solvent environment and/or a number of explicit water molecules, have been performed.78-82 We have performed a structural investigation of the carbonate system of dioxouranyl (VI) and (V), [U02(C03)3]4- and [U02(C03)3]5- in water.83 This study showed that only minor geometrical rearrangements occur upon the one-electron reduction of [U02(C03)3]4- to [U02(C03)3]5-, which supports the reversibility of this reduction. 

Ewing, R. C., Weber, W. J. Lian, J. (2004) Pyrochlore (A2B2O7) A nuclear waste form for the immobilization of plutonium and minor actinides. Journal of Applied Physics, 95, 5949-5971. 

Madic, C., Lecomte, M., Dozol, J.F., Boussier, H. 2004. Advanced chemical separations of minor actinides from high active nuclear wastes. EURADWASTE 04, Luxembourg, Belgium, March 24 to April 1. 

As no technology can selectively transmute minor actinides to a degree meaningful for waste management while they are contained in the spent nuclear fuel, these elements must be separated from the neutron-absorbing elements before being properly transmuted. In the case of trivalent minor actinides, this preliminary step is further necessary because of the following reasons ... 

Nuclear fuel reprocessing and partitioning allow recycling of useful fissionable materials such as uranium and plutonium, and remove harmful long-lived minor actinides (americium and curium). It is necessary also for safety storage of high-level liquid wastes(l). In order to improve efficiency of mutual separation between lanthanide and actinide elements, design of useful extractants are requisite. 

Minor actinides present in some nuclear waste produce large quantities of helium when they decay. When considering materials for storage of this waste, it is necessary to have some knowledge of helium diffusion in the material matrix. Helium diffusion into possible materials has been simulated by exposing them to a He ion beam (i.e., implantation). The implanted helium has then been profiled using the He(D,p)" He nonresonant nuclear reaction enabling diffusion coefficients to be determined. 

Nuclear waste, either in the form of spent fuel reprocessing, is associated with a radiotoxicity potential due to minor actinides (MA) and aon products (FP). The possibility of partitioning minor actinides out of the waste and transmuting them into less hazardous nuclides has been proposed and its technical feasibility is being studied. 

Although nuclear fission reaction does not occur between long-lived actinide nuclei and thermal neutrons in LWRs, it does with fast neutrons. Since nuclear fission reduces radioactive waste, it is beneficial in terms of a lowered environmental burden rather than effective utilization of resources. Cross section ratios of fission to capture of actinide nuclides have been compared between the dominant neutron energy range of a fast reactor (FR) and that of an LWR. The results show that the ratios of cross section for minor actinide (MA) nuclides of the FR in particular are one or more orders of magnitude larger than those of the LWR. Therefore, neutrons of the FR can burn more actinide nuclides in the core. 

In the Reprocessing Fuel Cycle (RFC) option, the unused uranium and the plutonium produced in the reactor are recovered leaving the minor actinides with the fission products as HLW. (The radiotoxicity of these wastes will be significantly less than that of the spent fuel although the toxic lifetime is determined by the minor actinides - neptunium, americium, curium - and, to a lesser extent, by some of the long-lived fission products content of the waste.) As mentioned previously, this was the scenario initially envisioned by the nuclear power industry to reprocess fuel for two reasons ... 

Madic, C., M.J. Hudson, J.-O. LUjenzin, J.-P. Glatz, R. Nannidni, A. Facchini, Z. Kolarik, and R. Odoj. 2002. Progress in Nuclear Engineering 40 3-4, 523-526. (Recent achievement in the development of partitioning processes of minor actinides from nuclear wastes obtained in the frame of the NEWPART Eurojjean Programme 1996-1999.)... 

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