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Projects: Projects for Investigator
Reference Number	NE/F017979/1
Title	Actinide uptake by carbonates: modelling of molecular processes in radioactive waste systems
Status	Completed
Energy Categories	Nuclear Fission and Fusion(Nuclear Fission, Other nuclear fission) 100%;
Research Types	Basic and strategic applied research 100%
Science and Technology Fields	PHYSICAL SCIENCES AND MATHEMATICS (Chemistry) 30%; ENVIRONMENTAL SCIENCES (Earth Systems and Environmental Sciences) 70%;
UKERC Cross Cutting Characterisation	Not Cross-cutting 100%
Principal Investigator	Dr MC (Michele ) Warren No email address given Earth, Atmospheric and Environmental Sciences University of Manchester
Award Type	R&D
Funding Source	NERC
Start Date	01 August 2008
End Date	31 July 2011
Duration	36 months
Total Grant Value	£328,435
Industrial Sectors	Power
Region	North West
Programme
Investigators	Principal Investigator	Dr MC (Michele ) Warren , Earth, Atmospheric and Environmental Sciences, University of Manchester (99.998%)
	Other Investigator	Professor F Livens , Chemistry, University of Manchester (0.001%) Professor DJ Vaughan , Earth, Atmospheric and Environmental Sciences, University of Manchester (0.001%)
Web Site
Objectives	1. To conduct first principles electronic structure simulations of actinide species for which good experimental data exist (specifically U(VI), U(IV), Cm(III)) and, subsequently, other transuranic species both (a) as impurities in bulk calcite and aragonite and (b) in known actinide carbonates, to determine their electronic and geometric structures and the stability of actinide ions in a range ofgeochemically important carbonate environments.2. To refine structural models for solid-state carbonate environments by combining existing EXAFS and TRLFS data and results from coprecipitation studies with the results of these simulations. 3. To develop a database of atomistic potentials by deriving potential parameters using the results of the first principles simulations conducted for the first objective. These potentials will be applicable to actinide incorporation at the carbonate surface and in the bulk, allow simulations of complex surface geometries and chemistries, and have applications beyond the immediate project. 4. To simulate calcite surfaces including more complex structures such as steps and etch pits, and the incorporation of actinides during calcite growth and dissoluti on. Electronic structure calculations will also be performed in a small number of test cases of such systems to validate the empiricalpotentials further. 5. To simulate adsorption of aqueous uranyl carbonato complexes at a selection of calcite surface geometries, chosen according to results for objective 4, drawing on published literature for additional potentials (such as those involving water m olecules) and possible hydration spheres and other complexes. This will not only place the study in the context of previous work but also complete the geochemical path from solution to bulk and vice versa.
Abstract	The actinide elements are major components of the radioactive wastes produced in all stages of the nuclear fuel cycle, from mine wastes and byproducts of processing to highly radioactive used nuclear fuel. Whilst uranium (U) is dominant, radioactive wastes may also contains neptunium (Np), plutonium (Pu), americium (Am) and curium (Cm). In particular, several forms of U, Np and Pu are long-livedand can exist in different chemical states, someof which are very mobile in the environment. In these circumstances, the interactions of actinides in solution with the surfaces of rocks and minerals will be the principal mechanism by which they will be prevented from entering the biosphere so it is important to understand how actinides interact with surfaces, and also how the surfaces themselvesbehave. One key mineral type is calcite (calcium carbonate), which commonly found as limestone, and which can also form in, for example, underground waste repositories as a result of the interactions of cement with the pre-existing rock. Another form of calcium carbonate, aragonite, is also important. Indeed, naturally occurring actinides in calcite and aragonite can be used to find the ages of rocks and minerals, and to reveal past environmental conditions, from actinide distributions to carbon dioxide concentrations. The behaviour ofactinides in carbonate environments is thus of great importance in many areas of environmental research. Experiments with actinides are very difficult because these elements are all radioactive and some, such as Pu and Am, are extremely hazardous. It wouldtherefore be very helpful if we could understand and predict actinide behaviour by using computer models. However, if we are to make credible predictions, these models have to be based ona detailed description of the actinide-calcite reaction at the molecular scale. The first aim of this project is therefore to develop a good understanding of the key processes involved in incorporation into calciteand reactions with calcite surfaces. The second aim is to use this understanding to define the principles that govern trends both through the actinide series and across different carbonates. We, and several others, have done experiments on actinide reactions with calcite and we have a good collaboration with INE Karlsruhe, a German nuclear waste research institute, which has excellent facilitiesfor experimental work. The results of these experiments can be used as the starting point for both developing and testing computer models. We will devise detailed descriptions of the actinides and their environments and then use these results to develop simpler models that canbe used in the large, complex simulations needed to investigate actinide-mineral interactions. We will collaborate closelywith INE, including exchanges of data and research staff, which will give us access to the latest experimental studies and allow us to carry out a coordinated programme of theoretical and experimental work.
Publications	(none)
Final Report	(none)
Added to Database	04/09/08