| AVS 65th International Symposium & Exhibition | |
| Actinides and Rare Earths Focus Topic | Friday Sessions |
| Session AC+MI+SA-FrM |
| Session: | Actinide and Rare Earth Theory and Related Measurements |
| Presenter: | Nikolas Kaltsoyannis, University of Manchester, UK, United Kingdom of Great Britain and Northern Ireland |
| Correspondent: | Click to Email |
Over half of the World’s stockpile of civil plutonium (c. 126 tonnes) is stored at Sellafield in the UK as PuO2 powder in sealed steel cans. There is evidence of gas generation in some of these cans. Many routes to gas production have been suggested, several of which involve complex, inter-connected and poorly understood PuO2/H2O interactions.
We have an ongoing project to study computationally the interaction of AnO2 (An = U, Pu) surfaces with water. Standard periodic boundary condition (PBC) implementations of DFT using generalized gradient approximation (GGA) functionals can fail to reproduce key features of actinide solids, e.g. predicting metallic properties in systems known to be insulating. This failure stems from incorrect description of the strongly correlated 5f electrons, which are overly delocalized by the GGA, and the standard solution to this problem is to correct the GGA functionals with an onsite Coulomb repulsion term known as the Hubbard U. An alternative solution is to employ hybrid DFT, in which some of the exact exchange energy of Hartree‑Fock theory is incorporated into the Hamiltonian. Such functionals typically produce more localized 5f electrons, and recover insulator behavior. They are, however, extremely expensive to employ in PBC calculations, and hence are rarely used. We have therefore sought a model which allows the routine use of hybrid DFT in AnO2/water systems, and have adopted the periodic electrostatic embedded cluster method (PEECM), in which a quantum mechanically treated cluster is embedded in an infinite array of point charges. We treat a cluster of AnO2 and adsorbing water molecules using hybrid DFT (PBE0) whilst the long-range electrostatic interactions with the bulk are modelled via embedding in point charges.
In this presentation, I shall describe the results of both PBC and PEECM studies of the interactions of water with both stoichiometric and reduced (oxygen vacancy) {111}, {110} and {100} surfaces of UO2 and PuO2. The geometries and energetics of single and multiple layers of water will be presented, together with our calculations of water desorption temperatures, from which we propose an alternative interpretation of experimental data.
[1] J.P.W. Wellington, A. Kerridge, J.P. Austin and N. Kaltsoyannis J. Nuc. Mat. 482 (2016) 124
[2] B.E. Tegner, M. Molinari, A. Kerridge, S.C. Parker and N.Kaltsoyannis J. Phys. Chem. C 121 (2017) 1675