A long-standing environmental, and potentially costly, issue involved in a nuclear fuel cycle is the disposal of the nuclear waste materials. After several hundred years, radiotoxicity is dominated by 239Pu (half-life is about 524,100 years) and 237Np (half-life is 52,000,000 years). Thus, a major part of the environmental and health risk is directly related to the mobility of these actinides in the geosphere. One important process, that can significantly limits the mobility of actinides, is the incorporation of actinides in mineral phases. Based on the similarities in the neptunium and uranium chemistries, Burns et al. [1] proposed incorporation of Np in uranium phases. In this study, we used density functional theory to understand the mechanism and energetics of charged-balanced Np5+ incorporation into newly synthesized uranium-based metal�organic frameworks (MOF). These MOFs include a homonuclear uranyl MOF (UO2)(C5H2N2O4) and two heterometallic MOFs, (UO2)2(C5H2N2O4)2(C5N2O4H)2Cu3(H2O)2-2H2O and (UO2)2(C5H2N2O4)2(OH)2Cu-2H2O [2]. The charge-balancing mechanisms considered here are a) H+ addition (U6+ ? Np5+ + H+) and b) coupled substitution such as (U6+ + Cu2+? Np5+ +Fe3+). The calculated reaction energy for one phase with H+ addition mechanism is as low as 78 kJ/mol. However, the source of Np and the sink of cations in the environment are usually aqueous complexes. Incorporation energy calculations considering aqueous source phases for Np and aqueous sink phases for U are a more realistic representation of the replacement mechanism and have a significant influence on the thermodynamics of the incorporation/replacement process [3]. 1. Burns, P.C., R.C. Ewing, and M.L. Miller, Journal of Nuclear Materials, 1997. 245 (1): p. 1-9.2. Cahill, C.L., D.T. de Lill, and M. Frisch, Crystengcomm, 2007. 9 (1): p. 15-26.3. Shuller, L.C., R.C. Ewing, and U. Becker, Journal of Nuclear Materials, 2013. 434 (1-3): p. 440-450.

Ab initio modeling of Np incorporation in uranium-based metal organic frameworks

248th National Meeting (2014)

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