Docking 90 Sr radionuclide in cement: An atomistic modeling study Mostafa Youssef a , Roland J.-M. Pellenq b,c,⇑ , Bilge Yildiz a,⇑ a Laboratory for Electrochemical Interfaces, Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA b Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA c <MSE> 2 , the CNRS-MIT Joint Laboratory, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA article info Article history: Available online xxxx Keywords: Molecular simulation Cement Nuclear waste storage Mechanical properties abstract Cementitious materials are considered to be a waste form for the ultimate disposal of radioactive mate- rials in geological repositories. We investigated by means of atomistic simulations the encapsulation of strontium-90, an important radionuclide, in calcium–silicate–hydrate (C–S–H) and its crystalline analog, the 9 Å-tobermorite. C–S–H is the major binding phase of cement. Strontium was shown to energetically favor substituting calcium in the interlayer sites in C–S–H and 9 Å-tobermorite with the trend more pro- nounced in the latter. The integrity of the silicate chains in both cementitious waste forms were not affected by strontium substitution within the time span of molecular dynamics simulation. Finally, we observed a limited degradation of the mechanical properties in the strontium-containing cementitious waste form with the increasing strontium concentration. These results suggest the cement hydrate as a good candidate for immobilizing radioactive strontium. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction The management and disposal of radioactive waste produced by the industry, medicine, radioisotope production facilities, fuel pro- cessing plants and the spent nuclear fuel are major challenges. In most countries the preferred technological approach is the disposal of the radioactive waste in repositories constructed in rock forma- tions hundreds of meters below the earth surface (Ansolabehere et al., 2003). Cementitious materials can be used for different purposes in the near field of the radioactive waste repository. For example, they can be used as a shielded cask to protect the waste package, as gro- uting for sealing cracks in the repository, as liners for the reposi- tory tunnels or most importantly as the inner most waste form itself (Evans, 2008; Komarneni et al., 1988; Wieland et al., 2006). As a waste form, the cement matrix is capable of immobilizing radionuclides. However, the mechanism of immobilization is very species sensitive (Evans, 2008) which implies that an effort has to be devoted to understand the interaction of each radionuclide with the cement matrix. Understanding on a fundamental level the long-term perfor- mance of cement-based materials as waste forms and developing capabilities to project their lifetimes is of utmost importance. To that end we believe that atomistic simulation is able to provide such a fundamental understanding and can be utilized as a predic- tive tool as well. Historically atomistic simulation was not used in cement science because of the absence of a realistic model to rep- resent the major binding phase of cement, the poorly-crystallized calcium–silicate–hydrate (C–S–H). However, with the recent ad- vances in our understanding of the structure of C–S–H at the nano- scale (Allen et al., 2007; Kalinichev et al., 2007; Pellenq et al., 2009), it is possible to incorporate atomistic simulation techniques in studying cementitious materials on equal footing with experiments. C–S–H is the cementitious phase responsible for the uptake of radioactive metal cations (Wieland et al., 2006). Moreover, cation exchange is regarded as one of the key processes that regulate the uptake of metal cations in C–S–H (Mandaliev et al., 2010a; Tits et al., 2006). The immobilization by cation exchange is not limited to the disordered C–S–H in the actual cement paste, but it can also take place in a crystalline family of C–S–H named tobermorite (Komarneni and Roy, 1983; Komarneni et al., 1988; Mandaliev et al., 2010b). In this study we consider the cationic exchange as a mechanism for radioactive metal cations immobilization in both the disordered C–S–H and a member of the tobermorite family which is 9 Å-tobermorite. The spent nuclear fuel is the most challenging source of radio- activity. At different times, different radionuclides are the domi- nant contributors to the overall radioactivity, radiotoxicity and decay heat emitted by the spent fuel. The two fission products strontium-90 and cesium-137 account for the bulk radioactivity 1474-7065/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pce.2013.11.007 ⇑ Corresponding authors. Address: Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA. Tel.: +1 617 253 7117 (R. Pellenq), +1 617 324 4009 (B. Yildiz). E-mail addresses: pellenq@mit.edu (R.J.-M. Pellenq), byildiz@mit.edu (B. Yildiz). Physics and Chemistry of the Earth xxx (2014) xxx–xxx Contents lists available at ScienceDirect Physics and Chemistry of the Earth journal homepage: www.elsevier.com/locate/pce Please cite this article in press as: Youssef, M., et al. Docking 90 Sr radionuclide in cement: An atomistic modeling study. J. Phys. Chem. Earth (2014), http:// dx.doi.org/10.1016/j.pce.2013.11.007