Iron-technetium alloys are of relevance to the development of waste forms for disposition of radioactive technetium-99 obtained from spent nuclear fuel. Corrosion of candidate waste forms is a function of the local cohesive energy ( ) of surface atoms. A theoretical model for calculating is developed. Density functional theory was used to construct a modified embedded atom (MEAM) potential for iron-technetium. Materials properties determined for the iron-technetium system were in good agreement with the literature. To explore the relationship between local structure and corrosion, MEAM simulations were performed on representative iron-technetium alloys and intermetallics. Technetium-rich phases have lower , suggesting that these phases will be more noble than iron-rich ones. Quantitative estimates of based on numbers of nearest neighbors alone can lead to errors up to 0.5？eV. Consequently, atomistic corrosion simulations for alloy systems should utilize physics-based models that consider not only neighbor counts, but also local compositions and atomic arrangements. 1. Introduction The development of long-term containment strategies for spent nuclear fuel requires a combination of careful experiments and theoretical studies so that the realistic lifetimes of these strategies can be confidently predicted. Currently, a number of different containment strategies are being considered, one of which involves the storage of certain fission products within a metallic alloy waste form [1]. Chief among these fission products is technetium-99, which has a half-life of ~105 years [2]. Studies have demonstrated that technetium can be alloyed with stainless steel with significant mass-loadings and that these waste forms have considerable corrosion resistance [3]. Predictions of the long-term stability of this material can be obtained through the coupling of the results of accelerated testing studies with a rigorous, physics-based theoretical model. Corrosion is believed to be the chief process by which stored, spent nuclear fuel will degrade over time and have the potential to release radionuclides to the environment [1, 4, 5]. Modeling corrosion at long-time scales requires a thorough and fundamental understanding of the electrochemical stability of such systems, particularly the surface phenomena (Figure 1) [6]. Surface processes that are important to consider include passivation (formation of protective oxide films), depassivation (the rupture of such films by localized dissolution, mechanical damage, or presence of deleterious chemistries), mass-transport (at the
References
[1]
W. L. Ebert, J. Cunnane, M. Williamson et al., “FY 2010 status report: developing an iron-based alloy waste form,” in Fuel Cycle Research and Development, Argonne National Laboratory, Chicago, Ill, USA, 2010.
[2]
N. Contributors, “Chart of nuclides,” in National Nuclear Data Center, A. A. Sonzogni, Ed., Brookhaven National Laboratory, New York, NY, USA, 2008.
[3]
D. Kolman, G. D. Jarvinen, C. D. Taylor et al., “Corrosion and passivity behavior of technetium waste forms,” in Corrosion, National Association of Corrosion Engineers, Houston, Tex, USA, 2011.
[4]
R. B. Rebak, “Material corrosion issues for nuclear waste disposition in Yucca mountain,” Journal of Management, vol. 60, no. 1, pp. 40–43, 2008.
[5]
D. Shoesmith, “Fuel corrosion processes under waste disposal conditions,” Journal of Nuclear Materials, vol. 282, no. 1, pp. 1–31, 2000.
[6]
D. Landolt, “Introduction to surface reactions: electrochemical basis of corrosion,” in Corrosion Mechanisms in Theory and Practice, P. Marcus, Ed., pp. 1–18, Marcel Dekker, New York, NY, USA, 2002.
[7]
P. F. Weck, E. Kim, F. Poineau, and K. R. Czerwinski, “Structural evolution and properties of subnanometer Tcn ( ) clusters,” Physical Chemistry Chemical Physics, vol. 11, no. 43, pp. 10003–10008, 2009.
[8]
F. Poineau, T. Hartmann, P. F. Weck et al., “Structural studies of technetium-zirconium alloys by X-ray diffraction, high-resolution electron microscopy, and first-principles calculations,” Inorganic Chemistry, vol. 49, no. 4, pp. 1433–1438, 2010.
[9]
C. D. Taylor, “Surface segregation and adsorption effects of iron-technetium alloys from first-principles,” Journal of Nuclear Materials, vol. 408, no. 2, pp. 183–187, 2011.
[10]
M. I. Baskes and R. A. Johnson, “Modified embedded atom potentials for HCP metals,” Modelling and Simulation in Materials Science and Engineering, vol. 2, no. 1, article 011, pp. 147–163, 1994.
[11]
W. Kohn and L. J. Sham, “Self-consistent equations including exchange and correlation effects,” Physical Review, vol. 140, no. 4, pp. A1133–A1138, 1965.
[12]
P. Hohenberg and W. Kohn, “Inhomogeneous electron gas,” Physical Review, vol. 136, no. 3, pp. B864–B871, 1964.
[13]
G. Kresse and D. Joubert, “From ultrasoft pseudopotentials to the projector augmented-wave method,” Physical Review B, vol. 59, no. 3, pp. 1758–1775, 1999.
[14]
G. Kresse and J. Hafner, “Ab initio molecular dynamics for liquid metals,” Physical Review B, vol. 47, no. 1, pp. 558–561, 1993.
[15]
L. Vitos, Computational Quantum Mechanics for Materials Engineers: The EMTO Method and Applications, Springer-Verlag, London, UK, 2007.
[16]
J. B. Darby, D. J. Lam, L. J. Norton, and J. W. Downey, “Intermediate phases in binary systems of technetium-99 with several transition elements,” Journal of The Less-Common Metals, vol. 4, no. 6, pp. 558–563, 1962.
[17]
J. P. Perdew, K. Burke, and M. Ernzerhof, “Perdew, Burke, and Ernzerhof reply,” Physical Review Letters, vol. 80, no. 4, pp. 891–891, 1998.
[18]
H. J. Monkhorst and J. D. Pack, “Special points for Brillouin-zone integrations,” Physical Review B, vol. 13, no. 12, pp. 5188–5192, 1976.
[19]
M. Methfessel and A. T. Paxton, “High-precision sampling for Brillouin-zone integration in metals,” Physical Review B, vol. 40, no. 6, pp. 3616–3621, 1989.
[20]
B.-J. Lee and M. I. Baskes, “Second nearest-neighbor modified embedded-atom-method potential,” Physical Review B, vol. 62, no. 13, pp. 8564–8567, 2000.
[21]
S. Plimpton, “Fast parallel algorithms for short-range molecular dynamics,” Journal of Computational Physics, vol. 117, no. 1, pp. 1–19, 1995.
[22]
D. M. Artymowicz, J. Erlebacher, and R. C. Newman, “Relationship between the parting limit for de-alloying and a particular geometric high-density site percolation threshold,” Philosophical Magazine, vol. 89, no. 21, pp. 1663–1693, 2009.
[23]
J. Erlebacher, “An atomistic description of dealloying porosity evolution, the critical potential, and rate-limiting behavior,” Journal of the Electrochemical Society, vol. 151, no. 10, pp. C614–C626, 2004.
[24]
R. S. Lillard, G. F. Wang, and M. I. Baskes, “The role of metallic bonding in the cristallographic pitting of magnesium,” Journal of the Electrochemical Society, vol. 153, no. 9, pp. B358–B364, 2006.
[25]
G. R. Love, C. C. Koch, H. L. Whaley, and Z. R. McNutt, “Elastic moduli and Debye temperature of polycrystalline technetium by ultrasonic velocity measurements,” Journal of The Less-Common Metals, vol. 20, no. 1, pp. 73–75, 1970.
[26]
A. F. Guillermet and G. Grimvall, “Thermodynamic properties of technetium,” Journal of The Less-Common Metals, vol. 147, no. 2, pp. 195–211, 1989.
[27]
H. Okamoto, Binary Alloy Phase Diagrams, ASM International, Russell Township, Ohio, USA, 2nd edition, 1990.
[28]
V. L. Moruzzi, J. F. Janak, and A. R. Williams, Calculated Electronic Properties of Metals, Pergamon Press, New York, NY, USA, 1978.
[29]
J. G. Darab and P. A. Smith, “Chemistry of technetium and rhenium species during low-level radioactive waste vitrification,” Chemistry of Materials, vol. 6, no. 5, pp. 1004–1021, 1996.
[30]
C. Taylor, M. Neurock, and J. R. Scully, “First-principles investigation of the fundamental corrosion properties of a model Cu38 nanoparticle and the (111), (113) surfaces,” Journal of the Electrochemical Society, vol. 155, no. 8, pp. C407–C414, 2008.
