XAFS and molecular dynamics study of natural minerals, analogues of ceramics for nuclear waste storage

Abstract

Natural actinides (U and Th) are harmful for the crystalline structure of natural minerals, due to their irradiation. Natural minerals can then become amorphous to x-ray diffraction (“metamict”) after being irradiated throughout a long period of time (108 years). Then, they are used as natural analogues of ceramics for nuclear waste storage. XAFS studies were performed in zircon, monazite and titanite to understand the effect of radiation damage on the local structure around Th, U, Zr and P and compared to available molecular dynamics (MD) simulations. In zircon, a local expansion around actinides (when substituting for Zr) is found. The radial expansion is a function of the metamictisation degree: up to ∼4 Å in crystalline zircon and larger in the metamict counterparts. Ab-initio calculations (FEFF7) were performed around Zr (∼23000 sites) and around U (1000 to 3000 sites) in various crystalline and alpha-decay damaged zircon MD simulations. The calculated averaged EXAFS spectra confirms this expansion, which validates the use of the potentials used in the simulations as well as the alpha decay damage model considered in these MD simulations. Tetravalent actinides were found to be 8-coordinated in the undamaged structure, whereas their coordination drops to 7 in the damaged structures. In contrast to zircon, no local expansion around actinides in monazite was detected, despite some polymerization around P is measured (related to radiation damage). Finally, in some phases (such as titanite), actinides are found as oxyde-type clusters (ThO2, UO2). Consequently, actinides do not “systematically” substitute for major actions in these structure, in contrast to the common belief in mineralogy.