XAFS And Molecular Dynamics Study of Natural Minerals, Analogues of Ceramics for Nuclear Waste Storage

Journal Article · Fri Oct 27 04:00:00 EDT 2006 · Phys.Scripta T115:928,2005

OSTI ID:894120

Harfouche, M; Farges, F; Crocombette, J P; Flank, A M

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 (10{sup 8} 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 {approx}4 {angstrom} in crystalline zircon and larger in the metamict counterparts. Ab-initio calculations (FEFF7) were performed around Zr ({approx}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 (ThO{sub 2}, UO{sub 2}). Consequently, actinides do not 'systematically' substitute for major actions in these structure, in contrast to the common belief in mineralogy.

• Research Organization: Stanford Linear Accelerator Center (SLAC)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC02-76SF00515
• OSTI ID: 894120
• Report Number(s): SLAC-REPRINT-2005-306
• Journal Information: Phys.Scripta T115:928,2005, Journal Name: Phys.Scripta T115:928,2005
• Country of Publication: United States
• Language: English

Related Subjects

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES
ABSORPTION
ACTINIDES
ALPHA DECAY
CERAMICS
FINE STRUCTURE
IRRADIATION
MINERALOGY
MONAZITES
NATURAL ANALOGUE
Other
OTHER
POLYMERIZATION
RADIATIONS
RADIOACTIVE WASTES
SPECTRA
STORAGE
TITANITE
X-RAY DIFFRACTION
ZIRCON