Description: The back-end of the nuclear fuel cycle has become the Achilles Heel of nuclear power. After more than 50 years of effort, there are, at present, no operating nuclear waste repositories for the spent nuclear fuel from commercial nuclear power plants or for the high-level waste from the reprocessing of spent fuel. The articles in this issue of Elements describe the status of geological disposal in salt, crystalline rock, clay, and tuff, as presently developed in five countries.

Description: This study provides new insights into the relation between thermally induced structural reorganization and the macroscopic mechanical properties of radiation-damaged titanite. The natural sample contains ca. 30% amorphous fraction. Low-temperature annealing affects only slightly the sample stiffness and leads to a softening resulting from the defect annihilation in crystalline regions. In the high-temperature annealing regime, amorphous domains recrystallize and this leads to further recovery of defects, reduction of interfaces, grain growth, and, in general, an increase in the long-range order. The thermally induced recrystallization is accompanied by massive dehydration leading to considerable stiffening and hardening. This interpretation of the recrystallization process in titanite based on the correlation of new results from nanoindentation and Raman-spectroscopic measurements complementing previous investigations using thermogravimetric and gas analyses by Hawthorne et al. (1991) and infrared spectroscopy by Zhang et al. (2001) . The new data combined with previous work leads to a detailed description of the annealing behavior of a radiation-damaged titanite, which is a complicated process that includes dehydration and atomic-scale structural reorganization. To minimize the influence of surface phenomena on the hardness measurements, the so-called "true" hardness was used instead of the standard hardness calculation ( Oliver and Pharr 1992 ). A comparison shows that the Oliver and Pharr method clearly underestimates the hardness.

Description: The OH stretching vibration frequencies of crystalline zircon that contains water have been investigated by quantum mechanical calculation using CRYSTAL09 and several hybrid functionals. Incorporation mechanisms considered for H in zircon include: (1) hydrogarnet and partial hydrogarnet-type substitution; (2) H-compensated trivalent substitution at the Si site; and (3) H-compensated trivalent substitution at the Zr site. The results provide a clear picture of the H locations in zircon and their associated IR stretching vibrations. Based on the results of structural relaxation and corresponding OH stretching calculations, we can assign the 3420 cm –1 band to a hydrogarnet or partial hydrogarnet-type substitution with polarization dependent on the extent of local relaxation (i.e., closer to E || c for partial hydrogarnet and less polarized for hydrogarnet). These results also show that the 3385 cm –1 ( E c ) IR band originates from H incorporation that accompanies a trivalent cation substitution at the Si-site. As an example of such a substitution, the [AlO 4 /H] defect was investigated. H-incorporation coupled with a trivalent cation substitution at the Zr-site appears to be responsible for generating the 3180 cm –1 ( E c ) IR band. The ab initio calculation on [YO 8 /H] and [LaO 8 /H] structures show that the 3180 cm –1 band represents an average between two distinct bands at ~3220–3250 and ~3100 cm –1 attributed to H locations close to or further away from the Zr-substituted atom. The energetics of these substitutions demonstrate that the overlapping of the two IR bands depends on the substituting cation, and this is controlled, in part, by the thermal history of the sample and/or by the experimental conditions under which the IR spectra are obtained. Furthermore, these results predict for the first time the existence of a peroxy-type O bond associated with the partial hydrogarnet substitution in the zircon structure that now accounts for the ~800 cm –1 bands.

Description: Fission track (FT) thermochornology is essentially based on empirical fits to annealing data of FTs revealed by chemical etching, because, until now, unetched, latent FTs could not be examined analytically at the atomic-scale. The major challenge to such an analysis has been the random orientation of FTs and their extremely small diameters. Here we use high-energy ions (2.2 GeV Au or 80 MeV Xe) to simulate FT formation along specific crystallographic orientations. By combining results from transmission electron microscopy (TEM) of single tracks and small-angle X-ray scattering (SAXS) for millions of tracks, a precise picture of track morphology as a function of orientation is obtained. High-resolution analysis reveals that orientation affects the shape of tracks in apatite and zircon through the preferential creation of damage along directions with highest atomic density. However, track radius does not depend on orientation, contradicting previous reports. Independent of track orientation, track radii, as measured at each point along the entire length of 80 MeV Xe ion tracks in apatite, can be understood using the thermal spike model of Szenes. Thus, the well-known track annealing anisotropy of apatite is not due to track radius anisotropy. The combination of ion-irradiations with TEM and SAXS analysis provides a unique opportunity to understand and model track formation and annealing under various geologic conditions.