Bruce K. McNamara

Fluorine-19 magnetic shielding tensors have been measured in a series of actinide tetrafluorides (AnF₄) by solid state nuclear magnetic resonance spectroscopy. Tetravalent actinide centers with 0–8 valence electrons can form tetrafluorides with the same monoclinic structure type, making these compounds an attractive choice for a systematic study of the variation in the electronic structure across the 5f row of the Periodic Table. Pronounced deviations from predictions based on localized valence electron models have been detected by these experiments, which suggests that this approach may be used as a quantitative probe of electronic correlations.

A novel relationship between noble metal phase particles and fission gas bubble production in used nuclear fuel is described. The majority of Te atoms within noble metal phase undergo radioactive decay to form stable Xe within a few hours after particle formation. This results in the production of clusters of Xe atoms contained within the solid metal matrix exhibiting an equivalent gas bubble pressure approaching 1 GPa. These high pressure bubbles are stabilized by the UO₂ within the bulk of the fuel. However, when these bubbles form near the fuel/cladding interface, in combination with local and temporal damage caused by fission recoil, they are capable of overcoming the fracture strength of the UO₂ and rupturing catastrophically. The force of the resulting bubble rupture is sufficient to eject noble metal phase particles several microns into the cladding. This proposed mechanism explains the observance of noble metal phase in cladding and is consistent with a host of morphological features found near the fuel/cladding interface.

Data demonstrates large variance in the volatilization temperatures of MoO₂ (likely as MoF₆) vs. UO₂ (as UF₆). This forms the basis of a gas-phase separation of ⁹⁹Mo from U.

Uranyl nitrate is a key species in the nuclear fuel cycle. However, this species is known to exist in different states of hydration, including the hexahydrate ([UO₂(NO₃)₂(H₂O)₆] often called UNH), the trihydrate [UO₂(NO₃)₂(H₂O)₃ or UNT], and in very dry environments the dihydrate form [UO₂(NO₃)₂(H₂O)₂]. Their relative stabilities depend on both water vapor pressure and temperature. In the 1950s and 1960s, the different phases were studied by infrared transmission spectroscopy but were limited both by instrumental resolution and by the ability to prepare the samples for transmission. We have revisited this problem using time-resolved reflectance spectroscopy, which requires no sample preparation and allows dynamic analysis while the sample is exposed to a flow of N₂ gas. Samples of known hydration state were prepared and confirmed via X-ray diffraction patterns of known species. In reflectance mode the hexahydrate UO₂(NO₃)₂(H₂O)₆ has a distinct uranyl asymmetric stretch band at 949.0 cm^–1 that shifts to shorter wavelengths and broadens as the sample desiccates and recrystallizes to the trihydrate, first as a shoulder growing in on the blue edge but ultimately results in a doublet band with reflectance peaks at 966 and 957 cm^–1. The data are consistent with transformation from UNH to UNT as UNT has two inequivalent UO₂²⁺ sites. The dehydration of UO₂(NO₃)₂(H₂O)₆ to UO₂(NO₃)₂(H₂O)₃ is both a structural and morphological change that has the lustrous lime green UO₂(NO₃)₂(H₂O)₆ crystals changing to the matte greenish yellow of the trihydrate solid. The phase transformation and crystal structures were confirmed by density functional theory calculations and optical microscopy methods, both of which showed a transformation with two distinct sites for the uranyl cation in the trihydrate, with only one in the hexahydrate.

This document describes the results of our investigations on the potential use of nitrogen trifluoride as the fluorinating and oxidizing agent in fluoride volatility-based used nuclear fuel reprocessing. The conceptual process uses differences in reaction temperatures between nitrogen trifluoride and fuel constituents that produce volatile fluorides to achieve separations and recover valuable constituents. We provide results from our thermodynamic evaluations, thermo-analytical experiments, kinetic models, and provide a preliminary process flowsheet. The evaluations found that nitrogen trifluoride can effectively produce volatile fluorides at different temperatures dependent on the fuel constituent.