In situ synthesis and characterization of uranium carbide using high temperature neutron diffraction

Abstract

We investigated the formation of UCx from UO2+x and graphite in situ using neutron diffraction at high temperatures with particular focus on resolving the conflicting reports on the crystal structure of non-quenchable cubic UC2. The agents were UO2 nanopowder, which closely imitates nano grains observed in spent reactor fuels, and graphite powder. In situ neutron diffraction revealed the onset of the UO2 + 2C  UC + CO2 reaction at 1440˚C, with its completion at 1500˚C. Upon further heating, carbon diffuses into the uranium carbide forming C2 groups at the octahedral sites. This resulting high temperature cubic UC2 phase is similar to the NaCl-type structure as proposed by Bowman et al. Our novel experimental data provide insights into the mechanism and kinetics of formation of UC as well as characteristics of the high

Neutron based evaluation in support of NEAMS

“…Wen et al [35] found, however, that the oscillation is not spherical, as described by Bowman, but rather an oscillation approximately around the static location of the C-C dumbbells in the pyrite structure. Analysis of our diffraction data [34] confirm the structure reported by Bowman et al, (also determined by neutron diffraction).…”

“…Further increase of the sample temperature allowed us to follow the thermal expansion of the UC x phase. However, even during the dwell times at constant temperatures we observed changes in lattice parameters [34]. We attribute this to the continuation of the migration of carbon atoms into the uranium carbide.…”

“…During heating (at 10˚C/min) data were collected in 2 min intervals. The experiment (excluding setup and alignment) was completed in ~11 h. Details about the experimental setup, data analysis, and sample characterization can be found elsewhere [34]. An overview of the diffraction data is given in Figure 4.…”

“…To explain the diminished diffraction intensity at 1890°C [34] and its subsequent recovery, molecular dynamics simulations of the cubic high temperature phase of the U-C system [35] suggest an explanation. With increasing temperature, the uranium carbide system absorbs increasing amounts of carbon without changing its crystal structure (except for an increase in lattice parameter).…”