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

• Main Authors: Reiche, H. M., Vogel, S. C.
• Format: Conference Proceeding
• Language: English
• Subjects: ACTINIDES; CRYSTAL STRUCTURE; GRAIN SIZE; GRAPHITE; LANL; LOSS OF COOLANT; MATERIALS SCIENCE; NEUTRON DIFFRACTION; NEUTRON FLUX; NEUTRON TEMPERATURE; NUCLEAR FUEL CYCLE AND FUEL MATERIALS; PHASE DIAGRAMS; RADIOACTIVE MATERIALS; SYNTHESIS; TEMPERATURE RANGE 1000-4000 K; URANIUM CARBIDES; URANIUM DIOXIDE; X-RAY DIFFRACTION
• ISSN: 0022-3115; 1873-4820
• DOI: 10.1016/j.jnucmat.2015.12.044

Advances in neutron flux, neutron instrumentation, and sample environments over the past years allowed the development of unique techniques to characterize material synthesis and processing. The available infrastructure at LANL allows to apply these techniques to radioactive materials or materials containing actinides. Here, we present capabilities and results to characterize materials in situ at temperatures above 2000 Celsius degrees such as in loss-of-coolant accidents. Temperatures in excess of 2000 C. degrees are not readily achieved and neutrons are one of a few probes to characterize materials under these conditions. Research in refractory materials, phase diagram studies or accident scenarios for nuclear materials are research areas where such extreme conditions are required. As an example, we present the formation of UC{sub x} from UO{sub 2+x} and graphite in situ using high temperature neutron diffraction with particular focus on resolving the conflicting reports on the crystal structure of non-quenchable cubic UC{sub 2}. The ability to follow the reaction UO{sub 2}+C → UC{sub x}+CO{sub y} in situ with neutron diffraction allows us to conclude that the reaction occurs within minutes over a temperature range of 60 Celsius degrees. The initiation of the reaction is far below the reaction temperature of 2000 C. degrees reported for this reaction in standard text books on actinide chemistry. This may be due to the unusually small ∼100 nm grain size of our UO{sub 2} initial powder. Furthermore, the holding time of several hours reported in the literature would be unnecessary. The neutron diffraction data collected for the cubic high temperature UC{sub 2} phase unambiguously confirmed the disordered C-C dumbbells and excluded structures previously determined by X-ray diffraction.