Advanced sample environments for in situ neutron diffraction studies of nuclear materials

Abstract

Generation IV nuclear reactor concepts, such as the supercritical-water-cooled nuclear reactor (SCWR), are actively researched internationally. Operating conditions above the critical point of water (374°C, 22.1 MPa) and fuel core temperature that potentially exceed 1850°C put a high demand on the surrounding materials. For their safe application, it is essential to characterize and understand the material properties on an atomic scale such as crystal structure and grain orientation (texture) changes as a function of temperature and stress. This permits the refinement of models predicting the macroscopic behavior of the material. Neutron diffraction is a powerful tool in characterizing such crystallographic properties due to their deep penetration depth into condensed matter. This leads to the ability to study bulk material properties, as opposed to surface effects, and allows for complex sample environments to study e.g. the individual contributions of thermo-mechanical processing steps during manufacturing, operating or accident scenarios.

I present three sample environments for in situ neutron diffraction studies that provide such crystallographic information and have been successfully commissioned and integrated into the user program of the High Pressure – Preferred Orientation (HIPPO) diffractometer at the Los Alamos Neutron Science Center (LANSCE) user facility. I adapted a sample changer for reliable and fast automated texture measurements of multiple specimens. I built a creep furnace combining a 2700 N load frame with a resistive vanadium furnace, capable of temperatures up to 1000°C, and manipulated by a pair of synchronized rotation stages. This combination allows following deformation and temperature dependent texture and strain evolutions in situ. Utilizing the presented sample changer and creep furnace we studied pressure tubes made of Zr-2.5wt%Nb currently employed in CANDU^® nuclear reactors and proposed for future SCWRs, acting as the primary containment vessel of high temperature heavy water (D₂O) inside the reactor core. The measured sample texture shows that upon traversing the phase transition, which proceeded according to the Burger orientation relationship, variant selection occurred during heating and cooling of the zirconium alloy. Experimental results of lattice strains depending on the crystallographic orientation can be used to calculate strain pole figures which grant insight into the three-dimensional mechanical response of a polycrystalline aggregate and represent an extremely powerful material model validation tool.

Lastly, I developed a resistive graphite high-temperature furnace with sample motion for in situ crystal structure and texture measurements of nuclear materials at steady-state temperatures up to at least 2200°C. This permits in situ observation of e.g. phase transitions and coefficients of thermal expansion, as well as phase formation and texture development during solidification. Utilizing this apparatus, I investigated the carbothermic reduction of UO₂ nanopowder forming uranium carbide, a promising Generation IV reactor fuel. The onset of the UO₂ + 2C → UC + CO₂ reaction was observed at 1440°C with the bulk portion being complete at 1500°C. I describe the novel synthesis for this nanoparticle UO₂ powder, which closely imitates observed nano grains in partially burnt reactor fuels. Of the three opposing structure models reported for the non-quenchable cubic UC₂ phase, stable between 1769°C and 2560°C, the NaCl-type structure according to Bowman is found to be correct. This is deemed major progress as the CaF₂-type structure was used for recent thermal modeling of safety critical factors in nuclear reactors. A temperature dependent increase in density due to carbon diffusion has been observed and quantified. I provide first experimental data of an unspecified, reversible order-disorder transition in this δ-phase with its onset at ∼1800°C which is likely due to rotating C₂ molecules in the sublattice.