Andrew Nelson

Nitride ceramics have been investigated for different applications in the nuclear industry, such as space nuclear power, fusion reactor diagnostics and plasma heating, inert matrix fuels, and accident tolerant fuels. Although thermal conductivity remains one of the most important properties to track following irradiation, traditional techniques such as laser flash and xenon flash are limited to bulk sample characterization, which requires lengthy and cost-consuming neutron irradiation. This work used spatial domain thermoreflectance (SDTR) for the micrometer-scale measurement of thermal conductivity in 15 MeV Ni ion-irradiated silicon nitride and zirconium nitride from 1 to 50 dpa and 300 to 700 °C. The SDTR-measured unirradiated thermal conductivity was found to be consistent with the published data on bulk samples. Electrically conductive ZrN exhibits modest reduction after irradiation which is minimal at the highest irradiation temperatures. In electrically insulating Si3N4, the reduction is more significant and unlike ZrN, the reduction remains significant even at a higher irradiation temperature. The thermal resistance evolution following irradiation was compared with lattice swelling, which was determined using grazing incidence x-ray diffraction, and radiation-induced defects were observed using transmission electron microscopy. A saturation value was observed between 15 and 50 dpa for thermal conductivity degradation in both nitride ceramics and a direct correlation with high-temperature defect recombination was observed, as well as the potential presence of additional carrier scattering mechanisms.

Reported is an experimental and computational investigation of the low temperature heat capacity, thermodynamic functions, and thermal conductivity of stoichiometric, polycrystalline CeO2. The experimentally measured heat capacity at T < 15 K provides an important correction to the historically accepted experimental values, and the low temperature thermal conductivity serves as the most comprehensive data set at T < 400 K available. Below 10 K, the heat capacity is observed to obey the Debye T3 law, with a Debye temperature of ΘD = 455 K. The entropy, enthalpy, and Gibbs free energy functions are obtained from the experimental heat capacity and compared with predictions from Hubbard-corrected density functional perturbation theory calculations using the Perdew, Burke, and Ernzerhof parameterization revised for solids. The thermal conductivity is determined using the Maldonado continuous measurement technique, along with laser flash analysis, and analyzed according to the Klemens-Callaway model.

The thermal and mechanical properties of cerium dioxide (CeO₂) were assessed using a range of experimental techniques. The oxygen potential of CeO₂ was measured by the thermogravimetric technique, and a numerical fit for the oxygen potential of CeO₂ is derived based on defect chemistry. Mechanical properties of CeO₂ were obtained using sound velocity measurement, resonant ultrasound spectroscopy and nanoindentation. The obtained mechanical properties of CeO₂ are then used to evaluate the Debye temperature and Grüneisen constant. The heat capacity and thermal conductivity of CeO₂ were also calculated using the Debye temperature and the Grüneisen constant. Finally, the thermal conductivity was calculated based upon laser flash analysis measurements performed on pellets fabricated using a range of feedstock purities to resolve discrepancies in the existing literature.

Trimethyltin fluoride (Me₃SnF) is a mild and selective reagent for the installation of actinide fluoride bonds as demonstrated by the room temperature synthesis of a variety of organometallic and inorganic thorium(IV), uranium(IV), and uranium(V) fluoride complexes {(1,2,4‐tBu₃C₅H₂)₂ThF₂, (C₅Me₅)₂U(F)(O‐2,6‐iPr₂C₆H₃), U(F)(O‐2,6‐tBu₂C₆H₃)₃, U(F)[N(SiMe₃)₂]₃, (C₅Me₅)₂UF₂(L) (L = O=PMe₃, O=PPh₃, O=PCy₃), and (C₅Me₅)₂U(F)(=N‐2,6‐iPr₂C₆H₃)} from their corresponding chloride, bromide, and iodide analogues. From these reactions, the new (C₅Me₅)₂UF₂(L) (L = O=PPh₃, O=PCy₃) uranium fluoride complexes were isolated and characterized by NMR spectroscopy and X‐ray crystallography.

In recent years, neutron radiography and tomography have been applied at different beam lines at Los Alamos Neutron Science Center (LANSCE), covering a very wide neutron energy range. The field of energy-resolved neutron imaging with epi-thermal neutrons, utilizing neutron absorption resonances for contrast as well as quantitative density measurements, was pioneered at the Target 1 (Lujan center), Flight Path 5 beam line and continues to be refined. Applications include: imaging of metallic and ceramic nuclear fuels, fission gas measurements, tomography of fossils and studies of dopants in scintillators. The technique provides the ability to characterize materials opaque to thermal neutrons and to utilize neutron resonance analysis codes to quantify isotopes to within 0.1 atom %. The latter also allows measuring fuel enrichment levels or the pressure of fission gas remotely. More recently, the cold neutron spectrum at the ASTERIX beam line, also located at Target 1, was used to demonstrate phase contrast imaging with pulsed neutrons. This extends the capabilities for imaging of thin and transparent materials at LANSCE. In contrast, high-energy neutron imaging at LANSCE, using unmoderated fast spallation neutrons from Target 4 [Weapons Neutron Research (WNR) facility] has been developed for applications in imaging of dense, thick objects. Using fast (ns), time-of-flight imaging, enables testing and developing imaging at specific, selected MeV neutron energies. The 4FP-60R beam line has been reconfigured with increased shielding and new, larger collimation dedicated to fast neutron imaging. The exploration of ways in which pulsed neutron beams and the time-of-flight method can provide additional benefits is continuing. We will describe the facilities and instruments, present application examples and recent results of all these efforts at LANSCE.

The organometallic uranium species (C5Me4R)2UBr2 (R = Me, Et) were obtained by treating their chloride analogues (C5Me4R)2UCl2 (R = Me, Et) with Me3SiBr. Treatment of (C5Me4R)2UCl2 and (C5Me4R)2UBr2 (R = Me, Et) with K(O-2,6-iPr2C6H3) afforded the halide aryloxide mixed-ligand complexes (C5Me4R)2U(O-2,6-iPr2C6H3)(X) (R = Me, Et; X = Cl, Br). Complexes (C5Me4R)2U(O-2,6-iPr2C6H3)(Br) (R = Me, Et) can also be synthesized by treating (C5Me4R)2U(O-2,6-iPr2C6H3)(Cl) (R = Me, Et) with Me3SiBr, respectively. Reduction of (C5Me4R)2UCl2 and (C5Me4R)2UBr2 (R = Me, Et) with KC8 led to isolation of uranium(III) “ate” species [K(THF)][(C5Me5)2UX2] (X = Cl, Br) and [K(THF)0.5][(C5Me4Et)2UX2] (X = Cl, Br), which can be converted to the neutral complexes (C5Me4R)2U[N(SiMe3)2] (R = Me, Et). Analyses by nuclear magnetic resonance spectroscopy, X-ray crystallography, and elemental analysis are also presented.

A novel approach was used to understand the effects of processing conditions on the conversion of zirconium dioxide to zirconium carbonitride using a carbothermic reduction to nitridation process. The conversion process was studied through the use of thermogravimetric analysis, X‐ray diffraction (XRD), and scanning electron microscopy, resulting in an understanding of the nature and progression of conversion. The rate of mass change as measured through thermogravimetric analysis was plotted as a function of reaction progression, guiding the use of XRD in further understanding the conversion process. This approach was then used to characterize the important effect of initial compact morphology on conversion. Coupled analysis techniques allow for advancement of a number of hypotheses regarding the rate‐limiting factors along the various steps in the carbothermic reduction to nitridation process. These results demonstrate the value of the proposed approach to aid in deconstruction of competing reaction mechanisms as necessary to understand the system studied here or other systems of scientific and industrial relevance.

A carbothermic reduction to nitridation process was developed which is capable of producing high‐purity thorium mononitride (ThN) in bulk quantities. This was accomplished through study of three distinct processing routes using thermogravimetric analysis. The information gathered was then used to guide development of a draft process, which was tested within a tungsten production furnace. Scaling issues were identified and corrected following the draft process. Finally, a partitioned process was developed in response to the draft process which separates the reduction from the nitridation and carbon cleanup steps. This partitioned process was demonstrated to be capable of producing phase‐pure ThN, with oxygen and carbon impurities of 990 ± 130 wppm and 240 ± 30 wppm, respectively.

Molybdenum disilicide (MoSi₂) has well documented oxidation resistance at high temperature (T > 1273 K) in dry O₂ containing atmospheres due to the formation of a passive SiO₂ surface layer. However, its behavior under atmospheres where water vapor is the dominant species has received far less attention. Oxidation testing of MoSi₂ was performed at temperatures ranging from 670–1498 K in both 75% water vapor and synthetic air (Ar‐O₂, 80%–20%) containing atmospheres. Here the thermogravimetric and microscopy data describing these phenomena are presented. Over the temperature range investigated, MoSi₂ displays more mass gain in water vapor than in air. The oxidation kinetics observed in water vapor differ from that of the air samples. Two volatile oxides, MoO₂(OH)₂ and Si(OH)₄, are thought to be the species responsible for the varied kinetics, at 670–877 K and at 1498 K, respectively. Increased oxidation (140–300 mg/cm²) was observed from 980–1084 K in water vapor, where passivation is observed in air.

The synthesis and full characterization by Nuclear Magnetic Resonance (¹H,¹³C{¹H} and¹¹⁹Sn{¹H}) of eleven Me₃SnX complexes in six common organic solvents is presented.

The thermal conductivity of stoichiometric CeO₂ was determined through measurement of thermal expansion from 313 to 1723 K, thermal diffusivity from 298 to 1473 K, and specific heat capacity from 313 to 1373 K. The thermal conductivity was then calculated as the product of the density, thermal diffusivity, and specific heat capacity. The thermal conductivity was found to obey an (A + BT)⁻¹ relationship with A = 6.776×10⁻² m·K·W⁻¹ and B = 2.793 × 10⁻⁴ m·W⁻¹. Extrapolations of applied models were made to provide suggested data for the specific heat capacity, thermal diffusivity, and thermal conductivity data up to 1723 K. Results of thermal expansion and heat capacity measurements agreed well with the limited low‐temperature data available in the literature. The thermal conductivity values provided in the current study are significantly higher than the only high‐temperature data located for CeO₂. This is attributed to the tendency of CeO₂ to rapidly reduce at elevated temperatures given the available partial pressure of O₂ in air at ambient pressure. The CeO₂ data are compared to literature values for UO₂ and PuO₂ to evaluate its suitability as a surrogate in nuclear fuel systems where thermal transport is a primary criterion for performance

Nickel ferrite (NiFe₂O₄) is a major constituent of the corrosion deposits formed on the exterior of nuclear fuel cladding tubes during operation. NiFe₂O₄ has attracted much recent interest, mainly due to the impact of these deposits, known as CRUD, on the operation of commercial nuclear reactors. Although advances have been made in modeling CRUD nucleation and growth under a wide range of conditions, the thermophysical properties of NiFe₂O₄ at high temperatures have only been approximated, thereby limiting the accuracy of such models. In this study, samples of NiFe₂O₄ were synthesized to provide the thermal diffusivity, specific heat capacity, and thermal expansion data from room temperature to 1300 K. These results were then used to determine thermal conductivity. Numerical fits are provided to facilitate ongoing modeling efforts. The Curie temperature determined through these measurements was in slight disagreement with literature values. Transmission electron microscopy investigation of multiple NiFe₂O₄ samples revealed that minor nonstoichiometry was likely responsible for variations in the Curie temperature. However, these small changes in composition did not impact the thermal conductivity of NiFe₂O₄, and thus are not expected to play a large role in governing reactor performance.

Multiphase borosilicate glass‐ceramics represent one candidate to contain radioactive nuclear waste separated from used nuclear fuel. In this work, the thermophysical properties from room temperature to 1273 K were investigated for four different borosilicate glass‐ceramic compositions containing waste loadings from 42 to 60 wt% to determine the sensitivity of these properties to waste loading, as‐fabricated microstructure, and potential evolutions in microstructure brought about by temperature transients. The thermal expansion, specific heat capacity, thermal diffusivity, and thermal conductivity are presented. The impact of increasing waste loading is shown to have a small but measurable effect on the thermophysical properties between the four compositions, contrasted to a much greater impact observed when transitioning from predominantly crystalline to amorphous systems. Thermal cycling below 1273 K was not found to measurably impact the thermophysical properties of the compositions investigated here.

In advanced nuclear applications, high temperature and a corrosive environment are present in addition to a high dose radiation field causing displacement damage in the material. In recent times it has been shown that Nanostructured Ferritic Alloys (NFA’s) such as advanced Oxide Dispersion Strengthened (ODS) steels are suitable for this environment as they tolerate high dose irradiation without significant changes in microstructure or relevant mechanical properties.

Ion beam irradiation is a fast and cost effective way to induce radiation damage in materials but has limited penetration depth. Therefore, small scale mechanical testing such as nanoindentation and micro compression testing in combination with FIB based sample preparation for micro structural characterization has to be performed allowing a full assessment of the materials’ behavior under radiation environment. In this work two different ODS materials have been irradiated using proton and combined proton and He beams up to 1 dpa at different temperatures. Nanoindentation and LEAP measurements were performed in order to assess the changes in properties of these alloys due to irradiation. The same techniques were applied to intermetallic nanostructured alloys in order to investigate the effectiveness of the metal-intermetallic interface to provide defect sinks for He and radiation damage. It was found that irradiation can cause the formation of intermetallic particles even at room temperature while increasing the material strength significantly.