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Nelson, Andrew Thomas

Publications and source records attributed to Nelson, Andrew Thomas.

17 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Uranium transport in acidic brines under reducing conditions

The behavior of uranium in environments, ranging from those of natural systems responsible for the formation of uranium deposits to those of nuclear reactors providing 11% of the world’s electricity, is governed by processes involving high-temperature aqueous solutions. It has been well documented that uranium is mobile in aqueous solutions in its oxidized, U 6+ state, whereas in its reduced, U 4+ state, uranium has been assumed to be immobile. Here, we present experimental evidence from high temperature (>100 °C) acidic brines that invalidates this assumption. Our experiments have identified a new uranium chloride species (UCl 4 °) that is more stable under reducing than oxidized conditions. Furthermore, these results indicate that uranium is mobile under reducing conditions and necessitate a re-evaluation of the mobility of uranium, particularly in ore deposit models involving this metal. Regardless of the scenario considered, reducing conditions can no longer be considered a guarantee of uranium immobility.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2018

Neutron Imaging at LANSCE—From Cold to Ultrafast

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.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

U 3 Si 2 behavior in H 2 O: Part I, flowing steam and the effect of hydrogen

Recent interest in U 3 Si 2 as an advanced light water reactor fuel has driven assessment of numerous properties, but characterization of its response to H 2 O environments is absent from the literature. The behavior of U 3 Si 2 in H 2 O containing atmospheres is investigated and presented in a two-part series of articles aimed to understand the degradation mechanism of U 3 Si 2 in H 2 O. Reported here are thermogravimetric data for U 3 Si 2 exposed to flowing steam at 250–470 °C. Additionally the response of U 3 Si 2 to flowing Ar-6% H 2 from 350 to 400 °C is presented. Microstructural degradation is observed following hours of exposure at 350 °C in steam. U 3 Si 2 undergoes pulverization on the timescale of minutes when temperatures are increased above 400 °C. In conclusion, this mechanism is accelerated in flowing Ar-H 2 at the same temperatures.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2017

U 3 Si 2 behavior in H 2 O environments: Part II, pressurized water with controlled redox chemistry

Recent interest in U 3 Si 2 as an advanced light water reactor fuel has driven assessment of numerous properties, but characterization of its response to H 2 O environments is sparse in available literature. The behavior of U 3 Si 2 in H 2 O containing atmospheres is investigated and presented in a two-part series of articles. This work examines the behavior of U 3 Si 2 following exposure to pressurized H 2 O at temperatures from 300 to 350 °C. Testing was performed using two autoclave configurations and multiple redox conditions. Use of solid state buffers to attain a controlled water chemistry is also presented as a means to test actinide-bearing systems. Buffers were used to vary the hydrogen concentration between 1 and 30 parts per million H 2 . Testing included UN, U 3 Si 5 , and UO 2 . Both UN and U 3 Si 5 were found to rapidly pulverize in less than 5 h at 300 °C. Uranium dioxide was included as a control for the autoclave system, and was found to be minimally impacted by exposure to pressurized water at the conditions tested for extended time periods. Testing of U 3 Si 2 at 300 °C found reasonable stability through 30 days in 1–5 ppm H 2 . However, pulverization was observed following 35 days. The redox condition of testing strongly affected pulverization. Characterization of the resulting microstructures suggests that the mechanism responsible for pulverization under more strongly reducing conditions differs from that previously identified. Hydride formation is hypothesized to drive this transition. In conclusion, testing performed at 350 °C resulted in rapid pulverization of U 3 Si 2 in under 50 h.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2017

Determination of elastic properties of polycrystalline U 3 Si 2 using resonant ultrasound spectroscopy

For this research, the elastic properties of U 3 Si 2 at room temperature have been measured via resonant ultrasound spectroscopy. Results show that the average value of Young's and the bulk modulus for U 3 Si 2 are 130.4±0.5 and 68.3±0.5 GPa, respectively. Further, a numerical model to assess thermal stress in an operating fuel is evaluated. Lastly, the thermal stress evolved in U 3 Si 2 is compared to UO 2 to facilitate an estimation of the probability of crack formation in U 3 Si 2 under representative light water reactor operating conditions.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2017

Exploiting the Reactivity of Actinide Fluoride Bonds for the Synthesis of a New Class of Bis(azide) Uranium Complexes

The terminal actinide fluoride bonds in (C 5 Me 5 ) 2 ThF 2 (py) (py = pyridine) and (C 5 Me 5 ) 2 UF 2 (O=PR 3 ) (R = Me, Ph) react with two equivalents of Me 3 SiN 3 in toluene to form the polymeric thorium bis(azide), [(C 5 Me 5 ) 2 Th(N 3 )2] ∞ , and a new class of monometallic uranium bis(azide) complexes, (C 5 Me 5 ) 2 U(N 3 )2(O=PR 3 ), respectively. Full characterization of the novel complexes (C 5 Me 5 ) 2 ThF 2 (py) and (C 5 Me 5 ) 2 UF 2 (O=PR 3 ) are reported, including the solid-state structures of (C 5 Me 5 ) 2 ThF 2 (py) and (C 5 Me 5 ) 2 U(N 3 ) 2 (O=PPh 3 ). Lastly, electronic absorption spectral data are also reported for (C 5 Me 5 ) 2 AnF 2 (py) (An = Th, U) and (C 5 Me 5 ) 2 U(N 3 ) 2 (O=PR 3 ) to confirm metal oxidation state and enable elucidation of the fluoride and azide ligand bonding in these complexes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Neutron Characterization of Encapsulated ATF-1/LANL-1 Mockup Fuel Capsules

Twenty pellets of mock-up accident tolerant fuels UN-U3Si5 were produced at LANL and loaded in two rodlet/capsule assemblies. Tomographic imaging and diffraction measurements were performed to characterize these samples at the Flight-Path 5 and HIPPO beam lines at LANSCE/LANL between November 2016 and January 2017 as well as in August 2017. The entire ~10 cm long, ~1 cm diameter fuel volume could be characterized, however due to time constraints only 2 mm slices in 4mm increments were characterized with neutron diffraction and a 28mm subset of the entire sample was characterized with energy-resolved neutron imaging. The double encapsulation of the fuel into two steel containers does not pose a problem for the neutron analysis and the methods could be applied to enriched as well irradiated fuels.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2017

Spherical Nanoindentation Stress-Strain Measurements of BOR-60 14YWT-NFA1 Irradiated Tubes

Spherical nanoindentation stress-strain protocols were applied to characterize unirradiated and fast neutron irradiated nanostructured ferritic alloy (NFA) 14YWT and compared against Berkovich nanohardness and available tensile data. The predicted uniaxial yield strength from spherical, 100 and 5 micron radii, indentation yield strength measurements was 1100-1400 MPa which compares well with the predictions from Berkovich nanohardness, 1200 MPa, and available tensile data, ~1100 MPa. However, spherical indentation measurements predict an increase in the uniaxial yield strength of ~1 GPa while Berkovich nanohardness measurements predict an increase of only ~250 MPa. No tensile data exists on the irradiated condition. It is believed the difference in the predicted uniaxial yield strength between spherical and Berkovich nanoindentation are due to a low number of tests on the irradiated sample combined with the significant heterogeneity in the microstructure, the differences in sensitivity to sample preparation on the irradiated sample between the two indentation protocols , and/or in how strain localizes under the indenter with the possibility of dislocation channeling under Berkovich hardness indents leading to strain softening. Nanoindentation capabilities to test neutron irradiated samples in a radiological area were realized.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Fabrication and thermophysical property characterization of UN/U 3 Si 2 composite fuel forms

High uranium density composite fuels composed of UN and U 3 Si 2 have been fabricated using a liquid phase sintering route at temperatures between 1873 K and 1973 K and spanning compositions of 10 vol% to 40 vol% U 3 Si 2 . Microstructural analysis and phase characterization revealed the formation of an U-Si-N phase of unknown structure. Microcracking was observed in the U-Si portion of the composite microstructure that likely originates from the mismatched coefficient of thermal expansion between the UN and U 3 Si 2 leading to stresses on heating and cooling of the composite. Thermal expansion coefficient, thermal diffusivity, and thermal conductivity were characterized for each of the compositions as a function of temperature to 1673 K. Hysteresis is observed in the thermal diffusivity for the 20 vol% through 40 vol% specimens between room temperature and 1273 K, which is attributed to the microcracking in the U-Si phase. Thermal conductivity of the composites was modeled using the MOOSE framework based on the collected microstructure data. In conclusion, the impact of irradiation on thermal conductivity was also simulated for this class of composite materials.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

Corrigendum to “Thermophysical properties of U 3 Si 2 to 1773 K”

An error was discovered by the authors in the calculation of thermal diffusivity in “Thermophysical properties of U 3 Si 2 to 1773 K”. The error was caused by operator error in entry of parameters used to fit the temperature rise versus time model necessary to calculate the thermal diffusivity. Lastly, this error propagated to the calculation of thermal conductivity, leading to values that were 18%–28% larger along with the corresponding calculated Lorenz values.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

Molybdenum Disilicide Oxidation Kinetics in High Temperature Steam

The Fuel Cycle Research and Development program’s Advanced Fuels Campaign is currently supporting a range of experimental efforts aimed at the development and qualification of ‘accident tolerant’ nuclear fuel forms. One route to enhance the accident tolerance of nuclear fuel is to replace the zirconium alloy cladding, which is prone to rapid oxidation in steam at elevated temperatures, with a more oxidation-resistant cladding. Several cladding replacement solutions have been envisaged. The cladding can be completely replaced with a more oxidation resistant alloy, a layered approach can be used to optimize the strength, creep resistance, and oxidation tolerance of various materials, or the existing zirconium alloy cladding can be coated with a more oxidation-resistant material. Molybdenum is one candidate cladding material favored due to its high temperature creep resistance. However, it performs poorly under autoclave testing and suffers degradation under high temperature steam oxidation exposure. Development of composite cladding architectures consisting of a molybdenum core shielded by a molybdenum disilicide (MoSi 2 ) coating is hypothesized to improve the performance of a Mo-based cladding system. MoSi 2 was identified based on its high temperature oxidation resistance in O 2 atmospheres (e.g. air and “wet air”). However, its behavior in H 2 O is less known. This report presents thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and x-ray diffraction (XRD) results for MoSi 2 exposed to 670-1498 K water vapor. Synthetic air (80-20%, Ar-O 2 ) exposures were also performed, and those results are presented here for a comparative analysis. It was determined that MoSi 2 displays drastically different oxidation behavior in water vapor than in dry air. In the 670-1498 K temperature range, four distinct behaviors are observed. Parabolic oxidation is exhibited in only 670-773 K water vapor, a temperature range in which the material pests in dry O 2 environments. From 877-1084 K in water vapor, MoSi 2 undergoes rapid mass gain resulting in oxidation throughout the bulk of the sample at 980 K and 1084 K. The resulting material displays swelling and warping after the 980-1084 K exposures. A pre-passivation heat treatment performed at 1395 K was found capable of producing a coarse SiO 2 layer that limited pesting at lower temperatures in water vapor over the time periods investigated.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

Deconvolution of mass gain and mass loss mechanisms during carbothermic reduction to nitridization reactions in the zirconium system through thermogravimetric analysis

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.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Hydrogen measurement during steam oxidation using coupled thermogravimetric analysis and quadrupole mass spectrometry

An analytical technique is presented with the goal of measuring reaction kinetics during steam oxidation reactions for three cases in which obtaining kinetics information often requires a prohibitive amount of time and cost. The technique presented relies on coupling thermogravimetric analysis (TGA) with a quantitative hydrogen measurement technique using quadrupole mass spectrometry (QMS). The first case considered is in differentiating between the kinetics of steam oxidation reactions and those for simultaneously reacting gaseous impurities such as nitrogen or oxygen. The second case allows one to independently measure the kinetics of oxide and hydride formation for systems in which both of these reactions are known to take place during steam oxidation. The third case deals with measuring the kinetics of formation for competing volatile and non-volatile oxides during certain steam oxidation reactions. In order to meet the requirements of the coupled technique, a methodology is presented which attempts to provide quantitative measurement of hydrogen generation using QMS in the presence of an interfering fragmentation species, namely water vapor. This is achieved such that all calibrations and corrections are performed during the TGA baseline and steam oxidation programs, making system operation virtually identical to standard TGA. Benchmarking results showed a relative error in hydrogen measurement of 5.7–8.4% following the application of a correction factor. Lastly, suggestions are made for possible improvements to the presented technique so that it may be better applied to the three cases presented.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Expanding the Chemistry of Actinide Metallocene Bromides. Synthesis, Properties and Molecular Structures of the Tetravalent and Trivalent Uranium Bromide Complexes: (C 5 Me 4 R) 2 UBr 2 , (C 5 Me 4 R) 2 U(O-2,6- i Pr 2 C 6 H 3 )(Br), and [K(THF)][(C 5 Me 4 R) 2 UBr 2 ] (R = Me, Et)

The organometallic uranium species (C 5 Me 4 R) 2 UBr 2 (R = Me, Et) were obtained by treating their chloride analogues (C 5 Me 4 R) 2 UCl 2 (R = Me, Et) with Me 3 SiBr. Treatment of (C 5 Me 4 R) 2 UCl 2 and (C 5 Me 4 R) 2 UBr 2 (R = Me, Et) with K(O-2,6- i Pr 2 C 6 H 3 ) afforded the halide aryloxide mixed-ligand complexes (C 5 Me 4 R) 2 U(O-2,6- i Pr 2 C 6 H 3 )(X) (R = Me, Et; X = Cl, Br). Complexes (C 5 Me 4 R) 2 U(O-2,6- i Pr 2 C 6 H 3 )(Br) (R = Me, Et) can also be synthesized by treating (C 5 Me 4 R) 2 U(O-2,6- i Pr 2 C 6 H 3 )(Cl) (R = Me, Et) with Me 3 SiBr, respectively. Reduction of (C 5 Me 4 R) 2 UCl 2 and (C 5 Me 4 R) 2 UBr 2 (R = Me, Et) with KC 8 led to isolation of uranium(III) “ate” species [K(THF)][(C 5 Me 5 ) 2 UX 2 ] (X = Cl, Br) and [K(THF) 0.5 ][(C 5 Me 4 Et) 2 UX 2 ] (X = Cl, Br), which can be converted to the neutral complexes (C 5 Me 4 R) 2 U[N(SiMe 3 ) 2 ] (R = Me, Et). Analyses by nuclear magnetic resonance spectroscopy, X-ray crystallography, and elemental analysis are also presented.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗