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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Energetics of oxidation and formation of uranium mononitride

Uranium mononitride (UN) is an advanced nuclear fuel currently being considered for use in several generation IV fast and thermal neutron spectrum core designs, with additional applications to thermal and electric nuclear propulsion reactors. Here, to better understand the thermal behavior and thermodynamic stability of UN, we investigated the bulk thermal oxidation process and thermochemical reactions, including the enthalpy of oxidation and standard enthalpy of formation, by conducting thermalgravimetric analysis – differential scanning calorimetry coupled with mass spectrometry (TGA-DSC-MS), and high temperature transposed temperature drop and oxide melt drop solution calorimetry. The bulk oxidation of UN (containing a small amount of α-UN 1.5+x ) in air was found to follow a step-wise process characterized by consecutive oxidative reactions UN-UN 1.5+x -UO 2 → UO 2 -UO 3 —N k → UO 3 —N k → UO 3 → U 3 O 8 . TGA results support that the UO 2 – U 2 N 3+x passivating layer delays the onset of rapid bulk oxidation of UN in air up to 662 K. Synchrotron X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) analyses were performed to characterize UN and its final oxidized product. The standard enthalpy of formation (ΔH° f ) of UN was determined to be –144.4 ± 5.9 kJ/mol·atom, in good agreement with previously determined values from Pt encapsulation and bomb calorimetric experiments. Lastly, a negative linear correlation between ΔH° f and the N/U molar ratio was established based on the thermochemical data obtained in this work and previously reported enthalpies of formations of β-UN 1.5-x and α-UN 1.5+x .

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2024

Performance stability of plastics for neutron-gamma pulse shape discrimination

The introduction of the first commercially available plastic scintillators with pulse shape discrimination (PSD) offered by Eljen Technology marked progress towards potentially replacing liquid scintillators in neutron detection. However, use of these plastics over several recent years has revealed an important flaw in these materials: the eventual degradation of scintillation light output and PSD performance. Here, studies described in this paper considered possible reasons for this degradation. Experiments conducted with numerous lab-prepared and commercial EJ-276 samples showed that the main factor affecting the instability is oxidation that involves highly reactive radicals generated during the polymerization process or from the further breakdown of polymer chains under oxygen/air exposure. Based on the obtained results, the stability of scintillation performance for PPO (2,5-Diphenyloxazole)-based PSD plastics has been improved through modifications of the composition via the utilization of scintillation dyes and compounds with antioxidant properties that diminish the effects of oxidation. Elements of these studies were used in the commercial production of the most recent EJ-276D PSD plastic version for fast neutron detection and 6Li-loaded plastics for thermal neutron and antineutrino detection applications. Performance projections of the new PSD plastics indicate a likely degradation of less than 10 % over 5–10 years in comparison to previous EJ-276 that might lose up to 30–40 % of the scintillation light during 1–2 years of storage or deployment under ambient conditions.

36 MATERIALS SCIENCE↗
NASA NTRS1997

Earth and Planetary Science Letters

Systematic measurements of the concentrations of cosmogen Ca-41 (half-life = 1.04 x 10(exp 5) yr) in the Apollo 15 long core 15001-15006 were performed by accelerator mass spectroscopy. Earlier measurements of cosmogenic Be-10, C-14, Al-26, Cl-36, and Mn-53 in the same core have provided confirmation and improvement of theoretical models for predicting production profiles of nuclides by cosmic ray induced spallation in the Moon and large meteorites. Unlike these nuclides, Ca-40 in the lunar surface is produced mainly by thermal neutron capture reactions on Ca-40. The maximum production of Ca-41, about 1 dpm/g Ca, was observed at a depth in the Moon of about 150 g/sq cm. For depths below about 300 g/sq cm, Ca-41 production falls off exponentially with an e-folding length of 175 g/sq cm. Neutron production in the Moon was modeled with the Los Alamos High Energy Transport Code System, and yields of nuclei produced by low-energy thermal and epithermal neutrons were calculated with the Monte Carlo N-Particle code. The new theoretical calculations using these codes are in good agreement with our measured Ca-41 concentrations as well as with Co-60 and direct neutron fluence measurements in the Moon.

K. Nishiizumi↗
DOE OSTI2020

Measured Thermal and Fast Neutrons Fluence Rates for ATF-1 Holders During ATR Cycle 166B

This report contains the thermal (2200 m/s) and fast (E>lMeV) neutron fluence rate data for the ATF-1 holders located in core for ATR Cycle 166B which were measured by the Radiation Measurements Laboratory (RML) as requested by the Power Reactor Programs (ATR Experiments) Radiation Measurements Work Order. This report contains measurements of the fluence rates corresponding to the particular elevations relative to the 80-ft. core elevation. The data in this report consist of (1) a table of the ATR power history and distribution, (2) a hard copy listing of all thermal and fast neutron fluence rates, and (3) plots of both the thermal and fast neutron fluence rates. All "BR" holder monitor wires for this cycle are 56.375 inches long. This length allows measurements from 31.17 inches above core midplane to 19.81 inches below core midplane. The distance from the end of the wires to the first count position was 4.25 inches for all wires counted from this cycle. The monitors were located in an outward small I-holes,1-22 and 1-23. Thermal neutron fluence was determined based on the activation of the CoAl wires and resonance corrections determined in the irradiation of Cycle 152A when the RML measured cadmium-covered as well as bare neutron monitors in the Southwest, Southeast, and H positions at the Center lobe to determine the division in neutron energy between epithermal and thermal neutrons. For more information please refer to letter “Radiation Measurements Laboratory measurements of in core ATR physics testing during cycle 152A. The measured values documented in RML procedure, “ACMM-3600, Flux Monitoring,” will be used indefinitely until further measurements supersede the current values. Fast neutron fluence was determined using the activation of nickel wires. Typically, a fission spectrum averaged cross section (E>1 MeV) is used as determined in detailed measurements described in “ATR Neutron Spectral Characterization,” which gives an effective cross section of 97 mb for the small I-holes. However, those measurements were performed with unfueled experiments. The fast fluence values reported here use a perturbed 114 mb effective>1 MeV spectrum averaged cross section for the small I-holes based on MCNP simulations for a previous fueled experiment in the1-24 position. Refer to “LWR- ATR Irradiation Measured Thermal and Fast Neutron Fluence Rates ATR CYCLE 133A 04/19/04 thru 05/17/04” for more information.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Comparisons of Nodal diffusion and whole-core transport methods for multiple cycles of a small Light water reacto

As part of a cooperative research and development agreement between the US department of Energy's Oak Ridge National Laboratory and Holtec International subsidiary SMR LLC, core simulator models of the SMR-160, a light water-cooled and -moderated, natural circulation, small modular reactor, were developed and compared. The model results indicate pin-by-pin power histories, critical boron concentration, and other key performance indicators useful for developing multi-cycle core loading patterns. One model uses industry standard nodal methods with a simple thermal-hydraulics solution, and the other uses an advanced deterministic neutronics solver with a coupled subchannel calculation. The goals of the comparison are to provide the reactor designer with a detailed and coupled core physics reference solution, inform the reactor designer of nodal model best practices based on the code-to-code comparison, and set up a workflow for performing subsequent detailed core simulator calculations. The results of this work will be used in the development of coupled neutronic, thermal hydraulic, and fuel performance models for advanced analysis of the SMR-160. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2023

Measured Thermal and Fast Neutron Fluence Rates ATR Cycles 171A IR TMIST 3B

This report contains the thermal (2200 m/s) and fast (E>1MeV) neutron fluence rate data for ATR Cycle 171A which were measured by the Radiation Measurements Laboratory (RML) as requested by the Power Reactor Programs (ATR Experiments) Radiation Measurements Work Order. This report contains fluence rate values corresponding to the particular elevations (relative to the 80-ft. core elevation) where the measurements were taken. The data in this report consists of (1) a table of the ATR power history and distribution, (2) a listing of the calculated thermal and fast neutron fluence rates, and (3) plots of calculated both the thermal and fast neutron fluence rates. The fluence rates reported are for the average power levels given in the table of power history and distribution. During the irradiation of Cycle 152A, the RML performed additional flux measurements to assess core reconfiguration issues. Measurements using cadmium covered as well as bare neutron monitors in the Southwest, Southeast, and H positions of the Center lobe were performed to determine the division in neutron energy between epithermal and thermal neutrons. This is defined as the resonance correction defined in this report. It is used to quantify both thermal and fast neutron flux. For more information please refer to letter, “RADIATION MEASUREMENTS LABORATORY MEASUREMENTS OF IN CORE ATR PHYSICS TESTING DURING CYCLE 152A” dated January 24, 2013, to Casey Stengel from C. C. Jensen. The measured values documented in RML Procedure, “ACMM-3600, Flux Monitoring,” will be used indefinitely until further measurements supersede the current values.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2023

Measured Thermal and Fast Neutron Fluence Rates ATR Cycles 171A ATF (Rev. 1)

This report contains the thermal (2200 m/s) and fast (E>1MeV) neutron fluence rate data for ATR Cycle 171A ATF which were measured by the Radiation Measurements Laboratory (RML) as requested by the Power Reactor Programs (ATR Experiments) Radiation Measurements Work Order. This report contains fluence rate values corresponding to the particular elevations (relative to the 80-ft. core elevation) where the measurements were taken. The data in this report consists of (1) a table of the ATR power history and distribution, (2) a listing of the calculated thermal and fast neutron fluence rates, and (3) plots of calculated both the thermal and fast neutron fluence rates. The fluence rates reported are for the average power levels given in the table of power history and distribution. During the irradiation of Cycle 152A, the RML performed additional flux measurements to assess core reconfiguration issues. Measurements using cadmium covered as well as bare neutron monitors in the Southwest, Southeast, and H positions of the Center lobe were performed to determine the division in neutron energy between epithermal and thermal neutrons. This is defined as the resonance correction defined in this report. It is used to quantify both thermal and fast neutron flux. For more information please refer to letter, “RADIATION MEASUREMENTS LABORATORY MEASUREMENTS OF IN CORE ATR PHYSICS TESTING DURING CYCLE 152A” dated January 24, 2013, to Casey Stengel from C. C. Jensen. The measured values documented in RML Procedure, “ACMM-3600, Flux Monitoring,” will be used indefinitely until further measurements supersede the current values.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Neutron Pulsed Die-Away Experiments at LLNL [Slides]

This presentation discusses why PNDA is ideal for TSL validations as it does not require fissile material, it has very simple target shapes and compositions, it is only sensitive to absorption and scattering of target medium, and that well conducted experiments have uncertainties of 0.1% - 0.5%. The presentation concludes by examining PNDA’s advantages and its role in nuclear data validations as it offers a cost-effective experiment for an integral benchmark, it provides the ability to focus on specific cross section data validation including thermal neutron absorption and thermal scattering laws, and the low experimental uncertainty makes it an excellent benchmark candidate. Additionally, it is easily tunable and can facilitate temperature dependent cross section validation by cooling or heating up targets.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1983

Neutron-transmutation-doped germanium bolometers

Six slices of ultra-pure germanium were irradiated with thermal neutron fluences between 7.5 x 10 to the 16th and 1.88 x 10 to the 18th per sq cm. After thermal annealing the resistivity was measured down to low temperatures (less than 4.2 K) and found to follow the relationship rho = rho sub 0 exp(Delta/T) in the hopping conduction regime. Also, several junction FETs were tested for noise performance at room temperature and in an insulating housing in a 4.2 K cryostat. These FETs will be used as first stage amplifiers for neutron-transmutation-doped germanium bolometers.

Palaio, N. P.↗
NASA NTRS1994

High-energy astrophysics: A theoretical analysis of thermal radiation from neutron stars

The unambiguous detection of thermal radiation from the surface of a cooling neutron star was one of the most anxiously awaited results in neutron star physics. This particular Holy Grail was found by Halpern and Holt, who used ROSAT to detect pulsed X-rays from the gamma-ray source Geminga and demonstrate that it was a neutron star, probably a radio pulsar beamed away from us. At an age of approximately 3.4 x 10(exp 5) years, Geminga is in the photon cooling era. Its surface temperature of 5.2 x 10(exp 5) K can be explained within the contexts of both the slow and fast cooling scenarios. In the slow cooling scenario, the surface temperature is too high unless the specific heat of the interior is reduced by extensive baryon pairing. In the fast cooling scenario, the surface temperature will be much too low unless the fast neutrino cooling is shut off by baryon pairing. Two other pulsars, PSR 0656+14 and PSR 1055-52, have also been detected in thermal X-rays by ROSAT. They are also in the photon cooling era. All of this research's neutron star cooling models to date have used the unmagnetized effective temperature-interior temperature relation for the outer boundary condition. Models are being improved by using published magnetic envelope calculations and assumed geometried for the surface magnetic field to determine local interior temperature-emitted flux relations for the surface of the star.

Applegate, James H.↗
DOE OSTI2021

Measured Thermal And Fast Neutron Fluence Rates ATR Cycle 169A

This report contains the thermal (2200 m/s) and fast (E>1MeV) neutron fluence rate data for ATR Cycle 169A which were measured by the Radiation Measurements Laboratory (RML) as requested by the Power Reactor Programs (ATR Experiments) Radiation Measurements Work Order. This report contains fluence rate values corresponding to the particular elevations (relative to the 80-ft. core elevation) where the measurements were taken. The data in this report consists of ( 1) a table of the ATR power history and distribution, (2) a hard copy listing of all thermal and fast neutron fluence rates, (3) plots of both the thermal and fast neutron fluence rates, and (4) an optical record (compact disk) containing a listing of the thermal and the fast neutron fluence rates, their assigned elevations and proper header identification of all monitor positions contained herein. The fluence rates reported are for the average power levels given in the table of power history and distribution. No neutron monitors were located in the N-2 position during this cycle. All "SR" holder monitor wires for this cycle are 55 inches long. This length allows measurements from 24.73 inches above core midplane to 25.25 inches below core midplane. The distance from the end of the wires to the first count position was 4.25 inches for all wires counted from this cycle.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Sub-critical assembly die-away analysis with organic scintillators

Determining the reactivity of systems containing fissile material is essential for verifying safety, and possession and enrichment levels. The goal of this experiment was to characterize a subcritical nuclear fuel assembly irradiated by a pulsed neutron generator through differential die-away analysis. Similar characterizations are commonly performed with thermal neutron detectors, but these detectors are bulky and cumbersome, the materials are not widely available, and timing can be clouded by the required thermalization time. In this work, an organic scintillator capable of detecting both neutrons and photons was used to measure the die-away profiles of both particles. These time profiles were then used to estimate the reactivity of the assembly. Here, the calculated reactivity estimates compare favorably to simulated estimates, and therefore provide a useful tool for fissile material characterization.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2020

Measured Thermal And Fast Neutron Fluence Rates ATR Cycle 166B

This report contains the thermal (2200 m/s) and fast (E>1MeV) neutron fluence rate data for ATR Cycle 166B which were measured by the Radiation Measurements Laboratory (RML) as requested by the Power Reactor Programs (ATR Experiments) Radiation Measurements Work Order. This report contains fluence rate values corresponding to the particular elevations (relative to the 80-ft. core elevation) where the measurements were taken. The data in this report consists of (1) a table of the ATR power history and distribution, (2) a hard copy listing of all thermal and fast neutron fluence rates, (3) plots of both the thermal and fast neutron fluence rates, and (4) an optical record (compact disk) containing a listing of the thermal and the fast neutron fluence rates, their assigned elevations and proper header identification of all monitor positions contained herein. The fluence rates reported are for the average power levels given in the table of power history and distribution. No neutron monitors were located in the N-2 position during this cycle.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2022

An automated fast neutron computed tomography instrument with on-line focusing for non-destructive evaluation

A fast neutron tomography imaging instrument has been designed, built, and tested at The Ohio State University 500 kW Research Reactor on a fast neutron beamline with a peak neutron flux ≈5.4 × 107 n·cm−2·s−1 at 1.6 MeV median neutron energy. The instrument and beamline are also configurable for thermal neutron imaging. The imaging apparatus is composed of a lens coupled, water-cooled Electron Multiplying Charge Coupled Device camera, a front-surface mirror, and a high light yield plastic Polyvinyl toluene scintillator. The instrument sits on a mobile cart. A total of 5 motion-control stages are built into the system for XYZ and rotational degrees of freedom for sample positioning; the fifth stage fine tunes the focal distance between the camera and the scintillator to achieve on-line focusing. A Python code with a user-friendly graphical user interface controls the fully automated image acquisition, not requiring user interaction, yet facilitating tracking of the image acquisition. A complete fast neutron computed tomography dataset with 360 projections requires less than 3 h, with 30 s per projection. On-line focusing is accomplished with a commercial, off-the-shelf, dielectrically actuated liquid lens. Finally, tomographic reconstructions are visualized using the Livermore Tomography Tools software package. The effective pixel size (width and height) is ≈0.1058 mm, yielding a minimum voxel size of 0.1058 × 0.1058 × 0.1058 mm3, and produces a spatial resolution of 231 μm when calculated from knife-edge measurements.

Bisbee, M. G. (ORCID:0000000313466697)↗
NASA NTRS1993

Redistribution of subsurface neutrons caused by ground ice on Mars

As part of an ongoing effort to understand data that will be returned by the Mars Observer Gamma Ray Spectrometer (MOGRS) experiment, we calculate neutron and gamma ray fluxes for simple models describing near-surface ground ice on Mars. Our goal is to determine the limits in information content of remotely sensed neutrons and gamma rays is delineating the abundance of ground ice and its distribution with depth. Such ice is expected based on simple theoretical calculations of the H2O environment of the martian surface and atmosphere. We find it produces a marked redistribution of neutrons with depth. Neutron-capture gamma ray production closely follows the flux of thermal neutrons and is likewise redistributed with depth. Calculated effects at Mars mapping orbit are sufficiently large that, if ice is present at the depths and in the quantities expected, the MOGRS should provide a robust detection of its presence. Although in the information content of gamma rays and neutrons alone is not sufficient to define the many parameters needed to specify its abundance and emplacement below the surface, their combined use might allow separation of the ice abundance from its vertical distribution within the regolith.

Feldman, W. C.↗
NASA NTRS2017

Bulk Hydrogen Content OF High-Silica Rocks in Gale Crater With the Active Dynamic Albedo of Neutrons Experiment

The Mars Science Laboratory (MSL) Curiosity rover recently traversed over plateaus of mafic aeolian sandstones (the 'Stimson' formation) that overlie mudstones (the 'Murray' formation). Within the Stimson formation we observed many lighter-toned, halo-forming features, that are potentially indicative of fluid alteration (see Fig. 1). These halo features extend for tens of meters laterally and are approx.1 meter wide. The halo features were characterized by Curiosity's geochemical instruments: Alpha Proton X-Ray Spectrometer (APXS), Chemin, Chemcam and Sample Analysis at Mars (SAM). With respect to the host (unaltered) Stimson rocks, fracture halos were significantly enriched in silicon and low in iron [1]. Changes in hydrogen abundance (due to its large neutron scattering cross section) greatly influence the magnitude of the thermal neutron response from the Dynamic Albedo of Neutrons (DAN) instrument [2]. There are also some elemental species, e.g. chlorine, iron, and nickel, that have significant microscopic neutron absorption cross sections. These elements can be abundant and variable results provide a useful estimate of the lower bound for bulk hydrogen content (assuming a homogeneous distribution).

Gabriel, T. S. J.↗
DOE OSTI2021

Fundamental Studies of Tritium Formation and Diffusivity in Pure and Defective Zircaloy-4 Getters

Zirconium (Zr) alloys have excellent mechanical properties at high temperature and are resistant to corrosion in different environmental conditions. These alloys are used as tritium (3H or T) getters in nuclear reactors due to their low absorption cross section to thermal neutrons and excellent thermo-mechanical properties. Zr alloys have better mechanical and thermal properties than many other refractory alloys including stainless steel. After the irradiation of TPBAR by neutron flux, 3H produced during irradiation diffuses though the pellets and is captured by the getter. 3H chemically reacts with Zr metal to form metal hydride (ZrTx). Hydrides thus formed are brittle and adversely affect the mechanical and thermal properties of alloys. The lattice mismatch at the interface of alloy and metal hydrides also creates a stress that significantly reduces the alloy’s performance and operation life. Therefore, understanding the behavior of 3H and its species becomes significant as fuel burnup is increased, which leads to increase in hydrogen pickup and oxide formation.

Duan, Yuhua↗
DOE OSTI2023

Thermal Spectrum Molten Salt-Fueled Reactor Reference Plant Model

This report details a fully coupled neutronics thermal hydraulics reference plant model for a thermal spectrum molten salt-fueled reactor. The Multiphysics model is developed on the Nuclear Regulatory Commission's Comprehensive Reactor Analysis Bundle available on the Idaho National Laboratory high-performance computer, which natively and seamlessly couples Griffin and Sam codes of the Multiphysics Object-Oriented Simulation Environment (MOOSE) based applications. Griffin provides the reactor physics capabilities, k-eigenvalue, delayed neutron precursor concentrations, and transient solutions for stationary and flowing fuels. Sam provides the solution of mass, momentum, and energy conservation equations of the whole system with a set of system components for one-dimensional single-phase flow. The neutronics feedback model relies primarily on fuel and moderator temperatures, fuel salt density, and delayed neutron precursors distributions. This report presents our results for the coupled steady state core and an unprotected loss of flow event. Although this model is prototypical regarding capabilities in the Comprehensive Reactor Analysis Bundle, its results are consistent with experimental data of the pump transient tests that were performed at zero power of the Molten Salt Reactor Experiment (MSRE).

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗