Keff Values Research Articles

Magnetic anisotropy of Co∕Cu (111) superlattices: calculation and experiment

The results of theoretical calculations and experimental data for the effective magnetic anisotropy Keff (obtained from ferromagnetic resonance measurements) of [Co(8Å)∕Cu(dCu)]20 (dCu=7–22Å) superlattices with a coherent interaction of the Co and Cu layers prepared by magnetron sputtering are presented. The experimental values of Keff are an oscillatory function of the thickness dCu interlayers. The computed and experimentally measured dependences Keff(dCu) are qualitatively similar; stable growth of the parameter Keff with increasing thickness of the nonmagnetic layers is observed (against the background of oscillations of the experimental curve), which is a direct consequence of the growth of axial distortions of the initial cubic structure of cobalt under the conditions of pseudomorphism. The reasons for the appearance of oscillations of Keff are discussed; the main one is the oscillatory behavior of the roughness of the interfaces of the layers with increasing dCu, resulting in synchronous oscillations of Keff, resistivity, magnetoresistance, and the ferromagnetic resonance linewidth.

Magnetic Investigation of CoFe$_{2}$O$_{4}$ Nanoparticles Supported in Biocompatible Polymeric Microsphere

Magnetic investigation of spinel ferrite nanoparticles dispersed in biocompatible polymeric microspheres is reported in this study. X-ray diffraction data analysis confirms the presence of nanosized CoFe2O4 particles (mean size of ~8 nm). This finding is corroborated by transmission electron microscopy micrographs. Magnetization isotherms suggest a spin disorder likely occurring at the nanoparticle's surface. The saturation magnetization value is used to estimate particle concentration of 1.6times1018 cm-3 dispersed in the polymeric template. A T1/2 dependence of the coercive field is determined in the low-temperature region (T < 30 K). The model of non-interacting mono-domains is used to estimate an effective magnetic anisotropy of Keff = 0.6times105 J/m3. The Keff value we found is lower than the value reported for spherically-shaped CoFe2O4 nanoparticles, though consistent with the low coercive field observed in the investigated sample.

A new heat transfer model of inorganic particulate-filled polymer composites

Thermal conductivity is an important parameter for characterization of thermal properties of materials. Various complicated factors affect the thermal conductivity of inorganic particulate-filled polymer composites. The heat transfer process and mechanisms in an inorganic particulate-filled polymer composite were analyzed in this article. A new theoretical model of heat transfer in these composites was established based on the law of minimal thermal resistance and the equal law of the specific equivalent thermal conductivity, and an relevant equation of effective thermal conductivity (Keff) for describing a relationship between Keff and filler volume fraction as well as other thermal parameters were derived based on this model. The values of Keff of aluminum powder-filled phenol–aldehyde composites and graphite powder-filled phenol–aldehyde composites were estimated by using this equation, and the calculations were compared with the experimental measured data from these composites with filler volume fraction from 0 to 50% in temperature range of 50–60 °C and the predictions by Maxwell–Eucken equation and Russell equation. The results showed that the predictions of the Keff by this equation were closer to the measured data of these composites than the other equations proposed in literature.

Subcritical Measurements of a Plutonium Sphere Reflected by Polyethylene and Acrylic

Subcritical measurements were conducted with an alpha-phase plutonium sphere using the 252Cf source-driven noise analysis method. Measurements were performed with both polyethylene and acrylic reflectors. For each reflector type, five different reflector thicknesses were investigated: 0 (bare), 1.27, 2.54, 3.81, and 7.62 cm. A certain ratio of spectral quantities that depends on the fluctuations in the fission chain multiplication process was measured for each configuration. In addition, two types of Monte Carlo calculations were employed to estimate the keff and spectral ratio values of each configuration. From the measured and computed quantities, the multiplication and uncertainty of the system can be inferred. The polyethylene measurements compared well to previous measurements conducted with the same plutonium sphere and polyethylene reflector thicknesses. The acrylic measurements provide benchmark data of an alpha-phase plutonium sphere reflected by acrylic.

The influence of reactor core parameters on effective breeding coefficient Keff

The values of effective breeding coefficient Keff in a reactor core of nuclear power plant are calculated for different values of parameters (core structure, fuel assembly component) by using the Monte Carlo method. The obtained values of Keff are compared and analysed, which can provide theoretical basis for reactor design.

Numerical Benchmarks for Very High-Temperature Reactors Based on the CNPS Critical Experiments

An evaluation of the Compact Nuclear Power Source (CNPS) experiments conducted at Los Alamos National Laboratory in the 1980s has been done using information available in the open literature. The MCNP4C Monte Carlo results for critical test configurations are in good agreement with the experimental values; the keff values are generally within 0.5% of the experimental values. The calculated total and differential rod worths and material worths were also found generally close to experimental values. These good results motivated the utilization of the experimental test data for the specification of two- and three-dimensional numerical benchmark cases that could be used for the verification and validation of core physics codes developed for Very High Temperature Reactor (VHTR) analysis, particularly the deterministic lattice and whole-core physics codes. To define the benchmark cases, the irregular arrangement of channels in the actual CNPS core was simplified to a regular Cartesian geometry arrangement in the benchmark cases, while preserving the important neutronics characteristics of the CNPS. The results of deterministic calculations using the HELIOS/DIF3D code package were compared to MCNP4C results to show the usefulness of the numerical benchmark cases.

Predicting Runoff and Sediment Yield from a Stiff-Stemmed Grass Hedge System for a Small Watershed

Grass hedges planted at regular intervals on the landscape offer many opportunities to reduce runoff and sediment from leaving fields. Objectives of this study were (1) to evaluate the ability of the WEPP watershed model to simulate grass hedge system effects of sediment trapping (TE), bench terracing (BT), and variable effective soil hydraulic conductivity (HC) on simulated hillslope runoff and sediment yield, and (2) to model the effects of measured effective hydraulic conductivity (Keff) values from a grass hedge management system by comparing predicted runoff and sediment yield values to those measured in a small watershed over an 11-year period. The study was conducted on a 6.6 ha watershed located in the deep loess hills region of southwestern Iowa. Narrow grass hedges of predominantly switchgrass (Panicum virgatum) were planted at 15.4 m intervals in 1991. The WEPP model simulated greater reductions in runoff (9%) and sediment yield (58%) from BT compared to TE and HC effects. Combination of all three effects gave the highest reductions in runoff (22%) and sediment yield (79%) compared to individual effects or any combination of two effects. The watershed model did not adequately simulate slope length reduction effects from the grass hedges. Runoff (r2 = 0.78) and sediment yield (r2 = 0.75) were comparable to observed data when measured Keff values for grass hedge, row crop, and channel areas were used as input data. Measured Keff data from grass hedge, row crop, and channel areas should be used for improved runoff and sediment yield predictions.

Criticality of a 237Np Sphere

A series of critical-mass experiments using a 6-kg neptunium sphere was performed on the Planet vertical-assembly machine at Los Alamos National Laboratory (LANL). The purpose of the experiments was to obtain a better estimate of the critical mass of 237Np. The configurations that were studied included surrounding the neptunium sphere with highly enriched uranium (HEU) shells as well as reflecting it with iron and polyethylene. An additional experiment using a 4.5-kg α-phase plutonium sphere surrounded with HEU was performed to demonstrate how well the computer transport code and the existing cross-section data for uranium and plutonium could reproduce the experiment. For some of the configurations, the prompt-neutron decay constants at delayed critical were measured. These experiments provided an integral measurement of the cross sections for 237Np in the fast-energy and possibly in the intermediate-energy regions. The measured keff from these experiments was compared with the calculated keff from the Monte Carlo N-Particle (MCNP) transport code using ENDF/B-V and ENDF/B-VI and cross-section data evaluated by the Nuclear Theory and Applications group (T-16) at LANL. In all the neptunium experiments, the calculated keff values based on ENDF/B-VI data were ~1% lower than the experimental keff. After adjusting the cross sections for neptunium and 235U to match the bare neptunium/HEU experiment as well as Godiva keff criticality and spectra indexes, the MCNP code yielded a value of 57 ± 4 kg for the bare critical mass of 237Np.

PATTERNED HIGH FREQUENCY THICK FILM MEMS TRANSDUCER

ABSTRACT This paper describes the resonant behaviour of PZT (lead zirconate titanate) micro electromechanical systems (MEMS) based thickness mode piezoelectric transducers fabricated using an innovative low temperature method. The results obtained from transducers with different thicknesses (23 to 42 μ m) on silicon and alumina substrates, were compared to the results obtained from an analytical MASON model. The composite sol gel and wet etching process used to fabricate the MEMS transducers has been described in previous work [1]. It involves producing a PZT powder/sol composite slurry which, when spun down, yields films a few micrometers thick. Repeated layering and infiltration produces PZT films up to 30 μ m thick. The low firing temperature used in this process (< 720°C) allows the use of silicon as a substrate, which offers the possibility to incorporate piezoelectric ceramics into MEMS. The PZT thick films are shaped using a wet etching technique, using a HF/HCl solution diluted in water. The stress dependency of the thickness and porosity of the film have been demonstrated to be the major issue regarding the cracking behavior of such film thicknesses. An investigation into the curvature of the wafer study was performed using a Veeco Dektak stylus profiler. Results obtained for a resonating device with a diameter of 635 μ m on two different substrates, such as silicon and alumina, are presented. After removal of the silicon substrate support devices were tested to determine the resonance and antiresonance frequencies. Theses were found to occur at approximately 69 and 73 MHz respectively for the 22 μ m thick PZT device. Resonance and antiresonace peaks can be observed at 70 and 77 MHz respectively for the 23 μ m thick PZT device. Estimated values of keff calculated from the positions of resonance and anti-resonance peaks show the keff to be approximately between 0.2 and 0.42 as a fonction of the film density. A 30 μ m thick film resonates at 50 MHz. This device has direct applications for near surface medical imaging. The spin coating technique used for the fabrication opens a new field of possibilities regarding the 3D structure of such transducers.

Large magnetic anisotropy in ferrihydrite nanoparticles synthesized from reversemicelles

Six-line ferrihydrite (FH) nanoparticles have been synthesized in the core of reverse micelles,used as nanoreactors to obtain average particle sizes . The blocking temperatures TBm extracted from magnetization data increased from≈10 to 20 K for increasing particle size. Low-temperature Mössbauer measurements allowed us toobserve the onset of differentiated contributions from the particle core and surface as theparticle size increases. The magnetic properties measured in the liquid state of the originalemulsion showed that the ferrihydrite phase is not present in the liquid precursor, butprecipitates in the micelle cores after the free water is freeze-dried. Systematic susceptibilityχac(f,T) measurements showed the dependence of the effective magnetic anisotropy energiesEa with particle volume, and yielded an effective anisotropy value ofKeff = 312 ± 10 kJ m−3.

A Deep Burn Fuel Management Strategy for the Incineration of Military Plutonium in the Gas Turbine–Modular Helium Reactor Modeled in a Detailed Three-Dimensional Geometry by the Monte Carlo Continuous Energy Burnup Code

In the future development of nuclear energy, the graphite-moderated helium-cooled reactors may play an important role because of their valuable technical advantages: passive safety, low cost, flexibility in the choice of fuel, high conversion energy efficiency, high burnup, more resistant fuel cladding, and low power density. General Atomics possesses a long experience with this type of reactor, and it has recently developed the gas turbine-modular helium reactor (GT-MHR), a design where the nuclear power plant is structured into four reactor modules of 600 MW(thermal). Amid its benefits, the GT-MHR offers a rather large flexibility in the choice of fuel type; Th, U, and Pu may be used in the manufacture of fuel with some degrees of freedom. As a consequence, the fuel management may be designed for different objectives aside from energy production, e.g., the reduction of actinide waste production through a fuel based on thorium. In our previous studies we analyzed the behavior of the GT-MHR with a plutonium fuel based on light water reactor (LWR) waste; in the present study we focused on the incineration of military Pu. This choice of fuel requires a detailed numerical modeling of the reactor since a high value of keff at the beginning of the reactor operation requires the modeling both of control rods and of burnable poison; by contrast, when the GT-MHR is fueled with LWR waste, at the equilibrium of the fuel composition, the reactivity swing is small.

Spin disorder and magnetic anisotropy in Fe3O4 nanoparticles

We have studied the magnetic behavior of dextran-coated magnetite (Fe3O4) nanoparticles with median particle size ⟨d⟩=8nm. Magnetization curves and in-field Mössbauer spectroscopy measurements showed that the magnetic moment MS of the particles was much smaller than the bulk material. However, we found no evidence of magnetic irreversibility or nonsaturating behavior at high fields, usually associated to spin canting. The values of magnetic anisotropy Keff from different techniques indicate that surface or shape contributions are negligible. It is proposed that these particles have bulklike ferromagnetic structure with ordered A and B sublattices, but nearly compensated the magnetic moments. The dependence of the blocking temperature with frequency and applied fields, TB(H,ω), suggests that the observed nonmonotonic behavior is governed by the strength of interparticle interactions.

Experiments on cascade-reactor models

There is great interest in our and other contries in cascade blankets. Theoretical and design work has been done on cascade blankets but up to now no experimental work has been done. The first experiments in the world were performed at the All-Russia Reserach Institute of Experimental Physics in 2003–2004 on deeply subcritical uranium— neptunium assemblies. Three series of experiments on cascade and the corresponding single-section blanket models have been peformed. The experiments have confirmed the theoretical conclusions that cascade blankets have positive properties and that 237Np is effective in them; they have provided valuable experience in operating cascade setups with an electron accelerator. The good agreement between the measurements and the calculations has increased confidence in the computational results obtained for cascade blankets with real values of keff. The results of the second and third experiments and a general conclusion concerninig all three series of experiments are presented in this paper.

05/01337 The experimental investigation of the impact of a particle or a rigid body on a cylindrical shell

HTR-10 is a 10 MWt prototype pebble-bed reactor (PBR) that presents a doubly heterogeneous geometry for neutronics calculations. An appropriate unit-cell treatment for the associated fuel elements is vital for creating problem-dependent multigroup cross sections. Considering four unit-cell options for resonance self-shielding correction in SCALE6, a series of HTR-10 core models were established using the CSAS6 sequence to systematically investigate how they affected the computational accuracy and efficiency of PBR criticality calculations. Three core configurations, which ranged from simplified infinite lattices to a detailed geometry, were examined. Based on the same ENDF/B-VII.0 cross-section library, multigroup results were evaluated by comparing with continuous-energy SCALE6/CSAS6 and MCNP5 calculations. The comparison indicated that the INFHOMMEDIUM results overestimated the effective multiplication factor (keff) by about 2800 pcm, whereas the LATTICECELL and MULTIREGION treatments overestimated keff values with similar biases at approximately 470–680 pcm. The DOUBLEHET results attained further improvement, reducing the keff overestimation to approximately 280 pcm. The comparison yielded two unexpected problems from using SCALE6/CSAS6 in HTR-10 criticality calculations. In particular, the continuous-energy CSAS6 calculations in this study present a non-negligible discrepancy with MCNP5, potentially causing a keff value overestimate of approximately 680 pcm. Notably, using a cell-weighted mixture instead of an explicit model of individual TRISO particles in the pebble fuel zone does not shorten the CSAS6 computation time in the high-fidelity HTR-10 model because of the presence of graphite balls and the complex modeling of peripheral reflectors.

A New Theoretical Formulation of Temperature Effect on Impurities Diffusion Coefficients in Molten Tellurium

The evolution of effective diffusion coefficients Deff of 13 different impurities in molten tellurium versus temperature T was studied. A new hyperbolic expression Deff = f(T) for these impurities was established for effective segregation coefficients Keff << 1 and for temperatures within the range 730 K–925 K. Experimental values of Keff determined by zone melting process were used to calculate a series of 4 effective diffusion coefficients Deff, at four different temperatures, for each impurity. These experimental results of Deff were fitted with the new expression of Deff = f(T) to confirm its validity. The diffusion mechanism of 13 impurities at the solid–liquid interface was then investigated under our previous experimental conditions (velocity v of interface, the starting concentration C0, and number n of passes). These investigations confirmed that the temperature increase of tellurium molten zone was not convenient to reach an optimal purification by zone melting process.

Kν Correlation Analysis. A Quantitative Two-Dimensional IR Correlation Method for Analysis of Rate Processes with Exponential Functions

A modified two-dimensional infrared correlation technique called kν correlation analysis is introduced. In this method, a mathematical asynchronous cross-correlation is performed between a set of N infrared spectra undergoing a dynamic intensity variation against a set of exponential functions that encompass a user-defined range of rate constants. The observed correlation intensities are a function of the rate constant of the exponential function and the spectral frequency. The kν correlation plots reveal the rate relationships between different molecular groups in terms of a quantitative and tangible parameter, k, which is the rate constant of the exponential function used in the correlation. A new parameter, the effective rate constant, keff, is defined as the point of maximum correlation intensity at particular frequencies in the plot of k vs ν. The calculated values of keff represent the relative rates at which the intensities of the spectral bands change during the course of the dynamic experiment. As a result, these keff values are comparable and can be used to assign quantitative rate relationships. By using simulated IR spectra, it was shown that the keff parameter is sensitive to the relative order in which the intensity change occurs, while the size of the correlation peaks gives an indication of the magnitude of the intensity change. Spectral bands whose rate of intensity change varies through a dynamic data set are distinguished by the presence of both positive and negative peaks in the kν correlation plots. We applied the kν correlation analysis method to the time-dependent IR spectra of the photoinitiated polymerization reaction of ethyl 2-cyanoacrylate. Analysis of the keff effective rate constants showed that vibrational modes corresponding to the monomeric and polymeric cyanoacrylate molecules react differently depending on whether an inhibitor is present.

Modeling the Transverse Thermal Conductivity of 2-D SiCf/SiC Composites Made with Woven Fabric

The hierarchical two-layer (H2L) model describes the effective transverse thermal conductivity (keff) of a two-dimensional (2-D) SiCf /SiC composite plate made from stacked and infiltrated woven fabric layers in terms of constituent properties and microstructural and architectural variables. The H2L model includes the effects of fiber-matrix interfacial conductance, high-fiber packing fractions within individual tows, and the nonuniform nature of 2-D fabric/matrix layers that usually include a significant amount of interlayer porosity. Previously, H2L model keff predictions were compared to measured values for two versions of 2-D Hi-NicalonTM/pyrocarbon (PyC)/isothermal chemical vapor infiltration (ICVI)-SiC composite, one with a “thin” (0.11-μm) and the other with a “thick” (1.04-μm) PyC fiber coating, and for a 2-D TyrannoTM SA/thin PyC/forced flow chemical vapor infiltration SiC composite. In this study, H2L model keff predictions were compared to measured values for a 2-D SiCf /SiC composite made using the ICVI process with Hi-Nicalon type S fabric and a thin PyC fiber coating. The values of keff determined for the latter composite were significantly greater than the keff values determined for the composites made with either the Hi-Nicalon or the Tyranno SA fabrics. Differences in keff values were expected for the different fiber types, but major differences also were due to observed microstructural and architectural variations between the composite systems, and as predicted by the H2L model.

Sensitivity- and Uncertainty-Based Criticality Safety Validation Techniques

The theoretical basis for the application of sensitivity and uncertainty (S/U) analysis methods to the validation of benchmark data sets for use in criticality safety applications is developed. Sensitivity analyses produce energy-dependent sensitivity coefficients that give the relative change in the system multiplication factor keff value as a function of relative changes in the cross-section data by isotope, reaction, and energy. Integral indices are then developed that utilize the sensitivity information to quantify similarities between pairs of systems, typically a benchmark experiment and design system. Uncertainty analyses provide an estimate of the uncertainties in the calculated values of the system keff due to cross-section uncertainties, as well as correlation in the keff uncertainties between systems. These uncertainty correlations provide an additional measure of system similarity. The use of the similarity measures from both S/U analyses in the formal determination of areas of applicability for benchmark experiments is developed. Furthermore, the use of these similarity measures as a trending parameter for the estimation of the computational bias and uncertainty is explored. The S/U analysis results, along with the calculated and measured keff values and estimates of uncertainties in the measurements, were used in this work to demonstrate application of the generalized linear-least-squares methodology (GLLSM) to data validation for criticality safety studies.An illustrative example is used to demonstrate the application of these S/U analysis procedures to actual criticality safety problems. Computational biases, uncertainties, and the upper subcritical limit for the example applications are determined with the new methods and compared to those obtained through traditional criticality safety analysis validation techniques.The GLLSM procedure is also applied to determine cutoff values for the similarity indices such that applicability of a benchmark experiment to a criticality safety design system can be assured. Additionally, the GLLSM procedure is used to determine how many applicable benchmark experiments exceeding a certain degree of similarity are necessary for an accurate assessment of the computational bias.

Axial macrosegregation in Ga‐doped germanium grown by the vertical gradient freeze technique with a rotating magnetic field

AbstractThe impact of a rotating magnetic field (RMF) on the axial segregation in Vertical Gradient Freeze (VGF) grown, Ga doped germanium is investigated. Growth experiments were performed using the VGF‐RMF as well as the conventional VGF technique. Carrier concentration profiles characterising the Ga segregation were measured by the Spreading Resistance method and calibrated using Hall values of carrier concentration and mobility. The Ga concentration rises more gradually under RMF action, i.e., the dopant segregation is significantly reduced by the rotating field. This effect is attributed to a better mixing of the melt. Numerical results on the flow velocity confirm this explanation. The RMF induced flow is much more intense than the natural buoyant convection due to the radial temperature gradient and leads to a pronounced decrease of the effective partition coefficient keff. In the early stages of growth a keff value close to k0 was obtained, i.e., the gallium was almost homogeneously distributed within the melt. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effect of Temperature on Segregation Coefficients of Impurities in Tellurium

The lowest values of effective segregation coefficients Keff of impurities in tellurium are the main parameters required to obtain an optimal purification ( ≥ 6N). A series of (13 × 4) values of Keff were determined at the same starting concentration C0 and velocity of interface v. Four different temperatures of the molten zone were used. The effect of some of these values was investigated for the first time. The influence of temperature in zone refining was analyzed through the relation of ln Keff versus 1/T. Experimental study showed two kinds of impurities. The first one was easily removed despite the variation of T while the other was strongly affected by T and trapped in the molten zone. The activation energies of 13 impurities in tellurium were also determined. The values of Keff seems to be acceptable about 773°K. For the molten zone, we proposed an adequate temperature range which was about 50°K higher than the melting temperature of tellurium. This was necessary to achieve an electronic grade purification of tellurium with a good efficiency by zone melting process.