Keff Values Research Articles

Criticality calculations of the HTR-10 pebble-bed reactor with SCALE6/CSAS6 and MCNP5

HTR-10 is a 10MWt 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 2800pcm, whereas the LATTICECELL and MULTIREGION treatments overestimated keff values with similar biases at approximately 470–680pcm. The DOUBLEHET results attained further improvement, reducing the keff overestimation to approximately 280pcm. 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 680pcm. 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.

Choice of suitable micellar catalyst for 2,2′-bipyridine-promoted chromic acid oxidation of glycerol to glyceraldehyde in aqueous media at room temperature

The micellar catalyzed 2,2′-bipyridine (bipy)-promoted oxidation of glycerol to glyceraldehyde by chromic acid is investigated under the criteria [glycerol]T ≫ [Cr(VI)]T at 30 °C. The critical micellar concentrations values of the three representative surfactants, N-cetylpyridinium chloride (CPC), sodium dodecyl sulphate (SDS), and TX-100, are determined by conductometric and spectrophotometric methods. The oxidized product glyceraldehyde is identified by 2,4-DNP test and FTIR spectral measurement. The pseudo-first-order rate constants (k obs, s−1) are calculated from the slope of plots of ln(A 450) versus time (t) which are linear. From these plots, the kinetic parameter k eff values are calculated and the k eff value of SDS-catalyzed bipy-promoted reaction path was found to be highest among all the combinations. In the bipy-promoted oxidation path, Cr(VI)–bipy complex is the main active oxidant which undergoes attack by the substrate to form the product. The active oxidant Cr(VI)–bipy complex reacts with glycerol to form a ternary complex which undergoes redox decomposition in a rate-limiting step. Here, the anionic surfactant SDS and the neutral surfactant TX-100 both catalyze the reaction in the presence of bipy, whereas the cationic surfactant CPC and neutral surfactant TX-100 inhibit the reaction in the absence of bipy. SDS is found to be the most suitable micellar catalyst for the bipy-promoted chromic acid oxidation of glycerol.

Growth of monodisperse nanospheres of MnFe2O4 with enhanced magnetic and optical properties

Highly dispersive nanospheres of MnFe2O4 are prepared by template free hydrothermal method. The nanospheres have 47.3-nm average diameter, narrow size distribution, and good crystallinity with average crystallite size about 22 nm. The reaction temperature strongly affects the morphology, and high temperature is found to be responsible for growth of uniform nanospheres. Raman spectroscopy reveals high purity of prepared nanospheres. High saturation magnetization (78.3 emu/g), low coercivity (45 Oe, 1 Oe = 79.5775 A·cm−1), low remanence (5.32 emu/g), and high anisotropy constant 2.84 × 104 J/m3 (10 times larger than bulk) are observed at room temperatures. The nearly superparamagnetic behavior is due to comparable size of nanospheres with superparamagnetic critical diameter Dcrspam. The high value of Keff may be due to coupling between the pinned moment in the amorphous shell and the magnetic moment in the core of the nanospheres. The nanospheres show prominent optical absorption in the visible region, and the indirect band gap is estimated to be 0.98 eV from the transmission spectrum. The prepared Mn ferrite has potential applications in biomedicine and photocatalysis.

Burnup extension of the Fixed Bed Nuclear Reactor using alternative fuels

Time evolution of criticality and burn up grades of the Fixed Bed Nuclear Reactor (FBNR) are investigated using alternative fuels. Neutron transport calculations are performed by using XSDRNPM/SCALE 5 codes. Firstly, the calculations were performed to search the optimum compositions of UO2/Reactor grade-PuO2; UO2/weapon grade-PuO2 and of UO2/MAO2 that would result in the similar initial value of keff as that of UO2, the original fuel of FBNR. Fuel component has been varied from 1% to 100%. The fuel compositions have been obtained with UO2: ① 35% RG-Pu O2; ② 19% WG-Pu O2; ③ 50% MAO2. After that, keff, burnup and operation time values of the fuels were compared. The temporal variation of the criticality keff and the burn-up values of the reactor have been calculated by full power operation for a period of 18years. The criticality in compositions of Reactor grade-PuO2/UO2; weapon grade-PuO2/UO2 and MAO2/UO2 starts by keff=1.2742, 1.2726 and 1.2783 for the fuels compositions, respectively. These mixed fuels would allow full power reactor operation for ⩾14years and burn up grade can reach ∼104,646, ∼101,990 and ∼80,798 MWD/T, respectively. On that way, the hazardous nuclear waste product can be transmuted as well as utilized as fuel.

A Numerical Study on Heat Conductivity Characterization of Aluminium Filled Polypropylene Composites

A numerical simulation of the heat-transfer process within polypropylene matrix composite filled with micro-sized aluminium particles using Finite Element Method is proposed in this paper. Three-dimensional spheres-in-cube lattice array models are constructed to simulate the microstructure of composite materials with aluminium content ranging from about 1.5 to 42 vol% and the effective thermal conductivities of the composites are estimated. A commercially available finite-element package ANSYS is used for this numerical analysis. The result shows that the effective thermal conductivity (Keff) increases with increase in the volume fraction of the aluminium in the composites. The simulated values are compared with calculated Keff values obtained from other established correlations such as Rule-of-Mixture (ROM), Maxwell’s model and with published experimental results. This study reveals that the incorporation of aluminium particles results in enhancement of thermal conductivity of polypropylene thereby increasing its heat transportation capability. It is found that with incorporation of about 42 vol% of 100 micron sized aluminium particles thermal conductivity of the composite increases from 0.239 W/m-K to 0.875 W/m-K. This study also shows that the effect of particle size with same volume percentage on thermal conductivity is marginal.

Surface anisotropy change of CoFe2O4 nanoparticles depending on thickness of coated SiO2 shell

We systematically investigated the effective surface anisotropy of CoFe2O4 nanoparticles dependant on the thickness of SiO2 shell. XRD (X-ray powder diffraction) patterns and TEM (transmission electron microscopy) micrographs were used to investigate the structure of particles and thickness of SiO2 shell, respectively. The thicknesses of SiO2 shell with 5.41 nm on CoFe2O4 nanoparticles were increased up to 14.04 ± 0.05 nm by changing the amount of added TEOS by, 0.10, 0.25, 0.50, 1.00, 1.50, and 2.50 mL. The increase of the SiO2 thickness shell decreased the effective anisotropy due to decline the effectiveness of the dipolar magnetostatic interactions, determined from Vogel–Fulcher equation, between the particles. The declines in the Keff values stabled at around 3.76 ± 0.11 × 105 J/m3 for TEOS amount higher than 1.5 mL.

Investigation of nonuniformity phenomenon in nanoscale SiO2 and high-k gate dielectrics

We proposed the concept of effective uniform area ratio (Keff) to evaluate the nonuniformity phenomena of SiO2 and HfO2 gate dielectrics below 3 nm. Keff can be considered as an indication of gate oxide uniformity. It is found that Keff increases with the thickness of SiO2, whereas decreases with increasing effective oxide thickness for HfO2. The reason for the observed phenomena is given in this work. The electrical and reliability characteristics were examined to check the feasibility of our concept. The tendency of Keff values was reconfirmed by the uniformity of leakage current and constant field stress test. It is believed that this methodology is useful for the future oxide quality test.

Neutronic analysis of the PBMR-400 full core using thorium fuel mixed with plutonium or minor actinides

Time evolution of criticality and burnup grades of the PBMR were investigated for reactor grade plutonium and minor actinides in the spent fuel of light water reactors (LWRs) mixed with thoria. The calculations were performed by employing the computer codes MCNP and MONTEBURNS 2.0 and using the ENDF/B-V nuclear data library. Firstly, the plutonium–thorium and minor actinides–thorium ratio was determined by using the initial keff value of the original uranium fuel design. After the selection of the plutonium/minor actinides–thorium mixture ratio, the time-dependent neutronic behavior of the reactor grade plutonium and minor actinides and original fuels in a PBMR-400 reactor was calculated by using the MCNP code. Finally, keff, burnup and operation time values of the fuels were compared. The core effective multiplication factor (keff) for the original fuel which has 9.6wt.% enriched uranium was computed as 1.2395. Corresponding to this keff value the reactor grade plutonium/thorium and minor actinide/thorium oxide mixtures were found to be 30%/70% and 50%/50%, respectively. The core lives for the original, the reactor grade plutonium/thorium and the minor actinide/thorium fuels were calculated as ∼3.2, ∼6.5 and ∼5.5years, whereas, the corresponding burnups came out to be 99,000, ∼190,000 and ∼166,000MWD/T, respectively, for an end of life keff set equal to 1.02.

A simulation study on thermal conductivity of glass bead embedded polymeric system

A numerical simulation of the heat-transfer process within epoxy matrix composites filled with solid glass beads (SGB) is made by using finite element analysis (FEA). A commercially available finite-element package ANSYS is used for this numerical analysis. Three-dimensional spheres-in-cube lattice array models are constructed to simulate the microstructure of composite materials for various SGB content ranging from about 1 to 18 vol.% and the effective thermal conductivities (Keff) of the composites are estimated. The results show that the FEA simulated effective thermal conductivity decreases almost in a parabolic manner with increase in the volume fraction of the SGB fillers in the composites. Finally, the simulations are compared with measured Keff values obtained from experiments. Guarded heat flow meter test method is used employing the instrument Unitherm™ Model 2022 as per ASTM-E1530 to measure the thermal conductivity of these composites fabricated by hand layup technique. This study shows that the incorporation of SGB results in reduction of heat conductivity of epoxy resin and thereby improves its thermal insulation capability. The experimentally measured conductivity values are compared with the numerically calculated ones and it is found that FEA simulations are fairly close to the measured Keff.

Thermal conductivity of PP/Al(OH)3/Mg(OH)2 composites

Polypropylene (PP) composites filled with aluminum hydroxide (Al(OH)3) and magnesium hydroxide (Mg(OH)2) were prepared using a twin-screw extruder. The effective thermal conductivity (keff) of the composites was measured by means of a thermal conductivity instrument under different testing temperatures. The results showed that the values of keff increased quickly with an increase the volume fraction (ϕf) of Al(OH)3/Mg(OH)2 powder when ϕf was less than 7.1%, then keff increased slightly as ϕf was varying from 7.1% to 13.3%, while the values of keff increased obviously when ϕf was more than 13.3%. Under the same conditions, the values of keff increased nonlinearly with increasing the diameter (d) of the Al(OH)3/Mg(OH)2 powder. The values of keff of the composites increased somewhat with a rise of test temperature. On the basis of the experimental results, the thermal conduction mechanisms of the composite were analyzed and discussed.

Utilization of thorium in a Gas Turbine – Modular Helium Reactor

Gas Turbine-Modular Helium Reactor (GT-MHR) is one of the new types of the reactors with high efficiency and increased safety features. The usage of different kinds of fissile material in this reactor can increase the life of it. Weapons-grade plutonium (WGrPu), which can be acquired from the old dismantled nuclear weapons, can be an option in a GT-MHR. In order to increase the sustainability of the WGrPu resources this fuel can be mixed with thorium, which is a fertile material that can be found in the nature and has resources three times more than uranium. In this study, possibility of utilization of the weapons-grade plutonium–thorium mixture was investigated and an optimum mixture ratio was determined. The behavior of this mixture and the original fuel was studied by using MCNP5 1.4, Monteburns 2.0 and Origen 2.2 tools. Calculations showed that, a GT-MHR type reactor, which is using the original TRISO fuel particle mixture of 20% enriched uranium+natural uranium (original fuel) has an effective multiplication factor (keff) of 1.270. Corresponding to this keff value the weapons grade plutonium/thorium oxide mixture was found 19%/81%. By using Monteburns Code, the operation time, which describes the time passed until the reactor reaches a keff value of 1.02, was found as 515days for the original fuel and 1175days for the weapons grade plutonium mixture. Furthermore, the burn-up values for the original fuel and WGrPu fuels were found as 47.69 and 119.27 GWd/MTU, respectively.

Highly crystalline superparamagnetic iron oxide nanoparticles (SPION) in a silica matrix

We report on magnetic properties of iron oxide nanoparticles in a silica matrix synthesized by the sol–gel method. The sample is characterized by using X-ray powder diffractometer (XRPD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and superconducting quantum interference device (SQUID) magnetometer. TEM reveals iron oxide nanoparticles (maghemite/magnetite) of small sizes of about 4nm, narrow size distribution and no particle agglomeration. The SQUID measurements show low blocking temperature TB=6K and superparamagnetic behavior above 30K. Obtained saturation magnetization MS=61.1emu/g is very high, among the highest values for iron oxides of particle size below 5nm. The field-cooled hysteresis measurements do not show displacement of the hysteresis loop thus indicating an absence of exchange bias, whereas AC susceptibility reveals non-interacting nanoparticles. The values of Keff and KS (effective and surface anisotropy constants) obtained in this work are smaller than those reported in the literature for systems where shell's disorder spin structure (surface effects) is observed. These results point to highly crystalline iron oxide nanoparticles with a low amount of internal defects and small surface disorder shell thickness which is uncommon for nanoparticles of this size. Superparamagnetic iron oxide nanoparticles (SPION) with such properties are convenient for the biomedical applications in targeted diagnostics and drug delivery.

Processing and Thermal Conductivity Characterization of Solid Glass Micro-Spheres Filled Polymer Composites

This paper describes the preparation and thermal conductivity characterization of solid glass micro-spheres (SGMs) filled polymer composites. SGMs of different sizes are embedded in epoxy resin to develop composites by hand layup technique. A numerical simulation of the heat-transfer within the composites is made by using finite element method (FEM). Three-dimensional spheres-in-cube lattice array models are constructed to simulate the microstructure of composite materials for various SGM content ranging from 0 to about 27 vol % and the effective thermal conductivities (Keff) of the composites are estimated. Keff values are also calculated using some of the existing theoretical models. Finally, guarded heat flow meter test method is used to measure the conductivity of these composites. The simulations are compared with Keff values obtained from experiments and it is found that the FEM simulations are fairly close to the measured Keff. This study shows that the incorporation of SGMs results in reduction of conductivity of epoxy resin and thereby improves its thermal insulation capability. Further, the size and content of SGMs influence the extent of reduction of Keff. Keywords: Composites; Glass Microspheres; FEM; Thermal Conductivity; Simulation

Processing and Thermal Conductivity Characterization of Solid Glass Micro-Spheres Filled Polymer Composites

This paper describes the preparation and thermal conductivity characterization of solid glass micro-spheres (SGMs) filled polymer composites. SGMs of different sizes are embedded in epoxy resin to develop composites by hand layup technique. A numerical simulation of the heat-transfer within the composites is made by using finite element method (FEM). Three-dimensional spheres-in-cube lattice array models are constructed to simulate the microstructure of composite materials for various SGM content ranging from 0 to about 27 vol % and the effective thermal conductivities (Keff) of the composites are estimated. Keff values are also calculated using some of the existing theoretical models. Finally, guarded heat flow meter test method is used to measure the conductivity of these composites. The simulations are compared with Keff values obtained from experiments and it is found that the FEM simulations are fairly close to the measured Keff. This study shows that the incorporation of SGMs results in reduction of conductivity of epoxy resin and thereby improves its thermal insulation capability. Further, the size and content of SGMs influence the extent of reduction of Keff. Keywords: Composites; Glass Microspheres; FEM; Thermal Conductivity; Simulation

Linking the effective thermal conductivity of snow to its shear strength and density

[1] The effective thermal conductivity of snow, keff, is a crucial climatic and environmental variable. Here, we test the intuition that keff is linked to microstructural and mechanical properties by attempting to relate keff to density ρsnow, and to shear strength σ measured with a handheld shear vane. We performed 106 combined measurements of keff, ρsnow and σ in the Alps, Svalbard, Arctic Alaska, and near the North Pole, covering essentially all snow types. We find a good correlation between keff and ρsnow which is not significantly different from that of Sturm et al. (1997). The correlation between keff and a combination of σ and ρsnow is stronger than with density alone. We propose an equation linking keff, (W m−1 K−1) ρsnow (kg m−3) and σ (Pa): keff = 7.114 10−5 ρsnow σ0.333 + 2.367 10−2. This equation places constraints on the calculation of keff, ρsnow and σ in avalanche warning models where σ is a key variable. For our samples, we calculate σ from measured values of keff and ρsnow using our equation and compare the value to that predicted by the French MEPRA avalanche warning model, which uses density and grain type as input data. MEPRA and the prediction of σ based on keff and ρsnow agree within 8%. MEPRA agrees with observations within 11%. Calculating σ from density only yields values 55% lower than measured, showing the interest of using additional data to predict σ.

Effect of interparticle interactions and size dispersion in magnetic nanoparticle assemblies: A static and dynamic study

Interparticle interactions in magnetic nanoparticles are studied by dc and ac magnetization measurements. For non-interacting nanoparticles, while the anisotropy constant of the nanoparticles Keff = 1.6 × 105 erg/cm3 is accurately determined by fitting zero-field-cooled and field-cooled measurements, we show that Keff values deduced only from time relaxation measurements must require simultaneous adjustments of the complex susceptibility by taking into account the size distribution of nanoparticles. This leads to Keff = 1.7 × 105 erg/cm3 in agreement with dc measurements. For interacting nanoparticles, comparisons with theoretical models show that energies due to magnetic dipolar interactions can only be predicted for weak and moderate interactions.

Cu Dual-Damascene Interconnects with Direct Chemical Mechanical Polishing Process on Porous Low-k Film

To reduce the effective dielectric constant (keff) value for 32 nm node technology and beyond, the effects of a direct chemical mechanical polishing (CMP) process on porous low-k film without a protective cap layer were investigated. It was confirmed that a capless structure on porous low-k film is effective in reducing the resistance–capacitance (RC) products, but it causes degradation of wire-to-wire breakdown voltage characteristics. The most important point of a direct CMP process is to control the amount of damage to the polished surface. In this study, two types of low-k film were compared in combination with a variety of CMP process conditions. As results, we found that a direct CMP process has a positive effect on wire-to-wire current leakage and time-dependent dielectric breakdown (TDDB) reliability where a porous low-k film deposited by modified conditions is used. By optimizing the deposition and curing conditions, it is possible to control the distribution of different pore sizes in porous low-k film, which allows us to realize a highly reliable capless structure.

Enhancement of Mesoscale Eddy Stirring at Steering Levels in the Southern Ocean

Abstract Meridional cross sections of effective diffusivity in the Southern Ocean are presented and discussed. The effective diffusivity, Keff, characterizes the rate at which mesoscale eddies stir properties on interior isopycnal surfaces and laterally at the sea surface. The distributions are obtained by monitoring the rate at which eddies stir an idealized tracer whose initial distribution varies monotonically across the Antarctic Circumpolar Current (ACC). In the absence of observed maps of eddying currents in the interior ocean, the advecting velocity field is taken from an eddy-permitting state estimate of the Southern Ocean (SOSE). A three-dimensional advection–diffusion equation is solved and the diffusivity diagnosed by applying the Nakamura technique on both horizontal and isopycnal surfaces. The resulting meridional sections of Keff reveal intensified isopycnal eddy stirring (reaching values of ∼2000 m2 s−1) in a layer deep beneath the ACC but rising toward the surface on the equatorward flank. Lower effective diffusivity values (∼500 m2 s−1) are found near the surface where the mean flow of the ACC is strongest. It is argued that Keff is enhanced in the vicinity of the steering level of baroclinic waves, which is deep along the axis of the ACC but shallows on the equatorial flank. Values of Keff are also found to be spatially correlated with gradients of potential vorticity on isopycnal surfaces and are large where those gradients are weak and vice versa, as expected from simple dynamical arguments. Finally, implications of the spatial distributions of Keff for the dynamics of the ACC and its overturning circulation are discussed.

Magnetic properties of Ni/Cu/Ni epitaxial films with perpendicular anisotropy

The effective magnetic anisotropy Keff has been studied at room temperature in Cu/Ni/Cu/Ni/Cu(001) epitaxial films with perpendicular magnetization as a function of the interleaving Cu block thickness tCu ranging from 0 nm to 6 nm and being the Ni blocks 3 nm thick. We have found that the value of Keff decreases when comparing the structures with tCu = 0 and 0.2 nm, although the strain increases. For larger values of tCu, Keff increases, although above tCu = 1.8 nm, Keff is nearly independent of tcu. These results can be explained using the measured in-plane strain ϵ and a surface anisotropy constant, Ks = −0.4 mJ/m2.

XT-ADS: Neutronics, Shielding, and Radiation Damage Calculations

This work is related to the design of the core of the eXperimental demonstration of the technological feasibility of Transmutation in an Accelerator-Driven System (XT-ADS) facility in the framework of the EUROpean Research Programme for the TRANSmutation of High Level Nuclear Waste in an Accelerator Driven System (EUROTRANS) project. The design specifications for the proton accelerator of the XT-ADS are 600 MeV and up to 3.5 mA for the beam energy and current, respectively. The proton beam impinges on a liquid target consisting of a lead-bismuth-eutectic mixture. The state-of-the-art Monte Carlo code MCNPX was used to assess the neutronics performance and shielding properties of the system. The nuclear data-processing system NJOY 99 was also used. The work consisted of the optimization of the core configuration (geometry, number, and location of the fuel and absorber assemblies) and the appropriate fuel composition in order to reduce radiation damage (namely, the displacement per atom values) on the core barrel and top grid plate, while maintaining the high neutron fluxes (1015 n·cm–2·s–1) and the keff of the system of ~0.95.The assessment of the core configuration and fuel composition was performed, resulting from the interplay among parameters such as the desired high neutron fluxes, the keff value wanted for safety and core performance reasons, the as-low-as-possible radiation damage of the core barrel and top grid plate, and the fuel composition, among others.