Verification of statistical method CORN for modeling of microfuel in the case of high grain concentration

Two methods of modeling a double-heterogeneity fuel are studied: the deterministic positioning and the statistical method CORN of the MCU software package. The effect of distribution of microfuel in a pebble bed on the calculation results is studied. The results of verification of the statistical method CORN for the cases of the microfuel concentration up to 170 cm^–3 in a pebble bed are presented. The admissibility of homogenization of the microfuel coating with the graphite matrix is studied. The dependence of the reactivity on the relative location of fuel and graphite spheres in a pebble bed is found.     

Nanoparticle Distribution in Three‐Layer Polymer–Nanoparticle Composite Films: Comparison of Experiment and Theory

The distribution of hydrophobic nanoparticles deposited on a hydrophilic polymer film is investigated by scanning electron microscopy, transmission electron microscopy, and atomic force microscopy before and after spin‐coating a polymer solution on the particle film and drying it at room temperature. Various polymers and solvents are used. To reach equilibrium, all investigated systems are annealed additionally above the glass transition temperature (T_g) of the polymers. The compatibility of the interacting components is estimated by calculating their surface energy, solubility, and mutual interaction parameters. The experimental results show a redistribution of the particles on both interfaces of the polymer film. This corresponds to the calculated immiscibility of particles and polymers. The distribution of the nanoparticles at the interfaces is related mainly to the vapor pressure of the solvent, that is, kinetic effects during spin‐coating. Only minor contributions result from surface energy, solubility, and interaction parameters.     

“Trapped fraction” effect in microfuel with plutonium fuel

Results of the investigation into the thermodynamics of two types of microfuel (with oxygen getter and without it) with plutonium fuel for various degrees of burnup are presented. The behavior of the trapped fraction of Ag, Ce, Cd, Cs, La, Mo, Pu, Pd, Ru, Sr, Te, and Y is investigated. The fraction of any fission product bound into stable chemical compounds with other components of the system and excluded from the diffusion process is called the trapped fraction. An abrupt increase in the trapped fraction of cesium and, correspondingly, a decrease in free cesium during the burnup of ～26% FIMA (fissions per initial metal atom) or more are found for the microfuel containing no oxygen getter. This leads to a substantial nonlinear burnup dependence of the trapped fraction and should be the cause of an abrupt decrease in the Cs output from the microfuel. It seems likely that the found effect is associated with the formation of carbonate Cs₂CO₃ in the plutonium fuel. In the case of microfuel containing the oxygen getter, no formation of cesium carbonate occurs and the trapped fraction of cesium is almost independent of the burnup.     

Electron-stimulated cesium atom desorption from the Cs/CsAu/Au/W system

This paper reports on a continuation of the investigation of electron-stimulated Cs-atom desorption from a tungsten surface on which cesium and gold films had been adsorbed at T = 300 K. Earlier studies revealed that Cs atoms start to desorb only after more than one monolayer of gold and more than one monolayer of cesium had been deposited on the tungsten surface. In this case, a coating consisting of a gold adlayer on tungsten, a CsAu compound possessing semiconducting properties, and a cesium monolayer capping CsAu (Cs/CsAu/Au/W) is formed on the tungsten surface at 300 K. The yield of atoms from this system exhibits a resonant dependence on the incident electron energy E _e , with an appearance threshold of 57 eV and a maximum at 64 eV. In this case, Cs atoms desorb in two channels, with one of them involving Cs desorption out of the cesium monolayer, and the other, from the CsAu monolayer. The Cs yield at E _e = 64 eV has been investigated in both desorption channels, with an additional cesium coating deposited on the already formed Cs/CsAu/Au/W layered system, as well as of the effect annealing produces on the yield and energy distributions of Cs atoms. It has been demonstrated that Cs atoms evaporated at 300 K on a layered coating with a cesium monolayer atop the CsAu layer on tungsten capped with a gold adlayer, rather than reflected from the cesium monolayer or adsorbing on it, penetrate through the cesium monolayer into the bulk of CsAu even with one CsAu layer present. The desorption yield does not vary with increasing cesium concentration at 300 K, but falls off gradually at 160 K. Annealing within the temperature range 320 K ≤ T _H ≤ 400 K destroys the cesium monolayer and the one-layer CsAu coating, but the multilayer CsAu compound does not break up in this temperature range even after evaporation of the cesium monolayer. It is shown that Cs atoms escape from the multilayer CsAu compound primarily out of the top CsAu layer.     

Radionuclides in deposition in the Ignalina NPP region in 2005

Results of radionuclide activity concentration measurements in deposition and ground-level air conducted at the station of the Institute of Physics situated 3.5 km from the Ignalina Nuclear Power Plant (Ignalina NPP) in 2005 are analyzed. Atmospheric depositional fluxes of ⁷Be, ⁶⁰Co and ¹³⁷Cs are estimated. Radionuclide particle deposition rates are calculated. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model of the global dispersion and deposition is used to explain variations in the ¹³⁷Cs activity concentration in the ground-level air in the Ignalina NPP region in 2005. An increase in the ¹³⁷Cs activity concentration of up to 17.4 μBq m⁻³ on 30 October-5 November is studied. Modelling results show that the Chernobyl NPP Unit 4 Sarcophagus and the radiocaesium resuspension from the Chernobyl NPP accident polluted regions are sources of ¹³⁷Cs to the environment of the Ignalina NPP. Results on solubility in water of aerosols — carriers of ¹³⁷Cs — are discussed.     

Monte Carlo simulations of magnetic particle suspensions with a simple assessment method for the particle overlap between magnetic spheroids

We have developed a simple assessment method for the overlap between spheroidal particles, which neither requires the complex manipulation of vectors and matrices that is indispensable in the ordinary methods, nor is based on a model potential. Moreover, we have developed an evaluation method for the interaction energy arising from the overlap of the steric layer coating spheroidal particles. This is based on a sphere-connected particle model, but some modifications are introduced in order to express an appropriate repulsive interaction energy at the deepest overlapping position. We have investigated the phase change in a magnetic spheroidal particle suspension for a two-dimensional system by means of Monte Carlo simulations. In the case of no external magnetic field, if the magnetic particle-particle interaction is sufficiently strong to favour cluster formation, long raft-like clusters tend to be formed in a dilute situation. With decreasing values of area fraction, a chain-like structure in a dense situation transforms into a raft-like structure within a narrow range of the particle area fraction. Similarly, the raft-like clusters are preferred in a weak applied magnetic field, but an increase in the field strength induces a phase change from a raft-like into a chain-like structure.

Highlights of the present paper:

1. A simple assessment method has been proposed for the overlap between two spheroidal particles.

2. The particle overlap assessment is free from a complex mathematical manipulation regarding vectors and matrices.

3. A modified sphere-connected model has been proposed in order to more accurately evaluate a repulsive interaction due to the overlap of the steric layers coating spheroidal particles.

4. 2D Monte Carlo simulations have been performed to elucidate the phenomenon of a phase change by magnetic spheroidal particles on a material plane surface.

5. A phase change between a raft-like and a chain-like aggregate structure is able to be controlled by the area fraction of particles and an external magnetic field.

     

Effect of γ-radiation on the strain characteristics of a high-filled wood-plastic composite

The strain rate and the characteristics of the jumps at micro- and nanolevels were measured by the high-precision interferometric method for a wood-plastic composite irradiated to doses of 0–100 kGy. Radiation was shown to strengthen the material and change the characteristics of strain rate and value jumps. Strain jumps and mean-square deviations of the measured strain rate from its smoothened time dependence were determined for micro- and nanosized jumps. The change of these characteristics depending on the radiation dose of specimens was traced. A relation between the characteristics of micrometer jumps and the macroscopic strain was revealed.

     

Surface states and accumulation nanolayer induced by Ba and Cs adsorption on the n-GaN(0 0 0 1) surface

The Ba and Cs adsorption on the n-GaN(0 0 0 1) surface has been studied in situ by the threshold photoemission spectroscopy using s- and p-polarized light excitation. Two surface bands induced by Ba (Cs) adsorption are revealed in surface photoemission spectra below the Fermi level. The surface-Fermi level position is found to be changed from significantly below the conduction band minimum (CBM) at clean n-GaN surface to high above the CBM at Ba, Cs/n-GaN interfaces, with the transition from depletion to electron accumulation occurring at low coverages. Photoemission from the accumulation nanolayer is found to excite by visible light in the transparency region of GaN. Appearance of an oscillation structure in threshold photoemission spectra of the Ba, Cs/n-GaN interfaces with existing the accumulation layer is found to originate from Fabry–Perot interference in the transparency region of GaN.     

Effect of impact-induced melting on interface microstructure and bonding of cold-sprayed zinc coating

Zn particles are employed to create different impact conditions, including impact-induced interface melting in cold spraying. The influence of particle impact conditions on the interfacial microstructure evolution, microhardness and the bonding of particles in cold-sprayed Zn coatings are studied. An examination of coating surface morphology provides convincing evidence for melting at particle interfaces. The results reveal that the nanostructured phase was formed at the interface areas between deposited particles in coating resulting from the recrystallization of deformed grains. Melting at interfaces significantly enhances the bonding between the substrate and the coating and between the deposited Zn particles in the coating through the formation of a metallurgical bond. In addition, high driving gas temperature causes the decreasing hardness of deposited Zn coatings. The effects of particle conditions on the impact-induced melting and bonding mechanisms are discussed.     

Determination of the soil-to-grass transfer of 137Cs and its relation to several soil properties at various locations in Serbia

Transfer coefficients of ¹³⁷Cs from soil to grass were determined for the terrain around the city of Kragujevac in central Serbia. Mass activity concentrations of ¹³⁷Cs in soil and grass samples were determined with a high-purity Ge-detector (HPGe). The activity concentration at the depth of 20 cm was found to be in the range of 14.92–124.05 Bq kg^?1, whereas the activity in grass for the same location was in the range of 4.60–84.95 Bq kg^?1. Transfer factors (TFs) were in the range of 0.07 up to 1.94.

Dependences of TFs on different soil characteristics were presented graphically. Weak dependences were determined between them. Absalom's model was used to predict TFs based on soil characteristics: pH value, contents of clay, exchangeable potassium and humus.

A comparison of measured and predicted values from Absalom's model is shown graphically. It has been found that Absalom's model might be carefully used for the prediction of ¹³⁷Cs in grass for specific regions.     

On-line production of Rb and Cs isotopes from uranium carbide targets

A series of on-line mass separation experiments have been performed at the IRIS facility to measure the yield and release of Rb and Cs neutron-rich isotopes produced by fission reaction of ²³⁸U. A 1 GeV proton beam was used to bombard uranium carbide targets with the densities of 11 g/cm³ and 1.5 g/cm³ held at temperatures in the range (2000-2230) ^°C. The release curves of Rb and Cs long-lived isotopes were measured from both kinds of targets. The overall production efficiency was determined making use of experimentally measured cross-sections of that isotope production. Comparison of the experimental yields of Rb and Cs isotopes with the calculated ones after corrections for losses due to finite release times suggests that the diffusion is the dominating process reducing the efficiency for short-lived isotopes. When normalized to the same thickness, an enhancement for the high-density rod target of the measured isotope yields is observed when going far from stability. This is possibly explained by the reactions induced by secondary neutrons. A significant odd-even effect with higher yields of Cs even neutron isotopes has been observed, confirming a similar effect obtained in earlier experiments.     

Influence of annealing treatment on the microstructure and mechanical performance of cold sprayed 304 stainless steel coating

In this study, 304 stainless steel coatings were deposited on interstitial-free steel substrate by cold spraying method. The effect of annealing treatment on microstructure, microhardness, ultimate tensile strength and fracture performance of the coatings were studied. The results showed that annealing treatment had made a dominant contribution to heal up the incomplete interfaces between the deposited particles. Both of the microstructure and the mechanical properties have been obviously optimized by annealing treatment. In addition, the coating microhardness decreased from 345 HV_0.2 for the as-sprayed coating to 201 HV_0.2 for the annealed coating. The coating ultimate tensile strength increased from 65 MPa for the as-sprayed coating to 357 MPa for the annealed coating, which resulted from the increase of the metallurgically bonded areas in the coating induced by annealing treatment. Fracture morphology of the coatings also revealed that annealing treatment changed the fracture character of the cold sprayed 304 stainless steel coating from brittle type to plastic type.     

Studies on the Preparation of Magnetic Photocatalysts

A crystalline titanium dioxide coating was deposited onto silica insulated magnetite particles to prepare a stable magnetic photocatalyst. The direct deposition of crystalline titanium dioxide was conducted by aging dispersions of insulated magnetite particles in a titanium sol–gel precursor mixture at 60–90°C. The coating process was found to be influenced by pH, alkoxide precursor concentration, aging time and reaction temperature. A mechanism for the formation of the titanium dioxide coating has been proposed. The photocatalytic performance of the prepared particles was found to be related to the preparation conditions.     

Effect of Ethanol on the Partition Coefficient Of Cyclohexylacetate Between Bulk and Cetyltrjmethylammonium Bromide Micellar Pseudophases

Variation of the partition coefficient of cyclohexylacetate (CA) was studied by a differential absorption spectroscopic method as a function of ethanol and (CTAB) cetyltrimethylammoniumbromide concentrations. Approach used was based upon the pseudo-two phase model, including effect of ethanol concentrations on the critical micelle concentration (CMC) of CTAB. Ethanol enhanced micelle formation of CTAB at concentrations below 1% by volume, had a slight inhibitory effect at higher concentrations and totally inhibited at a concentration above 15% by volume.

Various amounts of ethanol were used with a purpose of changing the solubility of CA in water. The differential absorbance (AA), was almost zero at low concentrations of CTAB, however an increase in AA began at a certain concentration of CTAB which can accordingly be assumed to be consistent with CMC of CTAB in the presence of ethanol. As CTAB concentration increased above CMC, ΔA reached a plateau. In this plateau, ΔA can reasonably be interpreted as the saturation of CA in micellar phase.

Plateau values of ΔA decreased with increasing ethanol concentration. This showed that the fraction of CA in the micellar phase depended on the ethanol concentration in terms of changing the solubility. Partition coefficients, K_c, were obtained from the plots of 1/ΔA against 1/[C_CA] +[C³ _m], which were linear in high CTAB concentrations. It was also seen that as ΔA, K_c decreased when the ethanol percentage were increased. In other words, the lower the solubility of CA in the bulk, the higher the partition coefficient.     

Properties of the lighest known caesium isotopes 114–118Cs

Neutron-deficient caesium isotopes with mass numbers from 114 to 118 have been studied at the ISOLDE facility. Half-lives have been determined from counting of β-particles, β-delayed protons, and γ-rays. A short-lived isomer ($\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 0.7}\text{sec}$) in ¹¹⁶Cs has been observed and the new delayed-particle precursors ¹¹⁶Cs (protons and α-particles), ¹¹⁵Cs (protons), and ¹¹⁴Cs (protons and α-particles) have been identified. Coincidences between delayed particles and γ-rays have been measured for ¹¹⁸Cs (pγ and αγ) and ¹¹⁶Cs (pγ). A search for the possible proton-radioactive isotope ¹¹³Cs has been performed and an upper limit for its production yield is given.     

Charge accumulation layers and surface states in ultrathin Cs,Ba/n-GaN(0001) interfaces

It is found that ultrathin cesium and barium coatings radically change the electronic properties of the surface and the near-surface region of epitaxial n-GaN(0001) layers. A charge accumulation layer serving as a quasi-two-dimensional electronic channel is first formed by adsorption on the surface of a semiconductor. It is revealed that photoemission from the accumulation layer is excited by visible light from the transparency region of GaN and is characterized by a high quantum yield. It is found that the photoemission thresholds hν _s and hν _p for s-and p-polarized excitation are equal to each other and correspond to the work function. The lowest work function for Cs,Ba/n-GaN interfaces is observed at Cs or Ba coverages close to 0.5 monolayer. Two bands induced by the local interaction of cesium (barium) adatoms with gallium dangling bonds are detected in the electronic spectrum of surface states of Cs,Ba/n-GaN interfaces. An oscillation structure is observed in spectral dependences of the photoyield. This effect is new for photoemission. A model of the effect is proposed. It is found that electronic and photoemission properties of the interfaces correlate with the structural perfectness of the epitaxial n-GaN(0001) layers.     

Studies on the influence of surface pre-treatments on electroless copper coating of boron carbide particles

Boron carbide is one of the hard ceramic particles which find application as structural materials and neutron shielding material due to its high neutron capture cross section. Copper coating on boron carbide particle is essential for the synthesis of metal-ceramic composites with enhanced sinterability and dispersibility. Surface characteristics of the substrate and the coating parameters play a foremost role in the formation of effective electroless coating. The effect of surface pre-treatment conditions and pH on electroless copper coating of boron carbide particles has been studied. Surface pre-treatement of B₄C when compared to acid treated and alkali treated particles were carried out. Uniform copper coating was observed at pH 12 in alkali treated particles when compared to others due to the effective removal of inevitable impurities during the production and processing of commercially available B₄C. A threshold pH 11 was required for initiation of copper coating on boron carbide particles. The growth pattern of the copper coating also varies depending on the surface conditions from acicular to spherical morphology.     

球床高温气冷堆燃料颗粒中放射性核素的累积释放份额研究

本文利用Fick扩散定律,给出经典方法和改进的两种累积释放份额的严格数学表达式,并分析与之相关的穿透时间的物理内涵,并分别用以上两种方法计算137Cs,90Sr,110mAg三种重要放射性核素通过SiC层扩散的累积释放份额.研究表明,传统的"穿透时间"概念并非放射性核素释放到SiC层外的时间,本文提出的改进方法可以更准确的描述放射性核素在燃料元件中的输运和扩散行为.     

Atom transport in random two sublattice structures: analogue of the random alloy sum rule

A simple and often used model of atom transport by the vacancy mechanism on two physically distinct interpenetrating sublattices assumes that each atom–vacancy exchange frequency depends only on the species of the atom and the sublattice from which it jumps. In the kinetic theory of this model, the phenomenological coefficients can be expressed as sums of partial correlation functions, each labelled by the sublattices associated with the atoms making the first and last jumps in the sequence of correlated jumps which it represents. A sum rule, a set of exact relations among these partial correlation functions, is derived for the model, assuming arbitrary vacancy content and any number of chemical species. It reduces to a widely used sum rule for the random lattice gas [L.K. Moleko and A.R. Allnatt, Phil. Mag. A 58 677 (1988)] in the limit that atom jump frequencies are made independent of sublattice. For the two sublattice model at very low vacancy contents, a more powerful sum rule is also derived; it is essentially the same as that of Belova and Murch [Defect Diffus. Forum 194/199 547 (2001)]. The efficiencies of the three sum rules are briefly compared. The low vacancy concentration sum rule is illustrated by numerical simulations for a binary two sublattice system.     

A brief review on the wear mechanisms and interfaces of carbon based materials

Abstract

The demand for reducing wear and friction has become the chief aim in the automotive industry nowadays. The usage of lubricant is not considered enough as there is still room for improvements. As a solution, much research has arisen towards what we called self-lubricating ideas, in order to reduce friction better than lubricant. This paper presents an overview wear mechanism and the interface of carbon-based materials. This paper will also discuss the interfaces by carbon as substrate and coating layer. The findings show that for metals, the predominant wear mechanisms were abrasion and fatigue. Meanwhile, for polymers and coating (DLC), they were abrasive along with adhesive wear. The surface roughness of the substrate plays a crucial role in increasing the excellent performance of the DLC coating. The interfaces of carbon elements definitely give huge impact on both self-lubricant materials and coatings where the coefficient of friction and wear rate changes drastically even with 1 wt.% addition. Nevertheless, a clear understanding of the factors that affect the tribological performance is very essential in performance improvement for potential applications.