Simulation of the defect pair VPb–VO in PbMoO4 crystals

A simulation study has been performed to investigate the defects in PbMoO₄, based on the interatomic potentials empirically fitted to the known crystal properties. The formation energies of the isolated point defects , , the cluster defects , and have been calculated. It is theoretically demonstrated that most of and in the as-grown PbMoO₄ crystal exist in the form of the vacancy pair , which would play important roles in the formation and transformation process of photo-chromic effect in PbMoO₄ crystal.     

Electron emission from carbon induced by the ions C, O, CO and

The secondary electron yield from carbon induced by the ions C⁺, O⁺, CO⁺ and has been measured as a function of ion energy in the range of 2 to 20 keV. It has been observed that electron yield from carbon increases with projectile energy. By comparing electron yields induced by equally fast atomic and molecular projectiles, a molecular effect as a yield reduction has been observed. The measured molecular effect was stronger than the predictions of sweeping-out-electron model.     

Seasonal variation of water-soluble inorganic species in the coarse and fine atmospheric aerosols at Dar es Salaam, Tanzania

The ionic composition of coarse, fine and total PM10 was investigated in aerosol samples collected from a kerbside in Dar es Salaam during the 2005 dry season and 2006 wet season. A “Gent” PM10 stacked filter unit sampler with sequential Nuclepore polycarbonate filters, providing coarse (8 μm) and fine (0.4 μm) size fractions, was deployed. The mean concentrations and associated standard deviation of fine, coarse and PM10 were, respectively, 17 ± 4, 52 ± 27, and 69 ± 29 μg/m³ during the 2005 dry season campaign and 13 ± 5, 34 ± 23 and 47 ± 25 μg/m³ for the 2006 wet season campaign. The higher PM mass concentrations during the dry season campaign are essentially due to soil dust dispersal, much biomass burning and temperature inversions. Chloride, Na⁺ and Mg²⁺ were the dominant ions in coarse fraction, indicating a significant influence of sea-salt aerosols. In the fine fraction, and and K⁺ were the most important ions. The mean equivalent PM2 concentration in the 2005 dry season campaign was two times higher than in the 2006 wet season campaign, probably due to reaction of NaCl (sea-salt) with HNO₃ as a result of higher levels of NO_x during the dry season and/or reduced volatilization of NH₄NO₃ due to lower temperature in the dry season. The results from our water-soluble ions study strongly suggests that biomass burning and secondary aerosols make a significant contribution to fine particulate mass in Dar es Salaam atmosphere. Thus, burning of waste and biomass are thought to be the major causes for the atmospheric particulate pollution in Dar es Salaam during the dry season.     

Contour integral computations for multi-component material systems subjected to creep

In the present paper the crack behavior of multi-component material systems is investigated under extensive creep condition. The validation of the creep fracture parameters C^* and C(t) is firstly examined at the microscale level. It is found that the C^* value is no longer path-independent when mismatch inclusions are embedded into the matrix. To characterize the crack fields in inhomogeneous material the integral value defined at the crack tip as is introduced to reflect the influence of the inclusion. The interaction effects between microcrack and inclusion are systematically calculated with respect to different mismatch factors, various inclusion locations and inclusion numbers. The analysis results show that the value is not only influenced by the inclusion properties but also depends on the microstructure near the crack tip.     

Onset of water accumulation in the upper plenum with a perforated plate

This paper describes an experimental study on the onset of water accumulation above a perforated plate in a vertical air-water countercurrent flow. We experimentally investigate the effects of holes with the relatively large diameter (5 cm), the number of holes (4 holes and 12 holes), the thickness of plate (1 cm and 4 cm), asymmetric water injection, and the location of the air vent line on the onset of water accumulation. The present results indicate that the onset of water accumulation is promoted by a smaller diameter of hole, a smaller number of holes, and symmetric water injection whereas it has negligible dependence on the location of air vent line in the upper plenum. It turns out that the nondimensional superficial velocity, , fits the data better than either the Kutateladze number, K_k, or the interpolation parameter, . We develop a Wallis-type correlation for the onset of water accumulation when the diameter of the holes ranges between 0.7 and 5 cm, and the number of holes is greater than six.     

Hydrogen influence on mechanical and fracture mechanics characteristics of zirconium Zr–2.5Nb alloy at ambient and elevated temperatures

This article deals with the investigation of the hydrogen concentration and temperature influence onto mechanical and fracture mechanics characteristics of RBMK-1500 Ignalina NPP unit 2 reactor fuel channel material—Zr–2.5Nb zirconium alloy (TMT-2) at temperatures from ambient up to 300 °C. The investigation of mechanical characteristics was performed on tensile specimens, fracture mechanics characteristics K_Q, , J_IC—on compact specimens (B = 4 mm) of hydrogen-free and saturated by hydrogen (52, 100 and 140 ppm) at 20, 170, 200 and 300 °C. The investigation showed that temperature increasing calls mechanical strength decreasing, whereas the reductions of area increase. Stronger influence of hydrogen concentration onto mechanical characteristics is noticed only at 20–170 °C temperature, however this influence diminishes as the temperature increases and weakest hydrogen influence is given at 300 °C. Fracture toughness characteristics K_Q, more depends on temperature than on hydrogen concentration. Critical J_IC integral values for the specimens containing hydrogen were given lowest at 20 °C, increases when temperature were raised up to 140 °C and were given highest when it reaches 300 °C.The analysis of and J_IC dependence due to the mechanical characteristics of zirconium alloy has showed that the modified plasticity Z_mod = (R_p0.2/R_m)Z satisfactorily approximates the influence of temperature and hydrogen concentration on variation of these characteristics.     

Nuclear Data Sheets for A = 25

This evaluation of A = 25 has been updated from previous evaluations published in 1998En04, 1990En08, and 1978En02. Coverage includes properties of adopted levels and γ rays, decay-scheme data (energies, intensities and placement of radiations), and cross reference entries. The following tables continue the tradition of showing the systematic relationships between levels in A = 23. These assignments are based on spectroscopic factors and energy differences where .     

Onset condition of gas and liquid entrainment at an inclined branch pipe on a horizontal header

An experimental and analytical investigation of onset condition of gas pull through and liquid entrainment in a single inclined branch to the arbitrary direction was performed in the present study. Since previous studies have been made only for specific angles of 0° and ±90°, it is hard to apply them to the case of branch off to the arbitrary direction. In the present study, series of experimental works was made with a horizontal header of 184 mm inner diameter with a single branch pipe which can be attached in nine different orientations of 0°, ±30°, ±45°, ±60°, ±90° angles and with two different diameters of 16.0 and 24.8 mm, respectively. Also, a simple mechanistic model is developed here based on the force balance at the top of the vortex on the surface of the stratified water. The onset condition due to the vortex breakup for the branch oriented with angle of θ was determined as and the onset condition due to the ascending film was which is similar to that of side branch. It was found that the present model predicted experimental data well in terms of the direction and diameter of the branch pipe. The present model and data may be useful in determination of the quality of flow through the inclined branch which is one of key parameter in the safety study of hypothetic small break loss of coolant accident in the nuclear power plant.     

Liquid entrainment and off-take through the break at the top of a vessel

In order to determine the bleed capacity of the safety depressurization system (SDS) of Advanced Power Reactor 1400 (APR1400) in the case of total loss of feed water (TLOFW), we performed an experimental study of liquid entrainment and liquid off-take from the swelled two-phase mixture surface in a vessel. A total of 220 experimental data on the entrainment and off-take are obtained using a test vessel with a height of 2.0 m and an inner diameter of 0.3 m and a top break with a diameter of 0.05 m. Two-phase mixture levels are measured by an ultrasonic sensor within ±1.77% with respect to the visual level data. Droplet entrainment data are obtained with and without the top break and are compared with the existing pool entrainment data. The present droplet entrainment data have higher values than the existing pool entrainment data due to (a) the pulling toward the break of the liquid deentrained on the top wall of the vessel and (b) gas acceleration in the vicinity of the break. In the present experiment, droplet entrainment, E_fg, strongly depends upon and is proportional to the seventh power of in the same way as the off-take data. The empirical correlation for the onset of off-take is developed in terms of the Froude number (Fr_g) at the break and the non-dimensional inception height (h_b/d). This correlation shows agreement with the present experimental data within ±15%. The present off-take quality data show agreement with Schrock's off-take quality correlation with the rms error of 15.8%.     

Neon implantation and the radiation enhanced diffusion of platinum for the local lifetime control in high-power silicon diodes

About 10 MeV helium and 120 MeV neon implantations were used for the local lifetime control of silicon power diodes with subsequent annealing at 200 °C. DLTS measurements show that the concentration ratio between VO^(–/0) pairs and divacancies after the implantation of neon is close to one in agreement with the data published for other heavy ions. The implantation dose to achieve the same point at the technology curve of the diodes under test was found about 10 times lower for the neon compared to helium. The radiation enhanced diffusion (RED) of platinum at 725 °C was evaluated both for the enhancement by implantation of helium and neon. The electrical parameters of silicon diodes (carrier lifetime, voltage drop, leakage current and reverse recovery) were compared. One order lower implantation dose of the neon compared to that of the helium was found necessary to obtain the same improvement of electrical parameters. The RED of Pt using the neon implantation was found functional in a similar way to that of the helium. The reduction of carrier lifetime, which would be normally sufficient for robust diodes, was found for the doses of neon at about 1 × 10¹³ cm⁻². However, the simultaneous increase of background doping concentration at the end of range of neon, which increases electric field, was found responsible for the decreased static breakdown voltage, decreased turn-off ruggedness and increased leakage current.     

On the effects of fission product noble metal inclusions on the kinetics of radiation induced dissolution of spent nuclear fuel

Radiation induced oxidative dissolution of UO₂ is a key process for the safety assessment of future geological repositories for spent nuclear fuel. This process is expected to govern the rate of radionuclide release to the biosphere. In this work, we have studied the catalytic effects of fission product noble metal inclusions on the kinetics of radiation induced dissolution of spent nuclear fuel. The experimental studies were performed using UO₂ pellets containing 0%, 0.1%, 1% and 3% Pd as a model for spent nuclear fuel. H₂O₂ was used as a model for radiolytical oxidants (previous studies have shown that H₂O₂ is the most important oxidant in such systems). The pellets were immersed in aqueous solution containing H₂O₂ and and the consumption of H₂O₂ and the dissolution of uranium were analyzed as a function of H₂ pressure (0–40 bar). The noble metal inclusions were found to catalyze oxidation of UO₂ as well as reduction of surface bound oxidized UO₂ by H₂. In both cases the rate of the process increases with increasing Pd content. The reduction process was found to be close to diffusion controlled. This process can fully account for the inhibiting effect of H₂ observed in several studies on spent nuclear fuel dissolution.     

CFD analysis of thermal–hydraulic behavior of heavy liquid metals in sub-channels

CFD analysis was carried out for thermal–hydraulic behavior of heavy liquid metal flows, especially lead–bismuth eutectic, in sub-channels of both triangular and square lattices. Effect of various parameters, e.g. turbulence models and pitch-to-diameter ratio, on the thermal–hydraulic behavior was investigated. Among the turbulence models selected, only the second order closure turbulence models reproduce the secondary flow. For the entire parameter range studied in this paper, the amplitude of the secondary flow is less than 1% of the mean flow. A strong anisotropic behavior of turbulence is observed. The turbulence behavior is similar in both triangular and square lattices. The average amplitude of the turbulent velocity fluctuation across the gap is about half of the shear velocity. It is only weakly dependent on Reynolds number and pitch-to-diameter ratio. A strong circumferential non-uniformity of heat transfer is observed in tight rod bundles, especially in square lattices. Related to the overall average Nusselt number, CFD codes give similar results for both triangular and square rod bundles. Comparison of the CFD results with bundle test data in mercury indicates that the turbulent Prandtl number for HLM flows in rod bundles is close to 1.0 at high Peclet number conditions, and increases by decreasing Peclet number. Based on the present results, the SSG Reynolds stress model with semi-fine mesh structures is recommended for the application of HLM flows in rod bundle geometries.     

Critical impact energies for scabbing and perforation of concrete target

In this paper, the influences of the relative target thickness (H/d) on those critical impact energies, at which local damage of various forms in concrete targets are initiated, are explored. The empirical formulae developed in the R3 Impact Assessment Procedure [BNFL, 2003. Reinforced Concrete Slab Local Damage Assessment, R3 Impact Assessment Procedure, vol. 3, Appendix H. Magnox Electric plc & Nuclear Electric Limited] are rationalized by different methods. A dimensional analysis was conducted to identify influential non-dimensional numbers, which were subsequently employed in the analyses of the experimental results relevant to scabbing and perforation by flat nosed missiles.The relationships between the non-dimensional impact energy at failure and the non-dimensional target thickness H/d are presented for all of the relevant experimental data in the “World Impact Data” collection [Bainbridge, P., 1988. World Impact Data—S.R.D. Impact Database Version Pre 3i, CCSD/CIWP(88)107(P)]. This collated hundreds of experimental data on local damage in concrete targets due to missile impact from various sources of nuclear industries, as well as experimental data from the UK electrical power industry used to develop empirical formulae in the R3 Impact Assessment Procedure [BNFL, 2003. Reinforced concrete slab local damage assessment, R3 Impact Assessment Procedure, vol. 3, Appendix H. Magnox Electric plc & Nuclear Electric Limited]. The experimental data in Bainbridge [Bainbridge, P., 1988. World Impact Data—S.R.D. Impact Database Version Pre 3i, CCSD/CIWP(88)107(P)] are compared with empirical and semi-empirical formulae for scabbing and perforation in order to examine the effects of H/d on the critical non-dimensional impact energy for these two local failures. An analytical formula based on a penetration-plugging model is employed to give the relationship between the critical impact energy and target thickness for perforation by a flat-ended projectile. Comparisons between these formulae and experimental data are presented.     

Validation study of computer code for sodium fire safety evaluation of fast reactor

A computer code solves coupled phenomena of thermal hydraulics and sodium fire based on a multi-zone model. It deals with an arbitrary number of rooms, each of which is connected mutually by doorways and penetrations. With regard to the combustion phenomena, a flame sheet model and a liquid droplet combustion model are used for pool and spray fires, respectively, with the chemical equilibrium model based on the Gibbs free energy minimization method. The chemical reaction and mass and heat transfer are solved interactively. A specific feature of is detailed representation of thermalhydraulics of a sodium pool and a steel liner, which is placed on the floor to prevent sodium–concrete contact. The authors analyzed a series of pool combustion experiments, in which gas and liner temperatures are measured in detail. It has been found that good agreement is obtained and the code has been validated with regard to pool combustion phenomena. Further research needs are identified for pool spreading modeling considering thermal deformation of steel liner and measurement of pool fluidity property as a mixture of liquid sodium and reaction products. The code is to be used mainly in the safety evaluation of the consequence of a sodium fire accident in a liquid metal cooled fast reactor as well as fire safety analysis in general.     

Ratcheting deformation of advanced 316 steel under creep–plasticity condition

A series of mechanical ratcheting tests under tension–torsion biaxial conditions has been conducted with an advanced 316 stainless steel at 923 K. Accumulation of torsional ratcheting strain was measured with a cyclic axial strain ranges of 0.005–0.02, cyclic axial strain rate of 10⁻⁵ to 10⁻³ s⁻¹ and steady torsional stresses of 24.5/ to 73.5/ MPa. The accumulation of ratcheting shear strain is mainly affected by the cyclic axial strain range and the steady shear stress, and increases with an increase of both these parameters. A simple evaluation of the accumulation of ratchet strain is proposed. Although this procedure is based on the separation of creep and plasticity and uses experimental data to skip the plasticity analysis, the obtained results show good agreement with the experimental results.     

The influence of the Coulomb explosion on the energy loss of and molecules channeling along the Si 1 0 0 direction

In this work we have measured the contribution of the Coulomb explosion to the electronic stopping power of molecular hydrogen ions and channeling along the Si 1 0 0 direction. We have used a SIMOX target, consisting of crystalline Si 1 0 0 with a buried layer of SiO₂. The measurements of the energy loss of H⁺, and have been carried out using the standard channeling Rutherford backscattering spectrometry. The energy loss has been measured around the Si 1 0 0 channel at a fixed energy per nucleon (150 keV/amu) as a function of the tilt and azimuthal angles. The present results show the effect of Coulomb explosion, which enlarges the protons traversal energy and consequently the channeling energy loss. This heating effect due to ions is about two times larger than molecules and amounts to about 5% of the total stopping power.     

Recombination of molecules in channeling conditions

The transmission of molecules through thin gold foils in channeling conditions is studied, both theoretically and experimentally, in the low energy range (E < 10 keV/u). For this purpose we have performed semiclassical computer simulations of the fragment trajectories inside the target and we are able to determine if they can reconstruct at the exit surface via the recombination model. Our calculated transmission yield shows interesting oscillations as a function of the dwell time, and also the calculated exiting number of with respect to H⁺ shows a very good agreement with our experimental measurements. We observed transmission yields almost three orders of magnitude larger than in the case of a random target.     

Fluence dependence of the surface roughness of InP after bombardment

InP(1 0 0) surfaces were sputtered under ultrahigh vacuum conditions by 5 keV ions at an angle of incidence of 41° to the sample normal. The fluence, , used in this study, varied from 1 × 10¹⁴ to 5 × 10¹⁸ cm⁻². The surface topography was investigated using field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). At the lower fluences ( 5 × 10¹⁶ cm⁻²) only conelike features appeared, similar in shape as was found for noble gas ion bombardment of InP. At the higher fluences, ripples also appeared on the surface. The bombardment-induced topography was quantified using the rms roughness. This parameter showed a linear relationship with the logarithm of the fluence. A model is presented to explain this relationship. The ripple wavelength was also determined using a Fourier transform method. These measurements as a function of fluence do not agree with the predictions of the Bradley–Harper theory.     

Novel porous media formulation for multiphase flow conservation equations

Multiphase flows consist of interacting phases that are dispersed randomly in space and in time. An additional complication arises from the fact that the flow region of interest often contains irregularly shaped structures. While, in principle, the intraphase conservation equations for mass, momentum, and energy, and their initial and boundary conditions can be written, the cost of detailed fluid flow and heat transfer analysis with explicit treatment of these internal structures with complex geometry and irregular shape often is prohibitive, if not impossible. In most engineering applications, all that is required is to capture the essential features of the system and to express the flow and temperature field in terms of local volume-averaged quantities while sacrificing some of the details. The present study is an attempt to achieve this goal by applying time averaging after local volume averaging.Local volume averaging of conservation equations of mass, momentum, and energy for a multiphase system yields equations in terms of local volume-averaged products of density, velocity, energy, stresses, and field forces, together with interface transfer integrals. These averaging relations are subject to the following length scale restrictions:

dℓL,