Fine Fragments Research Articles

Effect of particle size distribution and subgrade condition on degradation of railway ballast under impact loads

Ballasted rail tracks are generally exposed to impact loads generated from abnormal wheel-rail interface as well as sudden variations in support rigidity. These induced impact loads can lead to railway ballast degradation by attrition of the angular edges of the aggregate and breakage of single particle into finer fragments. In the present study, the degradation of ballast particles under impact loads is investigated by considering various fouling and breakage indices. For this purpose, impact test is conducted on ballast aggregates obtained from different quarries (rock types of basalt, marl, dolomite and trachyte) by varying the gradation of ballast aggregates, impact energy and subgrade type. According to the obtained results, the degradation of ballast specimens under impact loading is less for more broadly-graded ballast. In addition, providing a flexible subgrade as support condition leads to reduction in ballast degradation resulted from diminishing impact energy. Furthermore, the axial strain of ballast specimens reduces with decrease in degradation of aggregates under repeated impact loads.

Numerical modelling for blast-induced fragmentation in sublevel caving mines

The flow behavior of the ore and waste significantly affect the dilution in sublevel caving (SLC) mines. Drill and blast issues are identified as having a substantial impact upon SLC material flow. In the paper, blast-induced fragmentation in SLC was numerically investigated using the LS-DYNA code. A method was presented to evaluate fragmentation based on the damage description and a fragment identification routine implemented in the LS-PREPOST (a pre- and post-processing tool of LS-DYNA). The effects of the delay time and the primer position on fragmentation were investigated. The results indicated that a long delay time gives a finer fragmentation for the cases discussed in the paper. The results also showed that the middle primer and the top primer in SLC can give a fine fragmentation. The limitations of numerical modelling were also discussed.

An Attempt on the Cell Survival and Cell Differentiation by Fine Fragments of Tungsten Carbide and Steel Cutting Bars

An Attempt on the Cell Survival and Cell Differentiation by Fine Fragments of Tungsten Carbide and Steel Cutting Bars

The relationship between particle strength and particle breakage in loaded gravels

The particle breakage behaviour of coarse grains during shearing of a gravelly granular assemblage changes in accordance with the magnitude of the confining stress acting on the assemblage. The transition from particle rearrangement and sliding to particle breakage is studied for two gravelly materials subjected to a wide range of compressive and shear stresses (10kPa to 3.5MPa). The tested materials are a gravel fraction derived from crushed sedimentary sandstones and shales, and a gravel derived from a crushed rhyodacitic volcanic. At low confining stresses, breakage due to compression and shear are both small, with only some fine fragments produced. Confining stress increase results in a stage being reached where particle damage starts to increase greatly, under both compressive and shearing loads. The results indicate that, as stresses become very large, the particle breakage due to compression continues to increase, but the breakage due to shearing under the same vertical stress actually decreases. This is attributed to the attainment of a fabric which, through extreme damage during compression, allows subsequent shearing with reduced additional particle damage.

Steam explosion in nuclear reactors: Droplets of molten steel vs core melt droplets

Relative contributions of molten steel and core melt droplets to heat transfer from the droplets to ambient water at typical regimes of the droplet cooling and solidification are considered. It is shown that fine fragmentation of steel droplets overheated significantly with respect to the steel solidification temperature makes these droplets more dangerous for the steam explosion triggering, and this effect cannot be neglected in nuclear safety analysis. On the contrary, the solid crust on the surface of core melt droplets is formed before the first pressure drop in the steam envelope. As a result, the probability of fragmentation of these droplets is relatively small and there is no increase in the overall heat transfer due to the expected large total surface area of fine particles. The predicted low explosivity of core melt droplets is confirmed by the published data of laboratory experiments. It is recommended to revise the existing theoretical and computational models of steam explosion, paying more attention to the role of fine fragmentation of steel droplets.

Investigation of Fe-rich fragments in aluminum-steel friction stir welds via simultaneous Transmission Kikuchi Diffraction and EDS

An attempt has been made to investigate Fe-rich fragments and surrounding intermetallic compounds in aluminum-steel friction stir welds using conventional EBSD + EDS as well as advanced Transmission Kikuchi Diffraction (TKD). Results indicate that the fragments are covered by a layer of orthorhombic Al5Fe2. This layer is surrounded by a mixed region composed of sub-micrometer Al3Fe crystals in a matrix of Al nano grains. Employing high beam current TKD on electron-transparent samples enables investigation of sub-micrometer grains in the mixed region with a higher spatial resolution than conventional EBSD. As a result, nano-twinning in Al in the mixed region has been observed. Microstructural and texture results imply that Fe-rich fragments travel through the Al matrix under extreme shear forces, severely refining Al grains, and inducing nano-twinning. Due to extreme shear forces and Fe outward diffusion, fine steel fragments dissolve in the surroundings leading to the nucleation of nano Al3Fe crystals particularly along Al grain boundaries. Al3Fe crystals consume the Al matrix as they grow in size and fraction.

Discrete element method study of fuel relocation and dispersal during loss-of-coolant accidents

The fuel fragmentation, relocation and dispersal (FFRD) during LOCA transients today retain the attention of the nuclear safety community. The fine fragmentation observed at high burnup may, indeed, affect the Emergency Core Cooling System performance: accumulation of fuel debris in the cladding ballooned zone leads to a redistribution of the temperature profile, while dispersal of debris might lead to coolant blockage or to debris circulation through the primary circuit. This work presents a contribution, by discrete element method, towards a mechanistic description of the various stages of FFRD. The fuel fragments are described as a set of interacting particles, behaving as a granular medium. The model shows qualitative and quantitative agreement with experimental observations, such as the packing efficiency in the balloon, which is shown to stabilize at about 55%. The model is then applied to study fuel dispersal, for which experimental parametric studies are both difficult and expensive.

Collapse of a two‐dimensional brittle granular column: Implications for understanding dynamic rock fragmentation in a landslide

AbstractWe investigate numerically the failure, collapse, and flow of a two‐dimensional brittle granular column over a horizontal surface. In our discrete element simulations, we consider a vertical monolayer of spherical particles that are initially held together by tensile bonds, which can be irreversibly broken during the collapse. This leads to dynamic fragmentation within the material during the flow. Compared to what happens in the case of a noncohesive granular column, the deposit is much rougher, and the internal stratigraphic structure of the column is not preserved during the collapse. As has been observed in natural rockslides, we find that the deposit consists of large blocks laying on a lower layer of fine fragments. The influence of the aspect ratio of the column on the runout distance is the same as in the noncohesive case. Finally, we show that for a given aspect ratio of the column, the runout distance is higher when the deposit is highly fragmented, which confirms previous hypotheses proposed by Davies et al. (1999).

Investigating the maximum resolution of µXRF core scanners: A 1800 year storminess reconstruction from the Outer Hebrides, Scotland, UK

Micro x-ray fluorescence (µXRF) core scanning is capable of measuring the elemental composition of lake sediment at sub-millimetre resolution, but bioturbation and physical mixing may degrade environmental signals at such fine scales. The aim of this research is to determine the maximum possible resolution at which meaningful environmental signals may be reconstructed from lake sediments using this method. Sediment from a coastal lake in the Outer Hebrides, Scotland, has been analysed using calibrated element measurements to reconstruct storminess since AD 200. We find that a Ca/K ratio in lake-core sediments reflects the presence of fine calcium carbonate shell fragments, a constituent of sand in the catchment that is washed and blown into the lake. Variations in this ratio are significantly correlated with instrumental records of precipitation and low pressures, suggesting it is a proxy for storminess. Furthermore, identification of a c. 60-year cycle supports a climatic influence on Ca/K, as this cycle is frequently identified in reconstructions of the North Atlantic Oscillation and North Atlantic sea-surface temperature. Comparison with weather records at different resolutions and spectral analysis indicate that µXRF data from Loch Hosta can be interpreted at sub-decadal resolutions (equivalent to core depth intervals of 3–5 mm in this location). Therefore, we suggest that sub-centimetre sampling using µXRF core scanning could be beneficial in producing environmental reconstructions in many lake settings where sediments are not varved.

Experimental and Numerical Investigation of the Primary Fragmentation of a Lignite during Fluidized-Bed (FB) Devolatilization

The paper presents a comparison between experimental and model results of primary fragmentation of a lignite coal in a fluidized bed (FB). In the experiments, the char particle size distribution and the general indicators of primary fragmentation (intensity and index) were determined. The same parameters were calculated using a mathematical model of the process, fed by data of the fuel (the amount of volatiles and fixed carbon), fluidized bed temperature, and inlet particle size distribution. The size distribution and number of the char particles in fluidized bed significantly differ from the size distribution and number of inlet coal particles. Char population has a bimodal distribution—separate distributions for the smaller and larger sets of fragments. The experimental and model results show the same tendency: a coal particle partially breaks at the beginning of devolatilization, giving a large number of fine fragments, while, as the process continues, the rest of the parent particle sometimes breaks down into a smaller number of larger pieces, and sometimes does not fragment at all. Review of the Weibull distribution coefficients enables prediction of the char particle size distribution for the characteristic fluidized bed conditions and inlet coal particle sizes.

An Assessment of the Fuel Pulverization Threshold During LOCA-Type Temperature Transients

To investigate the potential disintegration to powder of high-burnup fuel pellets during a rapid temperature transient, the Nuclear Fuels Industry Research (NFIR) Program commissioned two independent scoping studies. The first investigated the effect of hydrostatic restraint pressure on fission gas release during a series of fast temperature ramps. In the second study laser heating was used to investigate the temperature at which small samples of fuel fragmented. From the observations made in these studies, local burnup and temperature thresholds of 71 MWd/kg HM and 645°C were identified for fuel pulverization during a loss-of-coolant accident (LOCA). It is shown that fine fragment production in integral LOCA tests performed in other independent investigations at Studsvik and Halden was generally well predicted using these thresholds of burnup and temperature. The NFIR investigations also reveal that the degree of pulverization and resulting fragment size are dependent on the temperature ramp rate. Moreover, they confirm that pulverization can be substantially reduced by the imposition of hydrostatic pressure.

Eulerian modelling of melt solidification impact during fuel–coolant interaction

Melt droplet solidification is one major phenomenon acting on the mitigation of the strength of vapour explosions in situations related to nuclear safety. The modelling of solidification effects is then an important challenge for the evaluation of fuel–coolant interaction. We present an attempt for modelling the crust that might develop around melt droplets and its effect on the fine fragmentation within an Eulerian formulation of the flow equations, with application to the MC3D code.A physically-based modelling of the melt droplets temperature profile and a fragmentation criterion for partly solidified droplets are presented. The implementation of these models in the specific case of Eulerian modelling of the melt droplet phase is described. Two additional transported model parameters based on the most important droplets features regarding the fuel–coolant interaction behaviour are derived. The first property to transport is the crust stiffness because it enables the correct prediction of the amount of droplets participating in the droplet fragmentation process. The second is related to the energy transfer from the droplet interior to the surface. This enables to improve the surface temperature determination and reflects the history of the droplet cooling. Also the potential effect of solidification on the vapour explosion strength is shown.

The challenge of modeling fuel–coolant interaction: Part II – Steam explosion

In the course of a severe accident in a nuclear power plant cooled or moderated by water, the core might melt and flow down into the water. Under certain circumstances, a steam explosion might develop during the mixing of the melt and the water. Such an explosion, if occurring in the reactor pit of a PWR or BWR, might challenge the containment integrity and is thus an important issue for nuclear safety. This paper aims at presenting both a status of research and understanding of the phenomenon and the main characteristics of the models developed in the 3-dimensional computer code MC3D. We make a particular emphasis on the underlying difficulties, uncertainties and needs for further improvements. We discuss more particularly the two major phenomena that are the fine fragmentation and the pressurization process. We also give insights on the impact of melt solidification on the fragmentation and on the issue of oxidation. The verification basis of the models is discussed and finally, an example of 3D calculation is presented to highlight the current code capabilities.

Zheltozems of Russia: Micromorphology, clay minerals, and pedogenetic analysis .

Zheltozems (literally “yellow earths”) that can be classified as Acrisols, Alisols, or Ultisols, occupy very small areas in Russia; they are strongly modified by human activities. The unchanged profiles are preserved only locally, in particular, in the Sochi arboretum. When compiling the soil map of the arboretum, different Zheltozems were described on gentle and moderately steep slopes and diverse parent materials: calcareous claystone and its heavy-textured derivates. The micromorphology of three characteristic Zheltozem profiles developed on these different parent materials was also studied. The features that they have in common are the following: in all profiles, a specific heavy-textured Bw, or BM ‘metamorphic’ horizon was identified with a certain set of micromorphological properties: a massive microstructure with fine planes merging into deformed biogenic pores, a monic groundmass with few silt grains that are sometimes oriented, and impregnative iron-oxides pedofeatures. The b-fabric was defined as stipple speckled, poro- and granostriated, locally (mono)striated ressembling stress coatings. There are almost no clay coatings except for very few fine internal ones that are light yellow and homogeneous; clay and iron pseudomorphs (alteromorphs) occur over some skeleton grains. Many of these properties are related to the clay mineral composition. There are varying quantities of smectites and kaolinite-smectites, whereby the former increase with depth. The presence of expanding minerals is known to contribute to the disintegration of coatings. Remnants of these are observed in typic iron-manganic nodules. In terms of WRB, this horizon is an intergrade between cambic and argic, which is not accounted for in soil diagnostics. As three different Zheltozems are studied in the same area, the following genesis of this specific intergrade can be given. The saprolite in one of the profiles is composed of fragments of weathered calcareous argillites (subangular) and sandstones (angular) separated by planes with infillings. The micromass in the argillites is locally depleted of carbonates; some fragments have rusty-black iron-manganic intercalations, external clay coatings and iron quasicoatings. The infillings contain fine carbonate nodules and fragments of clay coatings. The sequence of soils studied enabled us to identify the pedoplasmation phenomena, as described by Stoops and Schaefer (2010). This process starts with the decarbonatization, fragmentation and ferrugination of rock fragments, which are gradually assimilated by the groundmass, so that only a trend to linear arrangement of grains is preserved. The pedoplasmation starts in the saprolite of in situ argillite, next follows a transformation of the lithogenic fabric into a pedogenic one with the development of a Bw horizon. These phenomena are more obvious in the profile on the oldest parent material – clayey colluvium. The low porosity and dense fabric of the Bw horizon hinder the removal of carbonates from the underlying layers (saprolite), and this may be a reason for the incompatibility between abundant rainfall inherent to the wet subtropical climate in Sochi region and the high position of the line of effervescence with hydrochloric acid in Zheltozems on calcareous argillite.

Hydrodynamic fine fragmentation of partly solidified melt droplets during a vapour explosion

In this paper we investigate the fine fragmentation criterion for partly solidified droplets during the explosion phase of a vapour explosion. First, a modified Weber number is proposed for the determination of the critical conditions for the fine fragmentation. Next, this introduced dimensionless number, which considers the crust stiffness as a stabilizing force acting to retain the crust in the presence of hydrodynamic forces, is compared with experimental data. It is further investigated to see whether the limitation of the fine fragmentation by the crusts can help explain the strongly reduced explosion strength observed in experiments with corium as compared to alumina. Finally, the conclusions are used to assess the limitation due to solidification on the strength of the explosions that could occur during severe accidents in nuclear reactors.

Material Effect in the Nuclear Fuel–Coolant Interaction: Analyses of Prototypic Melt Fragmentation and Solidification in the KROTOS Facility

During a severe accident sequence in a pressurized light water reactor, the hot (~3000 K) molten materials (corium) coming from the degraded reactor core may generate a violent interaction if they come in contact with water. This melt-water interaction, called fuel-coolant interaction (FCI), may damage the structures and threaten the reactor integrity if there is a steam explosion. FCI occurs generally in two phases: a premixing phase, during which the molten corium jet is fragmented into large droplets and mixed with the water, and the explosion phase, during which the vapor film that has developed around the fuel droplets is destabilized and the droplets are finely fragmented. The presented work covers a set of experimental studies describing the morphology and nature of the solidified materials after interaction with water. Debris from experiments performed in the KROTOS (Commissariat à l’Énergie Atomique, Cadarache, France); PREMIX, ECO (Karlsruhe Institute of Technology, Karlsruhe, Germany); and MISTEE (Royal Institute of Technology, Stockholm, Sweden) facilities have been characterized by metallographic, analytical, and microscopic techniques. These post-test analyses are able to provide important information on the solidification path and other main phenomena involved during FCI. It was found that the behavior of metallic and oxide melts differs significantly from the standpoint of debris morphology. Oxide melts that underwent simple coarse fragmentation showed spherical or angular rocklike shape, unlike metallic melts. A statistical analysis was performed on the debris from the KROTOS tests; a data set of particles was described by the circularity, solidity, and porosity. The mechanism of water ingression (Kim and Corradini) inside the melt droplet was observed to be the key mechanism of fine (secondary) fragmentation. The particles participating in fine “thermal” fragmentation have significantly higher porosity, up to ~30% for prototypic corium in the KROTOS facility. It was calculated that only a part of the premixed melt participates in fine fragmentation, i.e., about one-third of the melt mass for the KROTOS tests using UO2-ZrO2 mixture.

Impact strength of small icy bodies that experienced multiple collisions

Frequent collisions are common for small bodies in the Solar System, and the cumulative damage to these bodies is thought to significantly affect their evolution. It is important to study the effects of multiple impacts such as the number of impacts on the impact strength and the ejection velocity of impact fragments. Here we conducted multiple-impact experiments using a polycrystalline water ice target, varying the number of impacts from 1 to 10 times. An ice cylindrical projectile was impacted at 84–502ms−1 by using a single-stage gas gun in a cold room between −10 and −15°C. The impact strength of the ice target that experienced a single impact and multiple impacts is expressed by the total energy density applied to the same target, ΣQ, and this value was observed to be 77.6Jkg−1. The number of fine impact fragments at a fragment mass normalized by an initial target mass, m/Mt0∼10−6, nm, had a good correlation with the single energy density at each shot, Qj, and the relationship was shown to be nm=101.02±0.22·Qj1.31±0.12. We also estimated the cumulative damage of icy bodies as a total energy density accumulated by past impacts, according to the crater scaling laws proposed by Housen et al. (Housen, K.R., Schmidt, R.M., Holsapple, K.A. [1983]. J. Geophys. Res. 88, 2485–2499) of ice and the crater size distributions observed on Phoebe, a saturnian icy satellite. We found that the cumulative damage of Phoebe depended significantly on the impact speed of the impactor that formed the craters on Phoebe; and the cumulative damage was about one-third of the impact strength ΣQ* at 500ms−1 whereas it was almost zero at 3.2kms−1.

Une amphore et des marmites. Pratiques et gestes cultuels au palais d’Amathonte

During the excavations taking place in the storage area of the Palace of Amathus since 1977, fourteen foundation deposits (thirteen cooking pots and one amphora) have been discovered in several places of the building, deposited into the floor preparation or under the pavement. If it still lacks a reliable chrono-typology of cooking pots, which can be dated from a time ranging from CA to the end of CC, the Bichrome IV amphora and its sealing can be dated from the late CA I (around 620) : this date would be that of the construction of the archaic palace. The paper studies the material contained in these deposits, which were nearly all sealed by plaster : silty clay, lamps, small dishes, charcoal and animal bones (sometimes found in the cooking pots, but not always). In addition to the fine clay and charcoal fragments, the amphora contained the bones – but stripped from their flesh – of the left quarter of a sub-adult Bos Taurus and a lump of coating. Unique deposits both in number and nature in Cyprus, they are compared with similar practices of the antique Near-Eastern world. It could be propitiatory practices related to the construction of a building or to rebuilding works.

Reconstructing the Chronology of the House XVII-XVIII Complex at Umm el-Jimal, East Jordan: Radiocarbon Dates of Organic Inclusions of Architectural Mortars

This article presents accelerator mass spectrometry (AMS) radiocarbon dates of organic inclusions of cement materials from the House XVII-XVIII Complex located in the Umm el-Jimal archaeological site, east Jordan, aiming at refining the unclear chronology of the house. Fine straws and small fragments of charcoal uncovered from preserved architectural lime mortars and plasters were dated without carrying out extensive excavations. The results indicate that the house most probably was initially plastered or built during the middle of the Byzantine period. The results agree with the historical and archaeological data indicating that Umm el-Jimal flourished during this period; therefore, it is probable that the house was established during this time to meet the housing demand for the increased number of its population.

Preparation and Characterization of Moso Bamboo Charcoal Modified with Nitric Acid

The surface properties of a commercial moso bamboo charcoal had been selectively modified by means of acidic treatment, using HNO3 with the concentrations of 17%, 34%, and 68% (w/w) respectively at 25°C for 8 hours. The iodine number, moisture content of all samples were tested, and the results showed that the iodine number of modified samples decreased slightly, while the moisture content increased sharply compared with raw sample, the sample modified with 68% HNO3 got the most remarkable change of above parameters. The Boehm titration、FTIR and SEM were also introduced to characterize the surface properties of raw and modified samples, the results revealed that the amount of surface functional groups, including various oxygen-containing and a few nitrogen-containing groups increased significantly of modified samples, the surface became more coarse and a large amount of fine fragment was yielded in pore space and vascular bundle of modified samples because of the corrosion behaviour of HNO3 in the modification process.