Surface Of Pebbles Research Articles

New ichnospecies of scratching traces from phosphatic nodules (Cenomanian, England)

Surfaces of phosphorite nodules and pebbles from the “Cambridge Green Sand” (Cenomanian, South England) yielded several discernible types of scratches. These originated before the burial of nodules/pebbles as evidenced by fossil epibiotic oysters cemented to cover the scratches. The individual forms of scratches differ in size and shape; therefore, the set of “scratching instruments” also had to be different. The scratches described differ from scratches generated by glacial processes, namely by the regularity of length and intervals, parallel orientation, existence of recurrent forms and placement along the nodule edges. We interpret the scratches as trace fossils of the ichnogenus Machichnus. Three new species, namely M. normani, M. harlandi and M. jeansi, are erected for them herein. The series of scratches originated probably by teeth on a couple of jaws; the makers possibly scraped bacterial or algal film off the surface of nodules that were covered with the phosphate gel. Both homodont and heterodont animals (probably fish) were involved.

Tritium release and retention properties of highly neutron-irradiated beryllium pebbles from HIDOBE-01 experiment

The current helium cooled pebble bed (HCPB) tritium breeding blanket concept for fusion reactors includes a bed of 1mm diameter beryllium pebbles to act as a neutron multiplier. Beryllium pebbles, fabricated by the rotating electrode method, were neutron irradiated in the HFR in Petten within the HIDOBE-01 experiment. This study presents tritium release and retention properties and data on microstructure evolution of beryllium pebbles irradiated at 630, 740, 873, 948K up to a damage dose of 18dpa, corresponding to a helium accumulation of about 3000appm. The measured cumulative released activity from the beryllium pebbles irradiated at 948K was found to be significantly lower than the calculated value. After irradiation at 873 and 948K scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses revealed large pores or bubbles in the bulk and oxide films with a thickness of up to 8μm at the surface of the beryllium pebbles. The radiation-enhanced diffusion of tritium and the formation of open porosity networks accelerate the tritium release from the beryllium pebbles during the high-flux neutron irradiation.

Tritium release kinetics in lithium orthosilicate ceramic pebbles irradiated with low thermal-neutron fluence

Tritium release kinetics in lithium orthosilicate (Li4SiO4) ceramic pebbles irradiated with low thermal-neutron fluence was studied by out-of-pile annealing experiments. It was found that the tritium produced in Li4SiO4 pebbles was mainly released as tritiated water vapor (HTO). The apparent desorption activation energy of tritium on the pebble surface was consistent with the diffusion activation energy of tritium in the crystal grains, indicating that tritium release was mainly controlled by diffusion process. The diffusion coefficients of tritium in the crystal grains at temperatures ranging from 450K to 600K were obtained by isothermal annealing tests, and the Arrhenius relation was determined to be D=1×10−7.0 exp(−40.3×103/RT) cm2s−1.

Lithium orthosilicate surfaces: Characterization and effect on tritium release

Within the European Union, slightly hyperstoichiometric lithium orthosilicate has evolved as one of the candidate solid breeder materials for the helium cooled pebble bed blanket, which will be tested in ITER. In the past, several long-term irradiation experiments proved that lithium orthosilicate shows excellent tritium release behavior when purged with helium with 0.1% hydrogen.In this work, short-term irradiation experiments at the High Flux Reactor in Petten with two standard pebble qualities, as-received and heat-treated lithium orthosilicate were carried out to investigate possible differences in tritium release. Since the surface of the pebbles may play a significant role, especially for short-term irradiation, and the mechanism of desorption from the surface determines the chemical form and very likely the release rate of tritium, the aim of this work is to link the chemical composition of the surface to the Thermal Programmed Desorption (TPD) tritium release experiments. Therefore X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry were applied to characterize the surface of the unirradiated pebbles and to obtain sputter depth profiles of up to 800nm. In addition, X-ray diffractometry was used to determine the composition of phases of the samples before and after irradiation.

CFD investigating the air ingress accident for a HTGR simulation of graphite corrosion oxidation

Abstract Through a compressible multi-component CFD model, this paper investigates the characteristics of graphite oxidation corrosion in the HTR-10 core under the postulated accident of gas duct rupture. In this accident, air in the steam generator cavity would enter into the core after pressure equilibrium is achieved between the core and the cavity, which is also called as the air ingress accident. Oxygen in the air would react with graphite on pebble surface, subsequently resulting in oxidation corrosion and challenging fuel integrity. In this paper, characteristics of graphite oxidation corrosion during the air ingress accident can be reasonably captured, including distributions of graphite corrosion amount on the different cross-sections, time histories of local corrosion amount at the monitoring points and overall corrosion amount in the core, respectively. Based on the transient simulation results, the corrosion pattern and its corrosion rate would approach to the steady-state conditions as the accident continuously progresses. The total amount of graphite corrosion during a 3-day accident time is predicted to be about 31 kg with the predicted asymptotic corrosion rate. This predicted value is less than that from the previous work of Gao and Shi ( Gao and Shi, 2002 ).

CFD investigating the air ingress accident occurred in a HTGR_simulation of thermal-hydraulic characteristics

Abstract The majority of this paper investigates thermal-hydraulic characteristics in a high-temperature gas-cooled reactor (HTGR) core under steady-state and accident conditions through a transient three-dimensional compressible CFD model. With the pebble-bed geometry, the HTR-10 core is selected for the present simulations. A loss-of-coolant accident (LOCA) with double-ended rupture of a gas duct tube is assumed and the core power would decay following this accident. In this LOCA, air in the steam generator cavity would enter the core via diffusion and natural convection mechanisms. Following air ingress into the core, O2 would react with graphite on pebble surface, causing the generation of CO2 and CO. The main thermal-hydraulic characteristics during this air ingress accident include O2 ingress into the core, CO2 and CO production histories, He concentration variations, and coolant temperature transient behavior, etc. Based on the present simulation results, the peak coolant temperature during the air ingress accident is slightly higher than that under the steady-state condition, revealing the safety of HTR-10 core in this postulated accident from the thermal point of view. In addition, corrosion phenomena related to the fuel pebbles are discussed and presented in the next paper.

Computational prediction of dust production in pebble bed reactors

Abstract This paper describes the computational modeling and simulation of graphite pebbles in frictional contacts as anticipated in a pebble bed reactor. For the high temperature gas-cooled reactor, the potential dust generation from frictional contact at the surface of pebbles and the subsequent lift-off and transport of dust and absorbed fission products are of safety concern at elevated temperatures under an air ingress accident. The aim of this work is to perform a computational study to estimate the quantity of the nuclear grade graphite dust produces from a typical anticipated configuration.

On the equilibria of finely discretized curves and surfaces

Our goal is to identify the type and number of static equilibrium points of solids arising from fine, equidistant $n$-discretrizations of smooth, convex surfaces. We assume uniform gravity and a frictionless, horizontal, planar support. We show that as $n$ approaches infinity these numbers fluctuate around specific values which we call the imaginary equilibrium indices associated with the approximated smooth surface. We derive simple formulae for these numbers in terms of the principal curvatures and the radial distances of the equilibrium points of the solid from its center of gravity. Our results are illustrated on a discretized ellipsoid and match well the observations on natural pebble surfaces.

Intralymphatic histiocytosis following placement of a metal implant

Intralymphatic histiocytosis (IH) is a rare condition first reported in 1994 by O'Grady et al. Less than 40 cases have been reported, with the majority occurring in patients with rheumatoid arthritis. We present a case of a 72-year-old man who developed an asymptomatic rash on his left upper arm 3 years after placement of a metal implant to stabilize a fractured humerus. Examinations revealed a poorly demarcated erythematous to brown indurated plaque with a pebbly surface overlying the inferior portion of the surgical scar. Biopsy revealed a mixed dermal infiltrate with a complex glomeruloid intravascular accumulation of histiocytes and neutrophils. Histiocytes were identified with immunostaining for CD68. Immunostains for CD31 and D2-40 confirmed the intravascular location of the histiocytes. This reactive process is closely related to reactive angioendotheliomatosis, but is best classified as IH. This is the only case of IH reported in association with a metal implant of the upper arm and the fourth case reported in association with a metal implant. Although the pathogenesis of IH is unknown, a role for lymphatic stasis secondary to chronic inflammation or surgery has been suggested. This is a benign process with a chronic course, and there are no known efficacious treatments.

Microfabrics of desert soils of Mongolia

The mineralogical composition of coarse fractions and the specificity of micro- and submicromorphology in the zonal sequence of desert soils of Mongolia (pale brown, gray brown, and extremely arid soils) were studied. A comparative analysis of analytical characteristics and macro- and micromorphological features of these soils made it possible to specify diagnostic criteria of elementary pedogenetic processes in the automorphic desert soils developed from the Quaternary slightly saline alluvial fan deposits. The microfabric of the soil mass near the pores in the crust horizons of desert soils and the microfabric of recent and relict calcite and calcareous-clay coatings on the surface of pebbles were described.

Turnover of quorum sensing signal molecules modulates cross‐kingdom signalling

N-acylhomoserine lactone (AHL) quorum-sensing molecules modulate the swimming behaviour of zoospores of the macroalga Ulva to facilitate the location of bacterial biofilms. Here we show that the intertidal surfaces colonized by Ulva are dominated by Alphaproteobacteria, particularly the Rhodobacteraceae family, and the Bacteroidetes family Flavobacteriaceae, and that this diverse assemblage both produces and degrades AHLs. N-acylhomoserine lactones could also be extracted from the surfaces of pebbles recovered from intertidal rock-pools. Bacteria representative of this assemblage were isolated and tested for the production and degradation of AHLs, and for their ability to modulate zoospore settlement at different biofilm densities. Of particular interest was a Shewanella sp. This strain produced three major AHLs (OC4, OC10 and OC12) in the late exponential phase, but the longer-chain AHLs were rapidly degraded in the stationary phase. Degradation occurred via both lactonase and amidase activity. A close relationship was found between AHL synthesis and Ulva zoospore settlement. The Shewanella isolate also interfered with AHL production by a Sulfitobacter isolate and its ability to enhance zoospore settlement in a polymicrobial biofilm. This influence on the attachment of Ulva zoospores suggests that AHL-degrading strains can affect bacterial community behaviour by interfering with quorum sensing between neighbouring bacteria. More importantly, these interactions may exert wider ecological effects across different kingdoms.

Characterization of pebble surfaces coated with biogenic manganese oxides by SIMS, XPS and SEM

The biogenic manganese oxide coating on the surface of a riverbed pebble was analyzed by means of secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). SEM observation combined with energy-dispersive X-ray spectroscopy (EDXS) confirmed that the coating consisted mainly of manganese oxide (thickness, 5 μm to a few tens of micrometers). SIMS and XPS analyses of the pebble surface suggested that the coating also contains iron oxides, silicate minerals and carbonaceous materials. The Mn 2p photoelectron spectrum indicated that the manganese oxide consists mainly of Mn(IV). Depth profiling by SIMS showed that the secondary ion intensities of Mn and Si increased and decreased, respectively, with sputtering time, which confirmed that the surface of the coating contains silicate mineral particles. In addition, the depth profile of Mn showed an oscillating pattern, suggesting a fine structure of the biogenic manganese oxide coating.

X-ray tomography investigations on pebble bed structures

The knowledge of pebble bed morphology and topology is important to understand the thermo-mechanical design of present ceramic breeder blankets. At Forschungszentrum Karlsruhe, pebble beds consisting of aluminium spheres with diameters of 2.3 and 5 mm, respectively, (simulating the blanket relevant 1 mm beryllium pebbles), were uniaxially compressed at different pressure levels. High resolution three-dimensional microtomography experiments were subsequently performed at the European Synchrotron Radiation Facility, Grenoble. Radial and axial void fraction distributions were found to be oscillatory next to the walls and non-oscillatory in the bulk. For non-compressed pebble beds, the bulk void fraction is fairly constant; for compressed beds, a characteristic gradient exists along the compression axis because of inner friction. The distribution of pebble contacts on the pebble surface is fairly homogenous in the pebble bed bulk. In the wall region, the pebble layer touching the wall is composed of zones with hexagonal structures, visualised by X-ray microtomography (MT) images and also reflected in the calculated angular contact distribution. With increasing distance from the wall, the regular structure vanishes and the bulk values are approached after the 4th layer away from the wall. Concerning the sum of contact surfaces per pebble, it is shown that the component normal to the compression axis (normal to the heat flow) is approximately independent of the pebble location. For strongly compressed pebble beds, this implies that by extrapolation of the bulk value of the thermal conductivity to the wall a further heat resistance might be neglected.

Analysis of solution lineations in pebbles: Kinematical vs. dynamical approaches

Solution lineations in conglomerates, resulting from indentation of non-soluble grains of the matrix into the surface of soluble pebbles, make up a morphological and genetic continuum with gradual transition between orthogonal stylolites, oblique slickolites and parallel striations. The distributions of incidence angles of matrix grains have been analyzed in individual pebbles in order to discern their kinematical or dynamical meaning. As a general rule, they fit theoretical models of flow trajectories determined by the bulk strain ( kinematical hypothesis). In contrast, they are not consistent with dynamical hypotheses based upon relationships with stress vectors. In particular, they do not fit the model of frictional sliding, which would give rise to a sharp discontinuity between slickolites parallel to the maximum principal stress σ 1 and true striations parallel to the resolved shear stress τ. Therefore, solution lineations all around a pebble cannot be considered as an analogue of multiple fault slip data, and they should not be generally analysed by methods of stress inversion based upon Bott's principle. Under certain conditions (high pebble solubility; active pressure-solution processes able to accommodate the strain rate; earlier cementation), the solution lineations tend to be parallel to each other and to the maximum shortening/compression axis. They therefore assume a double kinematical and dynanical meaning, and the deformation involves maximum volume reduction.

Turbulence-induced Heat Transfer in PBMR Core Using LES and RANS

This paper introduces the results of numerical simulations on flow fields and relevant heat transfer in the pebble bed reactor (PBR) core. In the core, since the coolant passes a highly complicated random flow path with a high Reynolds number, an appropriate treatment of the turbulence is required. A set of simple experiments for the flow over a circular cylinder with heat transfer was conducted to finally select the large eddy simulation (LES) and k-ω model among the considering Reynolds-averaged Navier-Stokes (RANS) models for PBR application. Using these models, the PBR cores, whose geometries were simplified to the body-centered cubical (BCC) and face-centered cubical (FCC) structures, were simulated. A larger pressure drop, a more random flow field, a higher vorticity magnitude and a higher temperature at the local hot spots on the pebble surface were found in the results of the LES than in those of RANS for both geometries. In cases of the LES, the flow structures were resolved up to the grid scales. Irregular distributions of the flow and local heat transfer were found in the BCC core, while relatively regular distributions for the FCC core. The turbulent nature of the coolant flow in the pebble core evidently affected the fuel surface temperature distribution.

Characteristics of air-blown gasification in a pebble bed gasifier

High temperature air-blown gasification is a new concept to utilize the waste heat from gasifier that is called multi-staged enthalpy extraction technology. This process was developed to solve the economic problems due to air separation costs for the oxygen-blown as a gasifying agent. In this study, we have constructed a pebble bed gasifier and operated it by controlling the pebble size and bed height with three different types of coal (Kideco, Datong and Drayton coal). As a result, we can produce syngas with a calorific value of 700 kcal/Nm3 at an air temperature of 650 °C; the performance of high temperature air gasification was strong in the order of Kideco coal, Datong coal and Drayton coal. Also, from the data of the exterior analysis of slag that is attached to the surface of pebbles, we can know that the iron component is considerably high. This means the increase in restored metallic iron component seems to contribute to the solidification of slag.

Effects of gelation and sintering conditions on granulation of Li 2TiO 3 pebbles from Li–Ti complex solution

Application of Li 2TiO 3 pebbles was proposed in the Japanese and European blanket design of a fusion reactor. The wet process and direct sol–gel methods are most advantageous from viewpoints of low-cost mass fabrication, reprocessing of lithium-bearing solution and removal of radioactive isotopes. Previously, the target density (80–85% T.D.) of Li 2TiO 3 pebbles was achieved in the preliminary fabrication test; however, the grain size of Li 2TiO 3 pebbles was larger than 5 μm and low sphericity. Therefore, the fabrication tests of Li 2TiO 3 pebbles by the direct-wet process were continued, focusing the effects on dissolving process of Li 2TiO 3, dropping of the solution into coagulant and drying/calcinating and sintering processes of the droplet for high sphericity and decreasing the cracks of Li 2TiO 3 pebble surface.

Aeolian dust accumulation by rock fragment substrata: influence of number and textural composition of pebble layers on dust accumulation

Rock fragments are a common occurrence in many arid and semiarid areas and can act as an effective dust trap. In the previous studies, dust accumulation on rock fragment substrata was investigated in a single stone layer, with stones of an identical size. The objective of the present study was to explore the effect of the number and the textural composition of pebble layers on aeolian dust accumulation in the semiarid loess area of NW China. The results of the experiment indicate that pebble layers have obvious effect upon dust accumulation. The effect was very pronounced for two layers of pebbles as compared to one layer. Dust accumulation on pebbles increased with increasing number of pebble layers. Total dust accumulation and accumulation between and underneath the pebbles in a single layer was significantly less than in two and three layers, however, no significant difference in dust accumulation was found between two and three layers. This suggests that a double layer of pebbles may be a better aeolian trap than single, triple or multi-layered pebble surfaces. The textural composition of a pebble surface has only minor effects on dust accumulation. Dust quantity on the pebbles was almost identical for all the combinations tested. Total dust accumulation and accumulation between and underneath the pebbles tended to increase with increasing proportion of small-size pebbles. The results of the study might be of use for proper management of gravel-mulched fields and stone barriers used for wind erosion control in the arid regions.

Mode of gypsum deposition in southeastern Iranian soils as revealed by isotopic composition of crystallization water

Variations in δD and δ 18O values of water of crystallization and meteoric water were used to elucidate the origin of mother brine and the gypsum mode of formation in selected southeastern Iranian soils. Thirteen representative pedons were sampled at various geomorphic positions around the city of Rafsanjan. Crystallization water in different parts of three gypsum pendants was also analyzed isotopically. The slope of the δD and δ 18O plot for crystallization water was found to be 2.8, showing that evaporation was the major process for gypsum accumulation in the studied area. Comparison of data for eight rainwater samples and the crystallization water shows that gypsum formation occurred in a climate that was probably lower in temperature than that of today. The mean δD and δ 18O values of gypsum crystallization water vary from −41.9‰ and +6.5‰ for rock pediment to −36.9‰ and +10.0‰ for mantled pediment and −28.8‰ and +13.8‰ for transitional surfaces and finally, −8.8‰ and +19.2‰ for playa margins. The existence of such a close relationship between the isotopic composition of gypsum crystallization water and geomorphic positions could be attributed to the deposition of gypsum in different landforms at various stages of evaporation and/or progressive evaporation of moving surface and subsurface waters. An increase in δD and δ 18O values was also observed with increasing depth in almost all soil profiles studied. Different climates at the time of gypsum formation in different depths seem to be the possible reason. With increasing depth from the pebble surface in gypsum pendants, δD and δ 18O values decrease, which is also attributed to climatic changes during pendant development.

Biogeochemistry of manganese oxide coatings on pebble surfaces in the Kikukawa River System, Shizuoka, Japan

The biogeochemistry of Mn-oxide coatings formed over submerged pebble surfaces on the streambed of the Kikukawa river system has been investigated. Located in central Shizuoka, Japan, this system drains strongly acidified soils under tea plantations. Besides containing high amounts of Mn (up to 450 μg/cm 2), the coatings are capable of scavenging and accumulating other elements including Ba, Zn, Ni, Co, W, Mo and Sb. When suspensions of the coating material were incubated with 0.2 mM Mn 2+, the Mn(II) ion was microbially transformed into Mn-oxides. When the same suspensions were spread on agar plates containing acetate, yeast-extract, and 1.0 mM Mn 2+ (AY agar medium) both Mn-oxidizing bacteria and fungi appeared, indicating the existence of a diversity of Mn-oxidizing microorganisms in the system. Plate counts using two agar culture media with varied nutrient levels indicated that the ability of these microorganisms to oxidize Mn(II) was strongly dependent on nutrient supply. The relatively nutrient-poor AY agar medium was more conducive to microbial growth than the K 1 agar medium with a higher organic nutrient content. Concentrations of Mn dissolved in the stream waters did not correlate well with the amounts of solid Mn on submerged pebbles. Thus, factors other than dissolved Mn concentration (e.g., organic nutrient supply and pH) determined the ability of microorganisms to oxidize Mn in the streambeds. A survey of dissolved Mn in streams and water draining tea plantations combined with chemical analysis of Mn in the underlying soils indicate that the soils have been strongly acidified through excessive applications of N-fertilizers. As a result, Mn was leached from the soil column into the Kikukawa river system. Biogenic Mn-oxide coatings on streambeds can therefore serve as an indicator of soil acidification and metal leaching from soils of the corresponding watershed.