Graphite Spheres Research Articles

Summary of Tritium Source Term Study in 10 MW High Temperature Gas-Cooled Test Reactor

Tritium (3H) has been increasingly researched when assessing the environmental impact of nuclear reactors and other nuclear facilities because it is widely present in nuclear systems and can easily enter the environment. The first pebble-bed gas-cooled test reactor in China, the 10 MW high temperature gas-cooled test reactor (HTR-10), uses helium, graphite, and graphite spheres containing embedded tristructural-isotropic–coated particles as primary coolant, reflectors, and fuel elements, respectively. Several experiments that involved the 3H source term in HTR-10 were performed, and they measured the 3H specific activity and its distribution in the irradiated graphite spheres from the core, 3H activity concentration in the primary helium, 3H activity concentration during the regeneration of the molecular sieve adsorber in the helium purification system, and 3H amount in the gaseous effluent discharge from the stack. The experimental data were summarized and compared with the theoretical predictions. The balance diagram of the 3H source term in HTR-10 is introduced in this paper. Sensitivity analysis was performed to illustrate the effect of the 3He abundance in the primary helium and Li content in the graphite reflectors on the 3H activity concentration in the primary coolant of HTR-10. The interactions between graphite and different hydrogen isotopes (1H, 3H, 1H2, 1H3H, and 3H2) were investigated using first-principles calculations and the diffusion theory. The results indicated that molecular 3H tended to diffuse in graphite.

Nodularity Control of Spheroid Ductile Cast Iron by Rayleigh Backscattering

Spheroid Ductile Cast Iron finds increasingly industrial use as a high strength material of high fatigue resistance. The material properties can be designed by density, shape and size of the graphite spheres—the nodularity—and the matrix microstructure. Until today, the near surface nodularity cannot be controlled nondestructively to take full advantage of the material quality especially at high loaded surface areas. A viable nondestructive approach we propose is based on ultrasonic backscattering preferably in the Rayleigh regime. First experiments with model samples proved significant contrast to distinguish materials with different graphite morphology and matrices. The claim of our tests is a fast and easy industrial control of nodularity in reference to a validated sample. The results encourage us for the development of a prototype equipment with optimized probe systems and inspection parameters. Quantified acceptance criteria still require systematic tests for optimized parameter settings and probe configurations on more representative samples provided by industry.

SiC/YSiC composite coating on matrix graphite sphere prepared by pack cementation and molten salt

SiC/YSiC composite coating was prepared on the surface of matrix graphite sphere by pack cementation and molten salt process to improve the anti-oxidation property of the matrix graphite. According to the experiment results, the composition of the composite coatings prepared with different Y content is the same. While, the crushing model and strength is different. The maximum crushing strength is 1716.2 N when Y content is 1 g. The different coating structure were analyzed according to the results. The coating is composed of inner original SiC layer and outer new formed Si layer when Y content is 0.25 g. When the addition of Y were 0.5 g and 0.75 g, the coating consists of a newly formed YC2 layer inside, a SiC layer in the middle and an external Si layer. The final coating structure is composed of SiC layer and residual Si and YC2 layers when Y content is 1 g.

Dependence of the Density of Entropy Production on the Rate of Changes in Temperature in the Linear Mode of Thermodynamics on the Examples of Graphite and Aluminum and Boron Nitrides

We analyze the experimental thermograms of specimens in the form of plates of aluminum and boron nitrides heated by a laser and a freely cooled graphite sphere. It is shown that all thermograms can be referred to the linear law of thermodynamics. The linear dependence of density of the entropy production on the rate of changes in temperature is established. It is proposed to consider the rate dT/dτ as an additional variable for the density of entropy production in the linear thermodynamic law and apply this rate for the representation of the properties determined in the nonstationary thermal mode.

A Comprehensive Study of the 14C Source Term in the 10 MW High-Temperature Gas-Cooled Reactor

ABSTRACTWhile assessing the environmental impact of nuclear power plants, researchers have focused their attention on radiocarbon (14C) owing to its high mobility in the environment and important radiological impact on human beings. The 10 MW high-temperature gas-cooled reactor (HTR-10) is the first pebble-bed gas-cooled test reactor in China that adopted helium as primary coolant and graphite spheres containing tristructural-isotropic (TRISO) coated particles as fuel elements. A series of experiments on the 14C source terms in HTR-10 was conducted: (1) measurement of the specific activity and distribution of typical nuclides in the irradiated graphite spheres from the core, (2) measurement of the activity concentration of 14C in the primary coolant, and (3) measurement of the amount of 14C discharged in the effluent from the stack. All experimental data on 14C available for HTR-10 were summarized and analyzed using theoretical calculations. A sensitivity study on the total porosity, open porosity, and percentage of closed pores that became open after irradiating the matrix graphite was performed to illustrate their effects on the activity concentration of 14C in the primary coolant and activity amount of 14C in various deduction routes.

INVESTIGATION ON TRIBOLOGICAL BEHAVIOR OF DUCTILE CAST IRONS WITH NANOSIZED PARTICLES

The work in this study is focused on investigation of the tribological behavior of ductile cast iron with nanosized particles: titanium nitride TiN; titanium nitride 30% + titanium carbonitride 70% (30%TiN+70%TiCN). The ductile cast iron composition is: Fe-3,55C-2,67Si-0,31Mn-0,009S-0,027P-0,040Cu-0,025Cr-0,08Ni-0,06Mg wt%. Before the addition to the melt nanosized particles were coated with nickel by the electroless nickel deposition method EFFTOMNICKEL .The nickel coating on the nanosized particles ensures their wetting in the melt as well as their uniform distribution into the cast. The optical and quantity metallographic observations and wear test are performed to study the influence of the nanoparticle additives on the cast iron tribological properties. It is observed that the quantity proportion changes between pearlite, ferrite and graphite phase in the cast iron structure. The graphite shape is retained the same, but the nanosized additives decrease the average diameter of the graphite spheres Dmid and increase the quantity of the graphite phase in the structure of ductile cast irons. The cast iron wear resistance in the presence of nanosized additives of (TiN+TiCN) and TiN increases to 55–69% in comparison to wear resistance of the cast iron without nanoparticles.

SiC coating on HTR graphite spheres prepared by fluidized-bed chemical vapor deposition

A dense SiC coating was prepared by fluidized-bed chemical vapor deposition (FB-CVD) process to protect matrix graphite sphere of high temperature gas-cooled reactor fuel element from oxidation. The effect of deposition temperature, atmosphere and the formation mechanisms of the SiC coatings were analyzed. Results show that deposition temperature has a significant influence on coating composition. At lower temperature, the co-deposition of Si occurs; while the co-deposition of C occurs at higher temperature. The atmosphere also can influence the coating phase composition. When the deposition is processed at 1773 K, there appears free Si in the SiC coating with the increase of the concentration of H2 in the atmosphere. The density of the deposited coatings can reach up to 90% of the theoretical density of SiC. The research results show that FB-CVD is an ideal choice for preparing dense SiC coating on the surface of high temperature gas-cooled reactor matrix graphite spheres.

Cold energy storage in a packed bed of novel graphite/PCM composite spheres

The low thermal conductivity of phase change materials (PCMs) are hindering wider use. To enhance the thermal conductivity of PCM (water), a novel approach of encapsulated water in a high thermal conductivity graphite sphere was investigated. For a single graphite/water composite sphere, the results showed that the effective thermal conductivity of the PCM (water) is increased by 12 times, and hence the freezing and melting times of the PCM is reduced by 53.7% and 44.4% respectively, compared to a non-graphite sphere filled with the same amount of water. In addition, a graphite/water composite sphere exhibits minimum super-cooling, indicating that the graphite matrix material plays a good role as a nucleating agent for water freezing. The thermal behaviour of a packed bed filled with 50 graphite/water composite spheres was tested. The influence of heat transfer fluid (HTF) inlet temperature and volumetric flow rate on the thermal behaviour of the packed bed latent heat storage system are experimentally investigated. The results showed that both had a significant effect on freezing time. The freezing time at HTF inlet temperature of −3.5 °C and volumetric flow rate of 1000 mL/min is 5.3 h. However, it is decreased by 45% and 65% when the HTF inlet temperature is decreased from −3.5 °C to −5.5 and −7.5 °C, respectively. It is observed that by adjusting the HTF inlet temperature and volumetric flow rate during the discharge process, the required output temperature and average melting rate can be achieved in practical applications. The developed ice storage system can be easily adapted to a variety of cooling applications.

Experimental study of the effective thermal conductivity in the near-wall region of a packed pebble bed

Abstract The effective thermal conductivity is an important parameter that represents the overall heat transfer in packed beds, including packed pebble beds. Experimental test facilities have been used to determine the effective thermal conductivity of packed pebble beds; however, the experimental results are often case specific and do not account for wall effects in the near-wall region. Recently the Near-Wall Thermal Conductivity Test Facility (NWTCTF) was developed at the North-West University in Potchefstroom, South Africa to perform detailed investigations of the conduction and radiation heat transfer phenomena in the near-wall region of a packed bed of spheres. The experimental test facility was introduced in a previous paper and some results were presented. This paper provides a short description of the test facility and then presents extensive experimental results obtained for structured and randomly packed beds consisting of 60 mm diameter graphite spheres at temperatures of up to 800 °C, together with associated uncertainties. Results obtained demonstrate the effect of the near-wall region, the packing structure and the presence of the wall on the effective thermal conductivity.

A comprehensive study on source terms in irradiated graphite spheres of HTR-10

With previously developed experimental methods which include the preparation and measurement process for the graphite sample, two new irradiated graphite spheres with surface γ dose rates of 51.00 μSv/h and 0.14 μSv/h from the reactor core of the 10 MW high temperature gas-cooled reactor (HTR-10) have been investigated experimentally. The total β counting rate, the β spectra and the γ spectra for each graphite sample of irradiated graphite spheres were recorded with a total α/β counting measuring apparatus, a liquid scintillation counter and a high-purity germanium detector connected to a multichannel analyzer, respectively. Combined with previous experimental data of two irradiated graphite spheres with surface γ dose rates of 25.10 μSv/h and 1.17 μSv/h, the types of key nuclides in the irradiated graphite sphere of HTR-10 were determined, which were H-3, C-14, Co-60, Cs-137, Eu-152 and Eu-154. The distributions for each nuclide in four irradiated graphite spheres were compared. The generation mechanisms of H-3, C-14, Co-60, Cs-137, Eu-152 and Eu-154 in the irradiated graphite sphere of HTR-10 were discussed and analyzed. Based on all the experimental data regarding impurities and uranium contamination in the matrix graphite of HTR-10 available, a sensitivity analysis was performed to explain the effect of impurities and uranium contamination on the specific activity of key nuclides in the graphite sphere. The influence of the neutron flux and the dwell time in the core on the specific activity of key nuclides was also considered. The differences of experimental specific activities among these irradiated graphite spheres were compared and explained. Current comprehensive studies on irradiated graphite spheres of HTR-10 can provide valuable information for the source term analysis, waste minimization and radiation protection of high temperature gas-cooled reactors (HTGRs).

Hollow graphite spheres embedded in porous amorphous carbon matrices as lightweight and low-frequency microwave absorbing material through modulating dielectric loss

Relative high filling ratio and poor reflection loss performance in low frequency are two major challenges in promoting microwave response of current carbon material which could be overcome by rational structure engineering. Herein, hollow graphite spheres embedded in porous amorphous carbon matrix has been prepared by etching Fe3Ni/C composites derived from Fe2Ni MIL-88 nanorods. Construction of conductive network and intrinsic high electrical conductivity contribute to the decrease of percolation threshold. Reflection loss performance in Ku and X band has been ameliorated by modulated dielectric loss mechanisms. Conduction loss models are established to verify the role of Fe3Ni nanoparticles, hollow graphite spheres and amorphous carbon matrix. Polarization modes have been tuned by interface strategy which leads to varied interfacial polarization loss in distinct frequency. Dipole polarization loss has also been controlled by strengthening or weakening interaction which can be influenced by filling ratio. As a result, with a low filling ratio of 10 wt%, effective absorption can be achieved in 12.76–18 GHz and 8.24–12 GHz. It is believed that this work would lay a foundation to design lightweight carbon-based microwave response materials in a wide frequency range.

Revisiting Models for Spheroidal Graphite Growth

Recent experiments resolved nucleation and growth of graphite during solidification of ductile cast iron in 4D using synchrotron X-ray tomography. A numerical model for microstructure formation during solidification is compared with the experiments. Despite very good overall agreement between observations of spheroidal graphite growth and model results, significant deviations exist towards the end of solidification. We use the experimental observations to analyse the relation between graphite growth rate and the state of the particle neighbourhood to pinpoint possible links between growth rate of individual graphite spheres and the overall solidification state. With this insight we revisit existing models for growth of spheroidal graphite and discuss possible modifications in order to correctly describe the critical final stage of solidification.

Research on the fuel loading patterns of the initial core in Chinese pebble-bed reactor HTR-PM

Modular high-temperature gas-cooled reactor (HTGR) is a kind of safe and advanced nuclear energy system, which can provide electric power and high-temperature process-heat efficiently. At present, a 200 MWe demonstration plant, named high-temperature gas-cooled reactor – pebble-bed module (HTR-PM), is being built in China and is planned to be critical in one or two years. In this paper, the physical consideration on the fuel loading patterns of the initial core of HTR-PM is given, and the initial core of HTR-PM is proposed to be made up of low-enrichment fuel spheres and graphite spheres. Then several representative schemes are simulated using VSOP code. The relationship between the key physical parameters (fuel enrichment, flux distribution, temperature coefficient, power per fuel sphere, charging rate) and the volume fraction of fuel spheres in the initial core is analyzed. As a result, the feasible region of the volume fraction of fuel spheres in the initial core is determined as [5/15, 11/15]. The work of this paper can provide support for the fuel management of HTR-PM in the future.

Experimental study to estimate the surface wear of nuclear graphite in HTR-PM

The aim of this study was to simulate the surface wear of graphite components in a pebble-bed nuclear reactor caused by the moving fuel balls and, thus, to help evaluate the integrity of the whole graphite structures over their service life. To this end, wear tests were conducted on the chewing machine with a specially designed load jig. The test specimens included graphite plates and graphite spheres, to represent the graphite bricks and fuel spheres, respectively. The main parameters, including volume loss of graphite plates and spheres, micro-morphology of the wear surface and weight and particle size of graphite dust generated, were studied in this work.

Research on microstructure and oxidation resistant property of ZrSi2-SiC/SiC coating on HTR graphite spheres

ZrSi2-SiC/SiC coating was prepared on the surface of high temperature gas-cooled reactor (HTR) matrix graphite spheres by two-step pack cementation and sintering process. The microstructure, oxidation resistance and thermal shock resistance properties of the as-prepared coatings with different original powder mixtures were investigated. Results show that dense microstructure of the ZrSi2-SiC/SiC coating and continuous ZrSiO4-SiO2-ZrO2 glass phase generated during the oxidation process were the key factors for the outstanding thermal properties. When the mole ratio of Zr:Si:C reaches 1:7:3 in the second pack cementation powders, the coated graphite spheres have optimum oxidation resistant ability. The weight gain is only 0.6wt% after 15 times thermal shock tests and 0.12wt% after isothermal oxidation test at 1500°C for 20h in air. The oxidation resistant mechanism of the coating was also discussed. The dense inner SiC layer and the outer glass layer generated during the oxidation process could protect the ZrSi2-SiC/SiC coating from further oxidation.

Microwave Deposition of Palladium Catalysts on Graphite Spheres and Reduced Graphene Oxide Sheets for Electrochemical Glucose Sensing.

This work outlines a synthetic strategy inducing the microwave-assisted synthesis of palladium (Pd) nanocrystals on a graphite sphere (GS) and reduced graphene oxide (rGO) supports, forming the Pd catalysts for non-enzymatic glucose oxidation reaction (GOR). The pulse microwave approach takes a short period (i.e., 10 min) to fast synthesize Pd nanocrystals onto a carbon support at 150 °C. The selection of carbon support plays a crucial role in affecting Pd particle size and dispersion uniformity. The robust design of Pd-rGO catalyst electrode displays an enhanced electrocatalytic activity and sensitivity toward GOR. The enhanced performance is mainly attributed to the synergetic effect that combines small crystalline size and two-dimensional conductive support, imparting high accessibility to non-enzymatic GOR. The rGO sheets serve as a conductive scaffold, capable of fast conducting electron. The linear plot of current response versus glucose concentration exhibits good correlations within the range of 1–12 mM. The sensitivity of the Pd-rGO catalyst is significantly enhanced by 3.7 times, as compared to the Pd-GS catalyst. Accordingly, the Pd-rGO catalyst electrode can be considered as a potential candidate for non-enzymatic glucose biosensor.

Experimental study on the content and distribution of key nuclides in an irradiated graphite sphere of HTR-10

An irradiated graphite sphere of the 10MW high temperature gas-cooled reactor (HTR-10) has been studied experimentally with a mechanical disintegration way and a suite of radiochemical analysis methods, including the total α/β counting measurement and the absolute activity measurement with a liquid scintillation counter and a high-purity germanium detector connected to a multichannel analyzer. The key nuclides in this irradiated graphite sphere were determined, which were H-3, C-14, Co-60, and Cs-137. The radial distributions of the specific activity for each nuclide were obtained experimentally and compared with theoretical calculations. The production mechanisms of H-3, C-14, Co-60, and Cs-137 in the irradiated graphite sphere of HTR-10 were discussed individually, and the generation sources for each nuclide were analyzed explicitly. Current results can supply important experimental data for source term analysis of HTR-10, and provide useful research reference for the decommissioning work and waste minimization of high temperature gas-cooled reactors (HTGRs).

Electrochemical Graphitization: An Efficient Conversion of Amorphous Carbons to Nanostructured Graphites.

This concept paper describes a new electrochemical method for the graphitization of amorphous carbons. The graphitization is achieved by a simple cathodic polarization of the carbons at merely ≈1100 K in molten CaCl2 . This electrochemical process is applicable to both graphitizable and non-graphitizable carbons, generating porous graphitic structures of high-crystallinity nanosheets. The transformation of carbon blacks, microspheres, and fibers to porous graphites, hollow graphite spheres, and pipes with porous shells have been demonstrated and the potential graphitization mechanism discussed.

Carbon-coated isotropic natural graphite spheres as anode material for lithium-ion batteries

Abstract Carbon coated isotropic natural graphite spheres (INGS/C) has been prepared by spray granulation using the by-product of spherical natural graphite (i.e., superfine graphite powders) as starting materials. In INGS/C, small graphite flakes are packed in disordered orientation and form an isotropic structure, which may effectively reduce local stress concentration appearing in the process of Li + insertion/extraction. The pyrolytic carbon decomposed from polyvinyl alcohol (PVA) and citric acid is connecting the small graphite flakes and coating on the surface of INGS, respectively, thus forms a continuous conductive network to improve the electron transmission and maintain the micro-structural integrity of isotropic natural graphite spheres as well. With the help of synergetic effect between the isotropic structure of INGS and the continuous conductive network, the INGS/C electrode exhibits superior rate capability of 346 mA h g −1 at 3 C at low potential below 0.25 V.

Dual layer SiC coating on matrix graphite sphere prepared by pack cementation and fluidized‐bed chemical vapor deposition

AbstractA dual layer silicon carbide (SiC) coating including inner porous SiC (p‐SiC) layer and outer dense SiC (d‐SiC) layer was fabricated on the matrix graphite (MG) spheres of high‐temperature gas‐cooled reactor fuel elements by pack cementation and fluidized‐bed chemical vapor deposition process to improve the oxidation‐resistant property. Microstructure of the coating demonstrates different density and structure of the two SiC layers with no obvious boundaries between them. Weight gain curves of oxidation tests at 1773 K for 200 hours show that the coating could effectively protected the MG sphere by isolating the air infiltration with p‐SiC layer as the main functional layer and d‐SiC layer as the transition layer to improve the bond strength. Due to the transition function of p‐SiC layer, the coated spheres could understand more than 50 times thermal shocking tests from 1773 K to room temperature with no stress cracking.