Graphite Blocks Research Articles

Decrease in the activity of irradiated graphite and liquid radioactive waste

Nowadays, the problem of nuclear waste deactivation is very urgent. The method proposed consists of exposing the aqueous solution of radionuclide to powerful nanosecond electromagnetic pulses by placing the treated solution between two electrodes connected to the outlets of the generator. Single-pole current pulses with a length of 1 ns, an amplitude of more than 5 kV, and a repetition frequency of 1 kHz are used. Such pulses can be obtained from the generator units with an amplitude of 5 kV to 15 kV. They have a pulse capacity of 1 to 4 MW, and the mains capacity is below 50 W. The pulse action leads to radiolysis of water and accelerated decay of radionuclides. As a result of radiolysis, hydrated electrons, hydrogen atoms, and various radicals are formed that causes precipitation of radionuclides, such as 137Cs and 90Sr. The description of several experiments and results of experimental investigations of the influence of powerful nanosecond electromagnetic pulses on the properties of water solutions with radioactive nuclides 137Cs and 90Sr and on items of irradiated graphite is presented. Under the influence of pulses on real liquid waste and aqueous solutions, radioactive nuclides in water are reduced by 5 to 50 times during 15 minutes. The effect described is preserved for 20 days. The reduction of beta activity twice during 25 min is observed in big blocks of the irradiated graphite. Flow-through unit for processing of liquid waste is developed, tested, and patented (RU 2726145. MPK G21F 9/28, July 7, 2020). This technology can be used to treat emergency solutions with tritium at Fukushima NPP. of the irradiated graphite.

Oxidation protective coating of Y2O3-Na2SixO2x+1 composite on graphite crucible for high temperature applications

Oxidation and corrosion protective coatings on graphite are essential for its high temperature applications. In the present work, oxidation protective coating on graphite was achieved with a combination of Y2O3 and SiO2 suspension with NaOH in methanol. Study showed that the optimized composition of the desired coating was Y2O3–15 wt% SiO2 (i.e., 2 g mixture of Y2O3 and SiO2 powder containing 0.3 g of SiO2 which is 15% of the total powder content if expressed in weight) with 5 M NaOH in 10 mL methanol. This protective coating on graphite was stable up to 1473 K for 30 min in Ar ambient before visible cracks appeared. The compositions of SiO2: NaOH were varied (5 mmol:5 M, 10 mmol:5 M and 5 mmol:10 M) to examine the roles of these two components in the overall adhesiveness of the coating. SiO2 and NaOH acted as inorganic binders and formed in-situ intermediate layers during sintering which held Y2O3 layer with the graphite surface. The coated graphite blocks and crucibles were characterized by X-ray diffractometer (XRD), field emission scanning electron microscope (FESEM), energy dispersive X-ray spectroscopy (EDS), differential thermal analysis and thermogravimetric analysis (DTA/TGA) and Fourier transform infrared spectroscopy (FTIR). The oxidation protective nature of the coating was tested by heating the coated crucible at 1273 K for 1-15 h in air atmosphere. The effective adhesion of oxidative coating on graphite was explained.

Study of the Molecular-Weight Distribution of Binder Pitches for Carbon Blocks

The present study aimed to identify the required characteristics of binder pitches in the filler-binder mixing process to effectively manufacture graphite blocks. To this end, a binder pitch was separated into pitch fractions of varying molecular-weight segments. The role and effectiveness of each pitch fraction were then analyzed with respect to their molecular-weight distribution. As a result, the optimal molecular-weight distribution was determined. More specifically, a coal-tar pitch was separated into solvent-soluble and solvent-insoluble fractions. The molecular-weight distribution was determined according to this classification, and the characteristics of each pitch fraction were examined. The pitch separation process was conducted using three solvents: hexane, toluene, and quinoline. The resulting pitch was separated into the following pitch fractions: hexane-soluble (HS), hexane-insoluble-toluene-soluble (HI-TS), toluene-insoluble-quinoline-soluble (TI-QS), and quinoline-insoluble (QI). Fourier transform infrared (FT-IR) spectrum, matrix-assisted laser desorption ionization-time of flight (MALDI-TOF), and softening point of each pitch fraction were measured. Also, pitch samples were refabricated while varying the mixing ratio of these pitch fractions, and carbon blocks were then prepared using them. The compressive strength and porosity of these blocks were measured and compared. The P154_B pitch with a high content of TI-QS was used to fabricate a green block. Due to the high viscosity of the binder used, the fluidity was not sufficiently high, and thus, the green block made of this pitch had relatively low strength. The other blocks had similar levels of strength. After the carbonization process, the carbon block with a high content of HS (P352_B-C) and the carbon block with the HS content removed (P073_B-C) showed lower compressive strength than their respective green-block counterparts (P352_B and P073_B). However, their strength was higher compared to those of the other carbon blocks. In the case of carbon block P073_B-C, the HS content was completely removed, and thus, the content of TI-QS (β-resin) was relatively high. Accordingly, this carbon block ended up with large amounts of components that had high coking values (CVs), and this contributed to limiting the formation of pores. Therefore, the compressive strength of this carbon block was high. In the case of the carbon block with a high content of HS (P352_B-C), a suitable level of viscosity was achieved because the HS components ensured high fluidity. As a result, blocks with higher density and compressive strength could be fabricated. The major findings of the present study confirm that producing carbon blocks with high mechanical properties requires binder pitches with a balanced combination of suitable viscosity to ensure sufficiently high fluidity and a proper level of CV to effectively suppress the formation of pores in the mixing and molding process.

Numerical Simulation Study on Failure of Graphite Heat Exchanger with Orifice Block

Phenolic impregnated graphite is a new kind of material with higher corrosion resistance and thermal conductivity. In the chemical industry, it is often used to make containers and pipes containing strong acid and alkali medium. It is the characteristic of phenolic resin impregnated graphite heat exchanger that the corrosion resistance and heat transfer efficiency of the heat exchanger are greatly improved. In this paper, a failure case of graphite heat exchanger in a chemical enterprise in Shanghai was analyzed by numerical simulation. According to the actual working conditions, the temperature field and flow field in the porous graphite heat exchanger were numerically simulated. The numerical simulation results of temperature field showed that the graphite block would be subject to thermal shock when the equipment was started and shut down, resulting in the increase of temperature difference of graphite block In addition, the pre tightening force of the flange of graphite heat exchanger was simulated by numerical simulation, and it was found that the position of stress concentration coincided with the crack position of actual failure, and the improvement suggestions were put forward for the structural design of graphite block.

Coupled Computational Fluid Dynamics–Discrete Element Method Study of Bypass Flows in a Pebble Bed Reactor

In the high-temperature reactor design, it is common practice to leave gaps between the graphite blocks of the reflectors to accommodate thermal dilatation and material swelling, as well as to provide an additional cooling source during operation. These gaps give rise to bypass flows entering the reactor core. The bypass flows can change friction factors and heat exchange coefficients obtained in the bulk of the pebble bed. In this paper, a coupled computational fluid dynamics–discrete element method (CFD-DEM) model is proposed. In this model, the pebbles are resolved by the CFD grid and the turbulent field is partially captured using a detached eddy simulation method. The DEM model is first validated against empirical correlations for the packing of the pebbles, and the coupled model is then tested against thermal measurements in the SANA experiment. Then the model is used to perform three-dimensional studies of the effects of the bypass flows in a representative pebble bed configuration. It is determined that the effect of cross flows can be approximately bounded to the first two layers of pebbles next to the reflector wall. Additionally, an increase of ~12% in the Nusselt number in the pebbles next to the reflector is predicted, with a maximum local increase in the pebble of ~100%.

Reconstructing the conductivity profile of a graphite block using inductance spectroscopy with data-driven techniques

The radial depth profile of the electrical conductivity of the graphite channels in the UK's advanced gas-cooled reactors (AGRs) can be reconstructed and estimated by solving a non-linear optimisation problem using the mutual inductance spectra of a set of coils. This process is slow, as it requires many iterations of a forward solver. Alternatively, a data-driven approach can be used to provide an initial estimate for the optimisation algorithm, reducing the amount of time it takes to solve the ill-posed inverse problem. Two data-driven approaches are compared: multi-variable polynomial regression (MVPR) and a convolutional neural network (CNN). The training data are generated using a finite element (FE) model and superimposed on a noise floor in the interval [20, 60] dB of the weakest amplitude point in the corresponding spectrum. A total of 5000 simulated datasets are generated for training. The results on smoothed test data show that the two models have a comparable mean percentage error norm of 17.8% for the convolutional neural network and 17.3% for multivariable polynomial regression. A further 500 unsmoothed profiles are tested in order to assess the performance of each algorithm on conductivity distributions where the conductivity of each layer is independent of another. The performance of both algorithms is then assessed on reactor-type test data. The results show that the two data-driven algorithms have a comparable performance when estimating the electrical conductivity depth profile of a typical reactor-type distribution, as well as vast deviations. More generally, it is thought that data-driven approaches for depth profiling of some electromagnetic quantity have the potential to be applied to other ill-posed inverse problems where speed is a priority.

Failure case study on porous block impregnated graphite heat exchanger

Phenolic impregnated graphite is a new kind of material with higher corrosion resistance and thermal conductivity. In the chemical industry, it is often used to make containers and pipes containing strong acid and alkali medium. In this paper, a failure case of graphite heat exchanger in a chemical enterprise in Shanghai was studied. The failure reason of the graphite block was tested by means of three-dimensional observation, thermogravimetry and scanning electron microscopy. The results show that the particle fineness of graphite block and the operating conditions of the graphite heat exchanger are the main reasons for the failure of the graphite heat exchanger. At last, some suggestions are put forward for the safe use of this kind of graphite heat exchanger.

Properties of graphite blocks consisting of ordered graphite flakes with bimodal particle size distribution

AbstractPetroleum pitch‐based graphite blocks (GBs) attract considerable attention owing to their low cost, high yield, and possible application as heat sink components. However, no studies have systematically investigated the particle size distribution of GBs, their structures, and the relationship between the structures and thermal properties resulting from graphitization. Hence, herein, we present a novel method for fabricating petroleum pitch‐based GBs using natural graphite flakes (GFs) of sizes ranging between 50 and 500 μm and a bimodal particle size distribution. Furthermore, the effects of the mean GF particle size, pitch binder content, and microstructure of the GBs with low GF contents on compression molding with elevated hot‐pressing temperatures after graphitization are investigated. To investigate the effect of packing density with a GF size of 500 μm, we mix a small‐sized GF (50 μm) and low pitch binder content by kneading. In particular, specimens in the in‐plane direction with a bimodal particle size distribution of GFs and 10 wt% pitch binder yield optimum density and thermal conductivity values of 1.74 g m−3 and 384 Wm−1 K−1, respectively, after high‐temperature graphitization at 2600°C. These findings provide insights for developing high‐performance GBs for use in heat sink components with high thermal conductivity.

Effect of Particle Orientation and Porosity on Thermal Conductivity of Petroleum Pitch Polymer-Based Carbon Molded Body

The present study was conducted to investigate changes in the thermal conductivity of petroleum pitch-based carbon molded bodies prepared by anisotropic (uniaxial) molding under different molding pressures. The carbon molded bodies were prepared using needle coke and petroleum-based binder pitch polymers (softening point: 150 ℃). Green blocks prepared under high molding pressure showed a higher particle orientation value up to 16.4 μm. Graphite blocks, prepared by graphitizing the green blocks at 2800 ℃ showed a similar trend. The pores in the carbon molded body were filled with low boiling point substances, generated by the thermal treatment of the binder pitch polymer or air that could not be discharged during the molding procedure. Therefore, when phonons encountered a pore, phonon scattering, rather than phonon transport, occurred, and thus the heat transport from the hot zone to a cold zone became slow. As a result, although the particle orientation was a little higher in the B_10-G sample than in the B_20-G sample (in the error range), the thermal conductivity was higher in the B_20-G sample, which may be because the B_10-G sample had a higher porosity than the B_20-G sample.

Effect of the average grain size of green pitch coke on the microstructure and properties of self-sintered graphite blocks

Effect of the average grain size of green pitch coke on the microstructure and properties of self-sintered graphite blocks

Recent Developments in Graphene and Graphene Oxide: Properties, Synthesis, and Modifications: A Review

AbstractGraphene was first discovered as a sheet structure mechanically exfoliated from a block of graphite, but in recent years researchers have extended their investigations into this two‐dimensional carbon nanostructure. Various applications of graphene‐based materials have included electronics, photonics, optoelectronics, sensors, and drug / gene delivery systems. These single atom carbon layers have the potential to be formed in different morphologies e. g., quantum dots, nanosheets, and nanoparticles, which can be tailored to achieve new breakthrough innovations. Nowadays, the utilization of graphene‐based nanomaterials in medicine is a hot research topic. This review discusses the structure, properties, methods of synthesis, and surface modifications of graphene and graphene oxide divided into covalent and non‐covalent approaches.

The Control of Volume Expansion and Porosity in Carbon Block by Carbon Black (CB) Addition for Increasing Thermal Conductivity

The graphite block as a phase change materials (PCMs) was manufactured by graphitization of a carbon block. Carbon blocks were prepared by filler (cokes or graphite) and binder (pitch). The binder-coated filler was thermally treated for carbonization. The gases generated from the evaporation of low molecular weight components in the binder pitch during the carbonization process were not released to the outside. Consequently, porosity and volume expansion were increased in artificial graphite, and thereby the thermal conductivity decreased. In this study, to prevent the decrease of thermal conductivity in the artificial graphite due to the disadvantages of binder pitch, the carbon block was prepared by the addition of carbon black, which can absorb low molecular weight compounds and release the generated gas. The properties of the prepared carbon blocks were analyzed by SEM, TGA, and thermal conductivity. The addition of carbon black (CB) decreased the porosity and volume expansion of the carbon blocks by 38.3% and 65.9%, respectively, and increased the thermal conductivity by 57.1%. The CB absorbed the low molecular weight compounds of binder pitch and induced the release of generated gases during the carbonization process to decrease porosity, and the thermal conductivity of the carbon block increased.

Graphite block derived from natural graphite with bimodal particle size distribution

This paper presents a novel method for the bimodal design of graphite blocks by optimizing the weight ratio of small-sized natural graphite (NG) particles (mean size of 5 μm) and large-sized NG particles (mean size of 500 μm). Graphite blocks were fabricated by subjecting a mixture containing large NG particles and a well-dispersed pitch/small-sized NG mixture (PSM) to pressure-mold heat treatment. The effect of the PSM contents on the structural, thermal, and mechanical properties of the graphite blocks was investigated. A correlation was found between the amount of small-sized NG particles, the micro-pore structure, and the distribution of large-sized NG particles. In particular, the graphite block, with small-sized NG particle content and pitch binder content of 21 wt% and 9 wt%, respectively, achieved maximum thermal conductivity and flexural strength values of 460 W m K−1 and 14 MPa, respectively, after pressure-mold heat treatment (graphitization) at 2600 °C. Our research advances the development of an eco-friendly solvent-free graphite block fabrication method that produces high-performance graphite blocks of varying size faster and more cost-effectively.

Research on Influence of Different Simulation Methods of Bypass Flow in Thermal Hydraulic Analysis on Temperature Distribution in HTR-10

In pebble-bed high temperature gas-cooled reactor, gaps widely exist between graphite blocks and carbon bricks in the reactor core vessel. The bypass helium flowing through the gaps affects the flow distribution of the core and weakens the effective cooling of the core by helium, which in turn affects the temperature distribution and the safety features of the reactor. In this paper, the thermal hydraulic analysis models of HTR-10 with bypass flow channels simulated at different positions are designed based on the flow distribution scheme of the original core models and combined with the actual position of the core bypass flow. The results show that the bypass coolant flowing through the reflectors enhances the heat transfer of the nearby components efficiently. The temperature of the side reflectors and the carbon bricks is much lower with more side bypass coolant. The temperature distribution of the central region in the pebble bed is affected by the bypass flow positions slightly, while that of the peripheral area is affected significantly. The maximum temperature of the helium, the surface, and center of the fuel elements rises as the bypass flow ratio becomes larger, while the temperature difference between them almost keeps constant. When the flow ratio of each part keeps constant, the maximum temperature almost does not change with different bypass flow positions.

Modeling and comparative study of heat exchangers fouling in phosphoric acid concentration plant using experimental data

Fouling still remains one of the most difficult problems for the use of heat exchangers. A methodological process of advanced analysis of experimental data on heat exchangers fouling allowing building predictive models is necessary to determine the fouling degree. Here, three different methods were used to predict the fouling resistance from some easily measurable variables of the system which are: Kern and Seaton, Partial Least Squares (PLS) and Artificial Neural Networks (ANN). Indeed, the fouling resistance was estimated according to the inlet and outlet temperature of the cold fluid, the temperature of the hot fluid, the density and the volume flow rate of the cold fluid and time for three types of heat exchangers, i.e. tubular stainless-steel and graphite blocks (Supplier (A) and Supplier (B)).The best modeling was determined by maximizing certain statistical accuracy indices. Results show that modeling by the use of Artificial Neural Networks is very performing compared with modeling by Partial Least Squares regression and Kern and Seaton. One of the key features of ANN model is their small levels of error in comparison with other models.

Wall resolved large eddy simulation of reactor core flows with the spectral element method

Numerical simulation is an intrinsic part of nuclear engineering research supporting the design of nuclear power plants. With the advent of Petascale computing (i.e., computers capable of more than 1 PFlop), the simulation of portions of reactor components with turbulence-resolving techniques is now possible. These simulations can provide invaluable insight into the flow dynamics, which is difficult or often impossible to obtain with experiments alone. The spectral element method in particular has emerged as a powerful method to deliver massively parallel calculations at high fidelity by using Large Eddy Simulation or Direct Numerical Simulation. In this work we review the fundamentals of the method and the reasons it is compelling for the simulation of nuclear reactor core flows. We review a series of wall-resolved large eddy simulations of reactor cores applied to three reactor types. In particular we discuss the simulation of the flow in a 17 × 17 rod bundle related to light water reactor (LWR) applications; the flow in a 37-pin wire-wrapped rod bundle related to liquid metal reactor (LMR) applications and the flow through a graphite block with application to molten salt reactors (MSR).

Heat exchangers fouling in phosphoric acid concentration

Industrial heat exchanger fouling is generally defined as the accumulation and formation of undesired materials on the exchange surface. Experimental measurements of degree of fouling are both difficult and time consuming and often do not provide accurate results. However, the modeling of the thermal resistance of fouling can inform us on the evolution of this phenomenon. The present work aimed at studying fouling three types of heat exchangers, namely stainless-steel tubular and two graphite blocks from different suppliers(A and B), in an industrial phosphoric acid concentration unit belonging to the phosphoric acid production plant of the Tunisian Chemical Group (GCT) using the measurements of the operating parameters over a period of two years. The overall heat transfer coefficients and fouling resistances were evaluated at different times using the experimental data. Our results show that the resistance of fouling reach a maximal value ranging between 1.38*10−4 and 2.55*10−4 m2. K.W−1 according to the type of the heat exchanger. Therefore, the experimental data of fouling resistances were compared with Kern and Seaton predictive model. Quantitative and qualitative agreement between measured and predicted fouling resistances is good with the coefficient of determination (R2) varying between 0.97 and 0.98.

Effect of the average grain size of green pitch coke on the microstructure and properties of self-sintered graphite blocks

Three self-sintered graphite blocks (SGs) were prepared from green pitch cokes with different average grain sizes (5.1, 3.1 and 1.9 μm) using cold isostatic compaction. The microstructures and properties of the SGs were characterized by XRD, SEM, mercury porosimetry, mechanical and thermal tests. Results indicated that the microstructures and properties of the SGs were closely related to the green coke granularity. When decreasing the average grain size of the green coke from 5.1 to 1.9 μm, the density, mechanical properties, Shore hardness and coefficient of thermal expansion increased while the degree of graphitization, crystallite diameter, thermal conductivity, median pore diameter and porosity decreased. The median pore diameters of the SGs were all in the nanometer range, which implied that they could effectively inhibit the infiltration of molten salt and might be used in molten salt reactors.

РАСЧЁТ БИОЛОГИЧЕСКОЙ ЗАЩИТЫ ТРАНСПОРТНОГО КОНТЕЙНЕРА ДЛЯ ОБЛУЧЁННОГО ГРАФИТОВОГО ЗАМЕДЛИТЕЛЯ ЯДЕРНЫХ РЕАКТОРОВ ТИПА РБМК-1000

In given paper is discussed a theme of the providing of the biological shield of a container which is intended for exportation from NPP spots the bloks of the irradiated graphite moderator of nuclear reactors RBMK-1000 at a stage of such reactors decommission. With usage of the computer program systems Microshield (MS) and MCC 3D has been performed a design of container’s biological shield. This container can be used as for graphite blocks transportation, as for their storage. Year 2018 was a final one for the first unit of Leyningradskay NPP with reactor RBMK-1000, which has been under operation from 1973. The stop of this unit laid the beginning of step by step process of decommissions of such type power reactors at our country territory. For power reactors, in which as moderator of neutrons is used graphite, at ending of their service time is necessary to remove it from NPP stop and to keep(store) it safely-before reprocessing on final disposal. For the transportation and for the safe storage of graphite blocks (GB) of irradiated graphite moderator (IGM) it is necessary to develop special container, which will provide proper level of a management with IGM at the spot of decommissioning unit, at a way to spot of IGM reprocessing or/and its final disposal. The structure of container should to satisfy to requirements of radiation protection, providing the necessary level of a biological shield and also should to have proper strength characteristics and ability to bear supposing dynamic loads without loss of integrity and without loss of hermetically. The practical meaning of given work consists in the possibility of usage of its results by designing and manufacturing of containers for transportation and storage of IGM. The usage of such containers is actual by decommission process of nuclear power units with reactor RBMK-1000, because such usage will allow to accelerate significantly this process and fulfill the requirements of the ecological safety. The usage in given work of the modern program systems of modeling of biological shield allows to evaluate numerically shielding possibility of the structure materials, which could be used for the biological shield of containers and allows to choose optimal option of loading of graphite blocks in container. In the work is presented information about positive properties of the high-strength cast iron with spherical graphite inclusion as a perspective structural material for container’s body, which able to provide high level of the ecological safety during the exploitation of containers for irradiated graphite blocks.

Electronic and structural relations between solid CaB6 and the molecular dianion B6H6(2-): A computational study

By means of the isolobality concept between a hydrogen atom and the radical dianion [B6H5• ]2− derived from an extraction of a hydrogen atom in the dianion [B6H6]2−, forming part of the K2(B6H6) structure, we rationalize the 3D boron skeleton architecture in the solids CaB6 and KB6. The above statement is supported by electronic structure computations based on the Electron Localisation Function (ELF) in the dianion [B6H6]2− and radical anion [B6H5•]2−. Valence-Bond (VB) theory is also used in the study of the B–B sigma bond connecting the octahedra in the CaB6 structure, with the model dimer [(H5B5)B–B(B5H5)]4−. Quantum-chemical geometry optimizations of clusters extracted from the CaB6 solid structure show the analogies between molecular chemistry and solid-state chemistry: In the same way as benzene is the building block of graphene and graphite, the anion [B6H6]2− is the building block of CaB6 provided every hydrogen atom in [B6H6]2− is connected to a further [(B•)6]2− octahedron in the three directions of the 3D space, and every void at the Ca positions is embedded with two electrons.