Gap Conductance Research Articles

Theoretical Study of (CO) n=1, 2 Adsorption on the (6,0) Zigzag Single-walled Carbon Nanotube

The behavior of the carbon monoxide and carbon dioxide [(CO) n=1, 2] adsorbed on the external surface of (6,0) zigzag single-walled carbon nanotube (SWCNT) was studied by using density functional calculations. Parallel and perpendicular bonding geometries of CO and CO2 adsorbed on the (6,0) CNT surface were considered. Geometry optimizations were carried out at the B3LYP/6–31G* level of theory using the Gaussian 98 suite of programs. We present the nature of the (CO) n=1, 2 interaction in selected sites of the nanotube. Our results show that the pristine carbon nanotubes cannot significantly detect (CO) n=1, 2 molecules. The calculated binding energy for CO in O-down configuration is higher than that in C-down, and the values of dipole moments for the most stable configuration of CNT-CO showed slight changes with respect to the pristine models. The calculated results showed that the C–O bond lengths remain almost unchanged in the most stable configuration of (CO)n=1, 2 –CNT. The calculated BE for CO2 in perpendicular orientation is higher than that in parallel orientation. Furthermore, the adsorption of CO and CO2 on the CNT are not changed the band gap and electrical conductance of the pristine CNT. The DOS and HOMO and LUMO for the CNT models showed that the pristine CNT cannot significantly detect CO and CO2 molecules.

Influence of Edge Defects, Vacancies, and Potential Fluctuations on Transport Properties of Extremely Scaled Graphene Nanoribbons

Atomistic quantum transport simulations of a large ensemble of devices are employed to investigate the impact of different sources of disorder on the transport properties of extremely scaled (length of 10 nm and width of 1–4 nm) graphene nanoribbons. We report the dependence of the transport gap, on- and off-state conductances, and on–off ratio on edge-defect density, vacancy density, and potential fluctuation amplitude. For the smallest devices and realistic lattice defect densities, the transport gap increases by up to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\sim$</tex></formula> 300%, and the on–off ratio reaches almost <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\sim\!\!\hbox{10}^{6}$</tex> </formula> . We also report a rather high variation of the transport gap and on–off ratio. In contrast, we find that the potential fluctuations have a negligible impact on the transport gap and cause a relatively modest increase of the on–off ratio.

Optimizing a gap conductance model applicable to VVER-1000 thermal–hydraulic model

The modeling of gap conductance for application in VVER-1000 thermal–hydraulic codes is addressed. Two known models, namely CALZA-BINI and RELAP5 gap conductance models, are examined. By externally linking of gap conductance models and COBRA-EN thermal hydraulic code, the acceptable range of each model is specified. The result of each gap conductance model versus linear heat rate has been compared with FSAR data. A linear heat rate of about 9kW/m is the boundary for optimization process. Since each gap conductance model has its advantages and limitation, the optimized gap conductance model can predict the gap conductance better than each of the two other models individually.

Strain-Induced Conductance Modulation in Graphene Grain Boundary

Grain boundaries (GBs) are ubiquitous in polycrystalline graphene materials obtained by various growth methods. It has been shown previously that considerable electrical transport gap can be opened by grain boundaries. On the other hand, polycrystalline graphene with GBs is an atomically thin membrane that can sustain extraordinary amount of strain. Here, by using atomistic quantum transport numerical simulations, we examine modulation of electrical transport properties of graphene GBs. The results indicate the modulation of transport gap and electrical conductance strongly depends on the topological structure of the GB. The transport gap of certain GBs can be significantly widened by strain, which is useful for improving the on-off ratio in potential transistor applications and for applications as monolayer strain sensors.

Thermal hydraulic performance assessment of dual-cooled annular nuclear fuel for OPR-1000

► A thermal hydraulic performance of a 12 × 12 annular fuel array is evaluated. ► The subchannel analysis code for the dual-cooled annular fuel, MATRA-AF is validated. ► We evaluate the sensitivity for geometry tolerances and operating parameter. ► We decide the essential design parameters to uprate the power generation by dual-cooled annular fuel. ► A thermal margin amount accommodating a 20% power-uprate seems viable. An internally and externally cooled annular fuel was proposed for an advance PWR, which can endure substantial power uprating. KAERI is pursuing the development for a reloading of power uprated annular fuel for the operating PWR reactors of OPR-1000. In this paper, the characteristics and verification of the MATRA-AF are described. The thermal hydraulic performance of a 12 × 12 annular fuel is calculated for the major design parameters and its performance is compared against the reference 16 × 16 cylindrical fuel assembly. In particular, the enhancements of the thermal hydraulic performance of dual-cooled annular fuel are estimated for the 100% normal power reactor core. The purpose of this study is to estimate a normal power for OPR-1000 with dual-cooled annular fuel, and ultimately to assess the feasibility of 120% core power. The parametric study was carried out for the fuel rod dimension, gap conductance, thermal diffusion coefficients, and pressure loss of the spacer grids. As a result of the analysis on the nominal power, annular fuel showed a sufficient margin available on DNB and fuel pellet temperature relative to cylindrical fuel. The margin amount seems accommodating a 20% power-uprate seems viable.

First-principles study of electronic and optical properties of the optical non linear LiMoO3(IO3)

The electro-optical properties, including: energy density of states function, dielectric function, refraction coefficient, extinction coefficient, band structure, energy gap and optical conductance of LiMoO3(IO3) structure with single-crystal data are computed and discussed in this paper. The LDA+U and generalized gradient approximations (GGA) with the full-potential linearized augmented plane wave method (FP-LAPW) in the framework of density functional theory (DFT) are used to compute the energy gap and other properties of this structure by considering the orbital dependent potential for coupled d orbital brought from experimental results and the U is applied to the Mo d-manifold. The results of energy gap are 2.1eV and 1.5eV for LDA+U and for GGA methods, respectively, which LDA+U method result is very close to experimental results.

FRED fuel behaviour code: Main models and analysis of Halden IFA-503.2 tests

The FRED fuel rod code is being developed for thermal and mechanical simulation of fast breeder reactor (FBR) and light-water reactor (LWR) fuel behaviour under base-irradiation and accident conditions. The current version of the code calculates temperature distribution in fuel rods, stress–strain condition of cladding, fuel deformation, fuel-cladding gap conductance, and fuel rod inner pressure. The code was previously evaluated in the frame of two OECD mixed plutonium–uranium oxide (MOX) fuel performance benchmarks and then integrated into PSI's FAST code system to provide the fuel rod temperatures necessary for the neutron kinetics and thermal–hydraulic modules in transient calculations. This paper briefly overviews basic models and material property database of the FRED code used to assess the fuel behaviour under steady-state conditions. In addition, the code was used to simulate the IFA-503.2 tests, performed at the Halden reactor for two PWR and twelve VVER fuel samples under base-irradiation conditions. This paper presents the results of this simulation for two cases using a code-to-data comparison of fuel centreline temperatures, internal gas pressures, and fuel elongations. This comparison has demonstrated that the code adequately describes the important physical mechanisms of the uranium oxide (UOX) fuel rod thermal performance under steady-state conditions. Future activity should be concentrated on improving the model and extending the validation range, especially to the MOX fuel steady-state and transient behaviour.

Thermal Analysis of Metallic Fuel for Future FBRs

Abstract Aconceptual design of metallic fuelfor future Fast Breeder Reactors (FBRs) has been studied. The fuel consists of U-Pu binary alloy cladded in T91 alloy with Zr layer between the fuel slug and cladding material and helium gas in semicircular groove region. Temperature distribution of this fuel has been evaluated for different Linear Heat Rate (LHR) for two types of geometrywith the help of Finite Element Method (FEM). Contact element has been considered between fuel and clad. Contact pressure due to differential thermal expansion between fuel and clad has been evaluated by FEM. Input data required for gap conductance was obtained from various models available for gaseous and solid to solid conductance. Effect of contact pressure and thermal conductivity of gas on gap conduction has been studied. Based on detailed temperature distribution it was found that Limiting value of LHR is first reached by clad eutectic temperature between T91 and U-Pu than fuel centerline temperature based on solidus temperature of fuel.

ICONE19-44013 ROD EJECTION ACCIDENT BY THE COUPLED SYSTEM CODE ATHLET-QUABOX/CUBBOX

The paper considers a Rod Ejection Accident (REA) which has been calculated by the coupled-code system ATHLET-QUABOX/CUBBOX. For the present study, a MOX/UOX mixed core loading was developed on the basis of a generic PWR. The results are particularly focused on the fuel enthalpy rise which is the main safety criterion for such transient. A parametric REA study has been performed, showing the influence of some important thermal-hydraulic and neutron-physical parameters. Simulations have been performed using realistic or artificially decreased delayed neutron fractions for two different core states (HZP and 30% of the nominal power). Effective fuel rod temperature influence (i.e. Doppler coefficient) has been studied by using different correlations (0.5/0.5 weighting factors or the typical T_<Doppler> = 0.7 T_<Surface> + 0.3 T_<Center>) or by changing the fuel gap conductance. It is shown that the maximum enthalpy (and enthalpy increase) does not always appear in the affected fuel assembly but can also appear in the neighboring ones. This result is a direct consequence of the burn up dependence of the enthalpy. The paper also considers the case of local delayed neutron parameters and briefly describes the future REA studies foreseen at GRS such as an investigation of quantitative uncertainty propagation from the nuclear data to the transient behavior.

Three-Dimensional Heat Transfer Analysis of Pin-Bushing System With Oscillatory Motion: Theory and Experiment

A three-dimensional computational thermal contact model is developed. The approach utilizes a combination of the transfer matrix and finite element methods. The frictional heat generated at the contact interface is instantaneously partitioned between the bushing and the shaft. Two methods to couple the heat and temperature at the contact interface are presented. One method automatically accounts for the heat division between contacting bodies by satisfying the heat equilibrium and temperature continuity at interactive surfaces. The other method introduces a fictitious layer between contacting bodies with a specified gap conductance to partition the frictional heat. Application of the model to the heat transfer analysis of journal bearing systems experiencing oscillatory motion is presented. Nonuniformly distributed frictional heat along the axial direction is considered. The model is capable of predicting the transient temperature field for journal bearings. It can also be used to determine the maximum contact temperature, which is difficult to be measured experimentally. Comparison of the simulated resulted along with experimental tests conducted in a laboratory is presented.

Effect of thermal gap conductance for MoO3 ampoules irradiated in a high neutron flux

Abstract During the irradiation process of MoO3 powder with in a high neutron flux, energy deposited in the powder must be released to avoid energy accumulation. The temperature increasing in the powder due to energy accumulation may cause powder phase change and under certain conditions the temperature may reach the melting point. An analytical model was developed to study the effect of gap conductance on the temperature distribution in the powder during the irradiation process. The gap conductance model was studied for helium and nitrogen gases at different pressures to obtain the optimum conditions verifying the safety in heat transfer point of view. It was found that the gap conductance is a function of gas pressure. The model provided the optimum conductivity for the gap filled with helium gas at 1 atm pressure.

FE prediction of residual stresses of investment casting in a Bottom Core Vane under equiaxed cooling

This paper presents an investment casting simulation of the Bottom Core Vane (BCV) component of an aero-engine subjected to equiaxed cooling. The simulation is carried out using the Finite Element Method (FEM). Two FE codes (ABAQUS and ProCAST) are used for performing the simulation and the results from both codes are compared and reviewed. The main goal is to predict the residual stresses in the BCV in a casting simulation. Inconel 718 material is used for the ingot (BCV) and a ceramic material is used for the mould. The gap formation and the gap conductance models, between the ingot and the mould, are implemented into the Finite Element Analyses (FEA). Sensitivity studies of the mechanical calculations in ABAQUS and ProCAST, mould filling, mould materials, convection and friction coefficients are conducted to investigate their influences on the predicted residual stresses.

Thermal gap conductance at low contact pressures (

Thermal contact conductance (TCC) measurements are made on bare and gold plated (⩽0.5 μm) oxygen free high conductivity (OFHC) Cu and brass contacts in vacuum, nitrogen, and argon environments. It is observed that the TCC in gaseous environment is significantly higher than that in vacuum due to the enhanced thermal gap conductance. It is found that for a given contact load and gas pressure, the thermal gap conductance for bare OFHC Cu contacts is higher than that for gold plated contacts. It is due to the difference in the molecular weights of copper and gold, which influences the exchange of kinetic energy between the gas molecules and contact surfaces. Furthermore, the gap conductance is found to increase with increasing thickness of gold plating. Topography measurements and scanning electron microscopy (SEM) analysis of contact surfaces revealed that surfaces become smoother with increasing gold plating thickness, thus resulting in smaller gaps and consequently higher gap conductance.

Encapsulated Thermoelectric Modules and Compliant Pads for Advanced Thermoelectric Systems

The described encapsulated thermoelectric (TE) module consists of a vacuum-tight stainless-steel container with dimensions of 55 mm × 50 mm × 11 mm in which a SiGe or BiTe TE module is placed. This construction enables maximum performance and durability because: (1) the thermal expansion mismatch between the hot and cold sides of the container can be accommodated by a sliding sheet that is present in the container; (2) the TE module inside is always kept in a vacuum environment so that oxidation cannot occur; and (3) the difference in pressure between the inside and outside of the container reduces the thermal contact resistance inside the container. The design also includes a compliant pad made of porous material that is infiltrated with braze filler material. If heated to a temperature above the melting temperature of the braze filler, the thermal gap conductance of the interface is enhanced due to the strong affinity of the braze filler to an adjacent member. It is possible for this interface to slide as long as the braze filler is in the liquid state. This design strategy provides high-flux, direct conduction paths to the heat source and heat sink as well as a large temperature gradient across the TE module itself (1.7 times that in the case of the conventional design strategy); therefore, the module can provide a power output that is three times greater than that of a conventional module.

Evaluation of a Dual-Cooled Annular Fuel Heat Split and Temperature Distribution

The heat flux and fuel temperature of an annular fuel rod was analyzed using a newly developed program, DUO_THERM. The dual-cooled annular fuel rod has both inner and outer coolant channels. The heat fluxes to inner and outer claddings can change during irradiation because of the variation in the gap conductances of the inner and outer sides. The heat flux is a very important design factor affecting fuel integrity and safety. However, in an annular fuel design, the prediction of heat flux is very complicated because it is closely connected with size changes of two gaps, which are different from each other during irradiation. A burnup increase and power change can cause pellet and cladding radial deformation and eventually change the gap width. Because an annular fuel has two gaps at both sides and their heat resistances are very large, an imbalance of the gap width can lead to heat flux asymmetry. Therefore, a pellet and cladding dimensional change including densification, swelling, creep, and thermal expansion must be considered in the calculation of heat flux and temperature. By using the DUO_THERM program, the changes in heat flux and temperature during an operation were evaluated with respect to a reference design of an annular fuel. The results of the evaluation showed that the heat flux of an annular fuel was greatly affected by the inner and outer gap conductance changes. The heat flux of the inner cladding was maximum at zero burnup, but that of the outer cladding was maximum at the burnup of the outer gap closing. In addition, the maximum fuel temperature location was changed by gap conductance asymmetry.

Effects of dimensionality on the ballistic phonon transport and thermal conductance in nanoscale structures

Using the scattering-matrix method, we investigate ballistic phonon transport and thermal conductance at low temperatures in a two-dimensional and a three-dimensional nanoscale structures modulated with a finite Cu/W superlattice. A comparative analysis for two-dimensional and three-dimensional models is made. The results show that the thermal conductance in three-dimensional model is larger than that in two-dimensional model for same structural parameters and temperature. In characteristics, however, the thermal conductance displays similar behaviors in both two-dimensional and three-dimensional models. Moreover, some interesting physical phenomena such as stop-frequency gap, resonant transmission, and universal quantized thermal conductance are observed in the structures. A brief analysis of these results is given.

Prospects of using different clad materials in a material test research reactor – Part 3 – The dynamic behavior

The effects of using different clad materials on the dynamics of a material test research reactor were studied. For this purpose, the aluminum clad of an MTR was replaced separately with stainless steel-316 and zircaloy-4. Simulations were carried out to determine the reactor performance under reactivity insertion and loss-of-flow transients. Nuclear reactor analysis code PARET was employed to carry out these calculations. It was observed that during the fast reactivity insertion transient, Al cladded fuel attained the maximum reactor power of 59.34 MW, while stainless steel-316 cladded attained 48.74 MW and zircaloy-4 cladded attained maximum power of 55.87 MW. During the slow reactivity insertion transient, Al cladded fuel attained the maximum reactor power of 12.38 MW, while stainless steel-316 cladded attained 12.23 MW and zircaloy-4 cladded attained maximum power of 12.34 MW. During the loss-of-flow transients, the reactor power of the stainless steel-316 cladded fuel remained slightly lower than the other two. The fuel temperature of stainless steel-316 and zircaloy-4 cladded fuels remained higher due to poor fuel–clad gap conductance.

Effect of parametric sensitivity on stability boundary of Chinshan NPP

Boiling water reactors are susceptible to instabilities under certain condition and require stability boundary to prevent instability. On this stability boundary, any perturbation or oscillation may easily induce unstable events. All of the parameters that may dominate global mode instability must be analyzed and evaluated. In each core reload design, a decay ratio analysis must be conducted to ensure system stability. LAPUR5.2 and SIMULATE-3 codes were employed to establish a process to conduct such analysis. This investigation contributes to the parametric sensitivity effect, which is a variation of the decay ratio with power/flow operating points along the stability boundary. The results indicate that fractional changes of decay ratios with variation in certain important parameters under different power/flow points. The density reactivity coefficient, gap conductance, recirculation loop gain and system pressure, at the points of high power/flow, all are associate with larger fractional changes in the decay ratio than the points of low power/flow.

Transverse Tunneling through DNA Hydrogen Bonded to an Electrode

Guanidinium ions tethered to an electrode form electrical contacts to DNA via hydrogen bonding with the backbone phosphates, thus providing a sequence-independent electrical connector for native DNA submerged in an aqueous electrolyte. DNA adlayers on a guanidinium modified electrode can be imaged by scanning tunneling microscopy with tens of pS gap conductance. The image resolution suggests that multiatom contacts contribute to the tunnel conductance, so we estimate that the single-nucleotide pair conductance may be on the order of 1 pS.

The role of interparticle contact in conductive properties of random particulate materials

The effective conductivity of material systems consisting of contacting particles is studied computationally using a model of monodisperse circular disks. The electrical or thermal conductivity is computed by modeling the relevant mechanical contact problem followed by a steady-state conduction analysis. The interparticle contact resistance is incorporated via the introduction of gap conductance in the contact pairs. Monte Carlo simulations show that the overall effective conductivity is much lower than that predicted by the model using an overlap assumption, even when the contact resistance is zero. The results also display a nonlinear relationship between the contact resistance and the effective conductivity. Therefore, in practical applications involving particulate inclusions of high density, the interfacial contact must be considered to reliably predict the relevant transport properties. Although the methodology is developed in the context of circular disks, it can be extended to deal with more general situations.