Gap Conductance Research Articles

Development of MERCURY for simulation of multidimensional fuel behavior for LOCA condition

During a loss of coolant accident (LOCA), the ballooning and rupture of fuel cladding can block coolant flow and reduce the coolability of a reactor, which can lead to violation of a safety criteria. It is crucial that fuel models consider how multidimensional thermomechanical behavior and burnup properties affect safety analysis and evaluation. In this study, a multidimensional entire fuel rod analysis module (MERCURY) based on the finite element method (FEM) was developed to simulate multidimensional fuel behavior during a LOCA. The MERCURY incorporated a transient thermal analysis model, a multidimensional gap conductance model, a nonlinear mechanical model, and a transient creep model as thermomechanical models. As fuel models, burnup-dependent material properties, an oxidation model at high temperature, a rod internal pressure model, and cladding burst criteria were developed. Each FEM-based model was verified against results using a commercial FEM package. Verifications demonstrated that the models were formulated and integrated correctly. As validation, the MERCURY simulated experiments (PUZRY) regarding cladding behavior out-of-pile at high temperature and high inner pressure, which is similar to the fuel condition during a LOCA. The simulation results show good agreement with measured hoop strain in experiment.

Hydrogen sulfide gas detection by Au-decorated ZnO nanotube: a computational study and comparison to experimental observations

Based on the experimental reports, Au-decoration on the ZnO nanostructures dramatically increases the electronic sensitivity to H2S gas. In the current study, we computationally scrutinized the mechanism of Au-decoration on a ZnO nanotube (ZON) and the influence on its sensing behavior toward H2S gas. The intrinsic ZON weakly interacted with the H2S gas with an adsorption energy of −11.2 kcal/mol. The interaction showed no effect on the HOMO–LUMO gap and conductivity of ZON. The predicted response of intrinsic ZON toward H2S gas is 6.3, which increases to 78.1 by the Au-decoration at 298 K. The corresponding experimental values are about 5.0 and 80.0, indicating excellent agreement with our findings. We showed that the Au atom catalyzes the reaction 3O2 + 2H2S → 2SO2 + 2H2O. Our calculated energy barrier (at 298 K) is about 12.3 kcal/mol for this reaction. The gap and electrical conductance Au-ZON largely changed by this reaction are attributed to the electron donation and back-donation processes. The obtained recovery time is about 1.35 ms for desorption of generated gases from the surface of the Au-ZON sensor.

An internal parallel coupling method based on NECP-X and CTF and analysis of the impact of thermal-hydraulic model to the high-fidelity calculations

An internal parallel coupling scheme was developed for the coupling between high-fidelity neutronics code NECP-X and the sub-channel thermal-hydraulic code CTF. A new coupling iteration scheme is proposed based on NECP-X/CTF, in which CTF will run after each transport sweep of NECP-X. The interface is developed for both CTF and NECP-X to transfer data via memory rather than files. Based on the newly developed high-fidelity neutronics/thermal-hydraulic coupling system NECP-X/CTF, the impact of the hydraulic cross-flow model, heat conduction model and gap conductance model was studied by calculating the VERA hot full power assembly and core cases. The numerical results show that the internal parallel coupling method is an effective approach for reactor core coupling simulation for the steady state calculation, the new coupling iteration method is more efficient than the traditional Picard iteration method. The sensitivity analysis demonstrates that the cross-flow and gap conductance are very important in coupling for the neutronics and thermal-hydraulic coupling.

Modeling of gap conductance for LWR fuel rods applied in the BISON code

ABSTRACT A new gap conductance model is proposed in this study as a combination of Toptan’s model and the Ross-Stoute model. A variance-based sensitivity analysis is performed to understand how simulation results depend on all input parameters of the proposed model. Additionally, new modeling options (e.g. fill gas thermal conductivity, temperature jump distance, thermal accommodation coefficient, etc.) are added into the nuclear fuel performance code, BISON. The need for further investigation of the gap heat transfer between fuel and cladding in BISON motivated this study to evaluate its impact on the code’s predictions. New gap conductance modeling is proposed. A series of integral-effects validation tests is performed: (1) to demonstrate the impact of the proposed model on the code’s fuel temperature predictions at the beginning of life and through the reactor’s life; (2) to ensure that the proposed model is capable of accurately modeling gap heat transfer characteristics in real-world problems; and (3) to investigate the impact of the estimation of fission gas release on the fuel temperature predictions with the proposed model. The results indicate that the proposed gap conductance model improves BISON’s predictions.

Current Analysis of Single Electron Transistor Based on Graphene Double Quantum Dots

The single electron transistors (SETs) as low power devices are suitable candidates for nanoscale circuit in future technology. These nanoelectronic devices operate based on an electron tunneling. However, coulomb blockade effect prevents single electron transfer between island and coulomb barriers in some conditions. This phenomenon causes zero-conductance region in low bias that is operation limitation of SET. This problem can be solved by using multiple islands in SET structure which their materials are two dimensional carbon based materials such as graphene. Increasing the number of islands effects on probability of electron tunneling. This factor not only rises speed of electron transfer but also it can reduce gap conductance in SET. In this research, current of graphene double quantum dots SET is analyzed and modeled. Moreover effects of graphene length, applied gate voltage and temperature on current SET are investigated. Furthermore effect of number of islands on SET current is evaluated with comparison their charge stability diagrams which are results of SET simulation by software.

Synthesis, Characterization, Thermal Study and Optical Property Evaluation of Co(II), Pd(II) Complexes Containing Schiff Bases of Thiophene-3-Carboxylate Ligand

In this study, a Schiff base ligand and its cobalt(II) and palladium(II) metal complexes were prepared. The structures of the obtained compounds were characterized through Fourier transform infrared spectroscopy, elemental analysis (microanalysis), ultraviolet–visible (UV–Vis) spectroscopy, proton (1H) and carbon (13C) nuclear magnetic resonance (1H-NMR, 13C-NMR), liquid chromatography–mass spectrometry (LC–MS), magnetic susceptibility measurements, thermogravimetric analysis and differential thermal analysis. UV spectra characteristics and basic optical parameters (e.g., absorbance band edge, optical band gap, refractive index and electrical conductance) of the ligand, Co(II) and Pd(II) complexes were investigated in dimethyl sulphoxide, N,N-dimethylformamide and ethanol (EtOH) solvents. The detailed analysis of the test results suggested that both the Pd(II) complex and, particularly, the Co(II) complex can be used in the preparation of diodes owing to their appropriate optical properties.

Gap conductance modeling II: Optimized model for UO2-Zircaloy interfaces

The model conventionally used to calculate heat transfer across the fuel-cladding gap in light water nuclear reactors is a modified version of the Ross-Stoute model. The model was modified to include gap distance in the formulation, which introduced additional uncertainties because the model parameters were not adjusted after the modification. In this study, this conventional model is optimized for uranium dioxide-Zircaloy interfaces using experimental data at high pressure for single- and multi-component gases. First, a calibration is performed for single-component gases. Second, the calibration is extended to multi-component gases, which allows for a demonstration of sources of uncertainty in the model. Third, a general form of the gap conductance model is optimized by combining both data sets. Difficulties arise due to: (i) inaccurate estimation of contact characteristics (e.g., number of solid contacts, deformation mechanism of surface irregularities, contact shapes) that are different for each experimental setup; (ii) the non-physical ratio of temperature jump distance to the gap distance for postulated model function form; (iii) an insufficient description of the appropriate heat transfer regime; and (iv) the pressure dependence of thermal conductivity for inert gases aside from helium. Lastly, a general model is optimized by setting the temperature jump distance at the wall to zero, which reduces possible uncertainties. This final analysis results in a more accurate prediction of the available experimental data. The Associated parameter uncertainty of the model is estimated by performing uncertainty propagation. Overall, the optimized model results in a larger gap conductance with significantly reduced error.

Gap conductance modeling I: Theoretical considerations for single- and multi-component gases in curvilinear coordinates

Accurate estimation of heat transfer across the gap is important in nuclear fuel performance because heat transfer across the fuel-to-cladding gap heavily impacts fuel temperatures and the thermo-mechanical performance of nuclear fuel rods. Better understood physics will allow a better prediction of the gap behavior. This paper focuses on providing an overview of the gap conductance model including theoretical considerations and underlying assumptions. The gap conductance is calculated considering three summed heat paths: fill gas conductance, direct thermal radiation, and solid contact conductance. Each heat transfer mechanism is described in detail. First, the models are generalized to curvilinear coordinates for diatomic/polyatomic molecules. Traditional models use one-dimensional Cartesian equations for a monatomic gas. Second, expressions for temperature jump distance and thermal accommodation coefficients are made consistent with the kinetic theory for both single- and multi-component gases. Lastly, fill gas thermal conductivity is updated to include its dependence on rod internal pressure.

The JFNK method for the PWR's transient simulation considering neutronics, thermal hydraulics and mechanics

Abstract A new task of using the Jacobian-Free-Newton-Krylov (JFNK) method for the PWR core transient simulations involving neutronics, thermal hydraulics and mechanics is conducted. For the transient scenario of PWR, normally the Picard iteration of the coupled coarse-mesh nodal equations and parallel channel TH equations is performed to get the transient solution. In order to solve the coupled equations faster and more stable, the Newton Krylov (NK) method based on the explicit matrix was studied. However, the NK method is hard to be extended to the cases with more physics phenomenon coupled, thus the JFNK based iteration scheme is developed for the nodal method and parallel-channel TH method. The local gap conductance is sensitive to the gap width and will influence the temperature distribution in the fuel rod significantly. To further consider the local gap conductance during the transient scenario, a 1D mechanics model is coupled into the JFNK scheme to account for the fuel thermal expansion effect. To improve the efficiency, the physics-based precondition and scaling technique are developed for the JFNK iteration. Numerical tests show good convergence behavior of the iterations and demonstrate the influence of the fuel thermal expansion effect during the rod ejection problems.

Micro-structurally informed finite element analysis of carbon/carbon composites for effective thermal conductivity

A micro-structurally informed two scale unit cell analysis has been presented for the prediction of thermal conductivities of 3D hybrid and 4D in-plane carbon/carbon composites. Realistic micro-structural features such as bundle cross sections, alignments, and macro voids have been included directly in the unit cells geometry through reconstructed X-ray computed tomography images of the composite. Asymptotic homogenization technique has been used on both scales (bundle and composite) along with periodic boundary conditions to obtain effective thermal conductivities. The imperfectly bonded conditions at the fiber/matrix and fiber bundle/matrix interfaces have also been modeled by considering the contact and equivalent gap conductance between their surfaces. The model has been validated through the experiment conducted in the in-plane direction of 3D hybrid composite. Finally, the effective thermal conductivities of 3D hybrid composite in out-of-plane directions and of 4D in-plane composite in x, y, and z-directions have been predicted for full temperature range (300–1500 K).

A study into the dependence of the cladding-fuel pellet gap conductance on burn-up and the effects on the reactor core neutronic performance

This paper presents the results of the research to study the dependence of the VVER-1000 (1200) cores neutronic characteristics on the cladding – fuel pellet gap conductance coefficient in the process of the fuel burn-up. The purpose of the study was to determine more accurately the dependence of the cladding – fuel pellet gap conductance coefficient on the fuel burn-up as shown in the Final Safety Report for the Bushehr NPP and to determine the extent of the effects this dependence had on the spatial distribution of the neutron field, on the xenon accumulation rate, and on the kinetic and dynamic behavior of the reactor facility. The paper presents the results of calculating the parameters using which the heat engineering safety of the reactor core is monitored in the process of the fuel burn- up (for a generalized fuel load of a VVER-1000) during the transition to an 18-month nuclear fuel cycle. This paper also includes the results of a numerical research to determine the cladding – fuel gap conductance coefficient depending on the fuel burn-up. These results have shown that, in reality, the gap conductance coefficient dependence on the burn-up does not affect greatly the steady-state characteristics. At the same time, it affects to rather a great extent the xenon accumulation rate, specifically in the event of an extended fuel life. In conditions of maneuvering (load following) modes accompanied by the xenon processes in the reactor core. These facts should be into consideration in design of engineering codes, that used to support the operation of the VVER-1000 (1200) and full-scale simulators.

On the validity of the dilute gas assumption for gap conductance calculations in nuclear fuel performance codes

Fill gas thermal conductivity’s dependence on pressure is neglected in today’s nuclear fuel performance codes. Current codes assume that gas behaves as a dilute gas, but the pressure effect is more pronounced at temperatures lower than ten times the critical temperature of each pure gas. The validity of this assumption for nuclear fuel performance is examined herein. Theories related to dilute and dense gas properties are presented, along with their validation against literature data at up to 30 MPa for selected inert gases. Underlying assumptions are clearly described for each model, and their possible impacts on gap conductance calculations are discussed. The dilute gas assumption is valid for helium because it behaves as a dilute gas. However, the assumption is not valid in most gap conductance calculations when the gap is mostly occupied with either lower conductivity gaseous fission products or an initial fill gas other than helium.

Post-heating response of concrete-filled steel tubular columns under sustained loads

Concrete-filled steel tubular (CFST) columns can generally be expected to better resist elevated temperatures as compared to unfilled steel hollow sections, whose evaluation after a fire is limited by the resulting deformation. A better understanding of the behaviour of CFST columns after a fire, which is dominated by the maximum temperature achieved by the concrete infill and plasticity of the steel, is required to properly estimate their residual strength and deformation and to adopt a reasonable strategy with minimum post-fire repair. In this paper, a fibre beam model for the simulation of the post-heating response of concrete-filled steel tubular (CFST) columns is presented. First, the model is validated against experimental results and subsequently it is employed to analyse the post-heating response of circular CFST columns under sustained loads. In reality, during a fire, the columns support load even during the cooling phase of a fire, so it is important to consider this loading condition when predicting the post-fire behaviour. The analysis presented in this paper comprises three stages: heating, cooling and post-fire (under sustained load) conditions. The model considers realistic features typical of the fire response of CFST columns, such as the existence of a gap conductance at the steel-concrete interface and the sliding and separation of the steel tube and the concrete. Based on the model, the response of CFST columns after heating is investigated via parametric analysis.

Mathematical modeling of nonlinear displacement processes in oil LEF-layer by methods of complex analysis and summary representations

The approach to the modeling of nonlinear displacement processes (one and two-phase filtration) in heterogeneous oil deformable layers is developed, taking into account the inverse effect of the potential of the velocity field and the flow function on the conductivity of the medium. We constructed a method and computational technology for solving the corresponding boundary value problems for nonlinear-layered triple-connected curvilinear domains, bounded by equipotential lines and flow lines, on the basis of the synthesis of numerical methods of quasiconformal mappings and summary representations for differential equations with discontinuous coefficients in combination with domain decomposition by Schwartz method. Quasi-ideal processes in nonlinearly double-layered horizontal LEF-layers, whose geometry of heterogeneity zones is unknown in advance, is described by the corresponding boundary value problems obtained on the basis of the Darcy law and the continuity equation with the coefficient of layer permeability, which is given by a piecewise-constant function with ruptures along the searched equipotentials and lines of flow. The coefficient of conductivity the medium is given as a piecewise-constant function, that is dependent on the searched quasipotential and function of flow, with unknown line dividing layers (lines gap conductance coefficient) that is along the searched equipotential lines and flow lines and that is finding in the process of solving the problem. The proposed algorithms automatically solve the problem of choice of nodes and building a dynamic grid, finding of the unknown dividing lines of areas constancy coefficient of conductivity the medium, the calculation of the velocity field and calculate other characteristic parameters of the process. The decomposition of the domain on the layers of the constancy of permeability coefficient allows us to solve problems in more «comfortable» subdomain than the original problem the whole domain, and allows make in parallel the computational process, since calculations in the subdomain at each iterative step are independent of each other and can be done in parallel with the use of modern computer technology.

Uncertainty Quantification and Propagation of Multiphysics Simulation of the Pressurized Water Reactor Core

In recent years, the demand to provide best-estimate predictions with confident bounds is increasing for the nuclear reactor performance and safety analysis. The Organisation for Economic Co-operation and Development Nuclear Energy Agency has been developing an international benchmark of the light water reactor (LWR) uncertainty analysis in modeling (UAM) for the examination of uncertainty quantification and propagation methodologies with various modeling and simulation code systems. The objective of the present work is to develop an uncertainty propagation mechanism based on the stochastic sampling method by taking into account the uncertainties of both basic nuclear data and fuel modeling parameters in the simulation of pressurized water reactors (PWRs) that can be incorporated in the conventional LWR simulation approach. More specifically, the Three Mile Island Unit 1–related exercises from the LWR-UAM benchmark have been modeled using the coupled TRACE/PARCS code system in the three-dimensional core representation. The input uncertainties of the neutronics simulation include few-group cross sections and kinetics parameters generated using the Sampler/Polaris sequence of SCALE 6.2.1. Several heat transfer–related variables for the fuel modeling were considered as sources of input uncertainty of the thermal-hydraulics simulations, including the thermal conductivity of fuel and cladding, fuel heat capacity, and the gap conductance. Dakota was used to sample input parameters of the coupled code system and to perform the uncertainty analysis. Two types of simulations were conducted: steady-state calculation at hot full-power condition and transient scenario initiated by the spatially asymmetric rod ejection accident. Quantities of interest for the steady-state calculation, including core multiplication factor and power peaking factors, were calculated with associated uncertainties. For transient calculations, best-estimate plus uncertainty results of the time evolution of core reactivity, core power, and peak fuel temperature were generated and analyzed. The Wilks’ formula was used to determine the necessary sample size to achieve a 95% confidence of the 95% limit of output quantities of interest. Although the uncertainty propagation and quantification method presented in this paper was developed for PWRs, it could be in general applicable to the multiphysics uncertainty quantification of other types of LWR cores.

Numerical modeling of fuel rod transient response under out of pile test conditions

As per revised Emergency core cooling system (ECCS) acceptance criteria, a precise prediction of fuel rod behavior is essential for realistic safety analysis of nuclear reactor. In this context, a one-dimensional code name ‘TRAFR’ (Transient Response Analysis of Fuel Rod) is developed to simulate the thermo-mechanical behavior of Zircaloy-4 cladding under transient conditions. The transient simulations for inert and oxidizing atmosphere were performed under out of pile test conditions and the predicted burst strains were in good agreement with the experiments conducted in past. Under inert atmosphere, the cladding rupture was delayed and burst strain was higher in all the phases due to the absence of oxidation kinetics. In the oxidizing atmosphere, the burst strain was considerably small at high temperature in mix phase (α+β) and β-phase due to increment in cladding strength and reduction in ductility. For the same internal pressure and clad surface boundary conditions, the temperature of failure was higher for oxidizing atmosphere due to heat generation by exothermic reactions at the surface of the cladding. The clad surface temperature rise rate decreased before the burst with increment in gap width between pellet and cladding owing to decrement in gap conductance. The code under-predicted the burst strain in the mix phase (α+β) and β-phase by ‘Baker-Just’ model. The reason for such deviation was abrupt parabolic oxide growth prediction with higher exothermic heat generation and subsequent faster reduction in cladding thickness which made ‘Baker-Just’ model approach more conservative than ‘Cathcart-Pawel’ model.

MCS based neutronics/thermal-hydraulics/fuel-performance coupling with CTF and FRAPCON

A Monte-Carlo neutronics/thermal-hydraulics/fuel-performance (N/TH/FP) multi-physics coupling system has been developed based on the MCS code recently for the purpose of large-scale high-fidelity analysis of light water reactors (LWRs). The full N/TH/FP coupling overcomes the drawbacks of the previous N/TH (MCS/CTF) and N/FP (MCS/FRAPCON) coupling systems, which suffered respectively from the approximations in CTF on the fuel thermal conductivity and gap conductance, and from the approximations due to the single closed channel enthalpy model in FRAPCON. Thus, compared to the previous coupling systems, the new full coupling system benefits from the transverse cross flow between neighboring sub-channels in CTF, the burnup-dependent fuel thermal conductivity formulation in FRAPCON, and the iteratively determined fuel pellet-cladding gap thermal conductance in FRAPCON. Two applications of the full coupling systems are presented. First, a single fuel rod case is tested to verify the accuracy and efficiency of the new coupled system. Then, the simulation of the BEAVRS whole core model with three-dimensional (3D) pin-by-pin power density and T/H feedbacks exchange between different solvers is performed using the MCS based multi-physics coupling system. The obtained results demonstrate the practical capability of the Monte Carlo based steady-state multi-physics coupling code system for large-scale high-fidelity LWR analysis.

Comparison of lightning protection performance of carbon/epoxy laminates with a non-metallic outer layer

This study shows how a lightning protection layer can be designed to effectively mitigate lightning damage in underlying composite structures. A parametric study was performed to characterize critical lightning protection layer properties that improve composite lightning damage resistance. Simulated 50 kA and 200 kA lightning strikes to pitch-based carbon fiber paper (PCFP)-protected AS4/3506 carbon/epoxy composites were considered in this study. The lightning protection characteristics of various PCFP outer layers were assessed by varying in-plane and through-thickness properties: electrical and thermal conductivities, and electrical and thermal gap conductances. The predicted matrix decomposition in the outermost AS4/3506 ply was significantly reduced by increasing the PCFP in-plane electrical conductivity. While predicted lightning damage decreased slightly with a decrease in thermal gap conductance, varying the electrical gap conductance and the in-plane and through-thickness thermal conductivities did not significantly affect the damage development. Among various PCFP properties, the PCFP in-plane electrical conductivity was the most critical factor in reducing thermal damage development (thus, protecting the underlying AS4/3506 laminate). This parametric study demonstrates that it may be possible to tailor lightweight non-metallic lightning protection layers as an effective alternative to traditional metallic projection layers.

Исследование зависимости теплопроводности газового зазора между оболочкой и топливной таблеткой от глубины выгорания и влияния на нейтроннофизические характеристики активной зоны

Исследование зависимости теплопроводности газового зазора между оболочкой и топливной таблеткой от глубины выгорания и влияния на нейтроннофизические характеристики активной зоны

Modeling burnup-induced fuel rod deformations and their effect on transient behavior of a VVER-440 reactor core

This paper describes the simplified model for burnup-induced fuel rod deformations occurring during base irradiation period, from manufacturing to the beginning of the transient. Model is intended to be used together with a codes that are used for analysis of whole core and plant transients. Model consist of thermal and mechanical parts. Due to the high conductance approximation for the gap conductance, the thermal part of the model can be solved independently from the mechanical model. In this way the use of iterative solvers is avoided and model is simple to implement. Model is verified against FRAPCON simulations and has been integrated to the reactor dynamics code HEXTRAN, in which it provides initial data for fuel behavior module FINIX for transient simulation. Effect of burnup initialization on simulation of reactor transients has been demonstrated with the reactor dynamics code HEXTRAN that uses FINIX as a fuel behavior module.