Behavior Of Reactor Research Articles

Computational fluid dynamics simulation of Fischer-Tropsch synthesis coupled with a novel reaction kinetics using trimetallic catalyst over a biomass-based support

A mathematical modeling based on the computational fluid dynamics (CFD) was performed in this work. The reactive flow behavior in a tubular reactor was modeled for Fischer-Tropsch synthesis (FTS) using a novel trimetallic catalyst supported on a low-cost and renewable carbonaceous material. The fixed bed reactor was simulated as a porous media using a commercial software package (COMSOL Multiphysics). In this study, major attention has been paid to the species transport and reaction kinetics, as a fundamental stage in CFD simulation of FTS. The reaction kinetics was evaluated using Eley-Rideal theory assuming the molecular adsorption of CO based on the enol mechanism. Furthermore, the products distribution was accounted in the kinetics study by defining a mathematical expression and the kinetics parameters were optimized using genetic algorithm (GA). The activation energy and heat of H2 adsorption were estimated as 62.63 and −29.52 kJ/mol, respectively. The role of operational conditions was critically evaluated on the partial pressure profiles of the species, and the CFD results were validated with the experimental data. While increasing total pressure improved the partial pressure of C5+ hydrocarbons to 1.2 bar, increasing temperature to 310 °C limited the chain growth. The employed modeling and novel catalyst design can provide promising overview of the FTS reaction mechanism and products selectivity in fixed bed reactors.

Experimental investigation of residence time and axial velocity of 6.0 cm graphite pebbles in a cold-flow pebble bed reactor using radioisotope-based technique

Understanding pebble flow characteristics and residence time distribution is essential for reactor design, fuel burnup estimations, and safety analysis of High-Temperature Gas-Cooled Pebble Bed Reactors. However, due to the complex nature of the dense granular flow, pebble flow remains inadequately understood and challenging to study through experiments. In this work, a three-dimensional experimental pebble-bed setup was developed to investigate pebble flow behavior in pebble-bed reactors. For the first time, 6.0 cm graphite pebbles were utilized, and features such as one pebble at a time discharge mode and continuous pebble recirculation were incorporated. The overall and zonal residence time distributions and the zonal-averaged axial velocity were investigated using a radioisotope-based residence time distribution (RTD) technique. The results showed that the overall residence time increases as the radius of the initial seeding position increases, but with a higher increase rate at the wall compared to the center region. The zonal residence time findings indicated a uniform profile in the upper section of the bed, except near the wall. However, non-uniformity in the radial profile increased in the lower sections, with the greatest variation occurring in the conical section. The axial velocity of the pebbles was found to be small and nearly uniform in the upper section of the bed. In the lower section, however, the velocity increased significantly for the center-region pebbles compared to those at the wall, resulting in a pronounced non-uniform radial profile. Moreover, a consistent hexagonal arrangement of pebbles was observed across the entire container wall after recirculating several bed inventories, which is worth further investigation.

Dynamic chaos of imaging measurements for characterizing gas–liquid nonlinear flow behaviour in a metallurgical reactor stirred by top‐blown air

AbstractA modern chaos detection strategy for imaging measurements is proposed in this work to quantify the thermal mixing state quality in a metallurgical reactor stirred by top‐blown air. Specifically, the improved C‐C algorithm is proposed to reconstruct the phase space of the nonlinear time series of the bubble characteristics under thermal conditions. Moreover, a chaos decision tree algorithm is introduced to extract the chaotic mixing characteristics of the thermal two‐phase mixing system for the first time. Experimental and calculated results show that all the possible mixing states of the mixing system are visualized by reconstructing the phase space with the help of the visualization technique of the thermal gas–liquid two‐phase flow. It is found that the attractor of the nonlinear time series of bubbles exhibited more serious variations while the chosen working condition was optimal. Furthermore, the obtained parameter represents the chaotic characteristics of the thermal gas–liquid two‐phase mixing system which has chaotic characteristics under various experimental conditions. Hence, the new strategy would be helpful and effective in beneficial exploration for understanding the nonlinear intensification mechanism of metallurgical thermal processes.

Modeling noise experiments performed at AKR-2 and CROCUS zero-power reactors

CORTEX is a EU H2020 project (2017-2021) devoted to the analysis of ’reactor neutron noise’ in nuclear reactors, i.e. the small fluctuations occurring around the stationary state due to external or internal disturbances in the core. One important aspect of CORTEX is the development of neutron noise simulation codes capable of modeling the spatial variations of the noise distribution in a reactor. In this paper we illustrate the validation activities concerning the comparison of the simulation results obtained by several noise simulation codes with respect to experimental data produced at the zero-power reactors AKR-2 (operated at TUD, Germany) and CROCUS (operated at EPFL, Switzerland). Both research reactors are modeled in the time and frequency domains, using transport or diffusion theory. Overall, the noise simulators managed to capture the main features of the neutron noise behavior observed in the experimental campaigns carried out in CROCUS and AKR-2, even though computational biases exist close to the region where the noise-inducing mechanical vibration was located (the so-called ”noise source”). In some of the experiments, it was possible to observe the spatial variation of the relative neutron noise, even relatively far from the noise source. This was achieved through reduced uncertainties using long measurements, the installation of numerous, robust and efficient detectors at a variety of positions in the near vicinity or inside the core, as well as new post-processing methods.For the numerical simulation tools, modeling the spatial variations of the neutron noise behavior in zero-power research reactors is an extremely challenging problem, because of the small magnitude of the noise field; and because deviations from a point-kinetics behavior are most visible in portions of the core that are especially difficult to be precisely represented by simulation codes, such as experimental channels. Nonetheless the limitations of the simulation tools reported in the paper were not an issue for the CORTEX project, as most of the computational biases are found close to the noise source.

Large Eddy Simulation of Convective Heat Transfer in a Random Pebble Bed Using the Spectral Element Method

Abstract The development of fluoride-cooled high-temperature reactors has drastically increased the demand for an in-depth understanding of the heat transfer (HT) in packed beds cooled by liquid salts. The complex flow fields and space-dependent porosity found in a pebble bed require a detailed understanding to ensure the proper cooling of the reactor core during normal and accident conditions. As detailed experimental data are complicated to obtain for these configurations, high-fidelity simulation such as large eddy simulation and direct numerical simulation (DNS) can be employed to create a high-resolution heat transfer numerical database that can assist in addressing industrial-driven issues associated with the heat transfer behavior of fluoride-cooled high-temperature reactors. In this paper, we performed a series of large eddy simulation using computational fluid dynamics (CFD) code NekRS to investigate the heat transfer for a bed of 1741 pebbles. The characteristics of the flow, such as average, rms, and time series of velocity and temperature, have been analyzed. Porous media averages have also been performed. The simulation results show a good agreement between non-conjugate heat transfer and conjugate heat transfer. The generated data will be used to benchmark heat transfer modeling methods and local maxima/minima of heat transfer parameters. It will also be used for supporting convective heat transfer quantification for Kairos Power and benchmarking lower fidelity models.

Residence time distributions in microchannels with assistant flow inlets and assistant flow outlets

Residence time distributions (RTD) have been extensively employed to characterize the flow behavior in reactors, which significantly impact reaction conversion and selectivity. RTDs are investigated in microchannels with assistant flow methods by computational fluid dynamics, including assistant flow outlets (AFO), assistant flow inlets (AFI), and assistant flow inlets and outlets (AFIO). The inlet velocity ranges from 0.01 to 1 m/s, with flow rate weights ranging from 0.4 to 0.8 for the central flow outlet and from 5/8 to 1/13 for the central flow inlet. For AFO, as the decreasing flow rate weight of the central flow outlet, the peak of RTDs occurs closer to mean residence time, i.e., dimensionless mean residence time (θ) θ = 1. Meanwhile, the peak value grows higher, and the RTDs of tracking particles become narrower. There is a reduction of the dimensionless variance (σθ2) of particle residence time by tens of times compared with straight channels. For AFI, the central flow is further encapsulated in the center, leading to better RTDs and smaller σθ2 at a low velocity, but it is excessively disturbed at a high velocity, resulting in the increasing σθ2. AFIO not only achieves better RTDs but also diverts assistant flow to guarantee the weighting of central flow at the central outlet. This method using assistant flows can be extended to various size ranges and structure designs to promote the RTDs in continuous flow systems despite still laminar conditions.

Dynamic Model of a Virtual Air Gap Reactor

Variable reactors have been a vital component of power networks for decades, where they have been used as fault-current limiting devices or for reactive power compensation. Traditionally, modifying the inductance of predominantly mechanically operated variable reactors requires seconds to minutes. In contrast, virtual air gap (VAG) reactors can change the inductance within milliseconds, potentially improving power system stability. Existing dynamic models of VAG reactors cannot capture the entire system dynamics, limiting their applicability for simulations in the time-domain. This research presents two dynamic VAG reactor models, one with and one without core losses. The models capture all significant system dynamics using electromagnetic principles and VAG reactor flux linkage behavior. The proposed models were experimentally validated using a small VAG reactor. Over a broad operating range, both models accurately reproduce the dynamic behavior, transient response, and dominant harmonics of the small VAG reactor. Consequently, the models may be used for a variety of applications, such as time-domain simulations, harmonic analysis, and the development of suitable controllers for VAG reactors. In addition, engineers may use the core loss omitting model as a VAG reactor design tool, as the actual reactor is not required for modeling.

Monitoring and simulation of the fuel irradiation behavior in nuclear reactors based on phononic crystal structure

We have presented in the current work a novel idea for simulating the irradiation behaviors of the nuclear fuel pellets in nuclear reactors by using a one-dimensional defective phononic crystal (1D-DPnC) design was presented. The transmission spectra of the incident mechanical waves were considered basic data for expressing the characteristics of different nuclear fuel-pellets. Herein, the density, sound speed, and Young’s modulus of the fuel-pellets represent the key parameters that are influenced by the irradiation behaviors of these pallets. Mixed plutonium–uranium oxide (MOX) nuclear fuel is considered the main fuel in the present study. In addition, a comparison is performed for this fuel with other types of nuclear fuels. Moreover, the mechanical properties of these MOX-pellets are dependent on the porosity, the ratio of oxygen-to-metal (O/M), and the plutonium (Pu-content). The theoretical treatments depend on the transfers matrix method to compute the transmission spectra through the 1D-DPnC. The numerical findings provided that the MOX-pellet has the highest performance compared to other fuel pellets and with sensitivity equal to 59.388 × 103 Hz s/m. It was also reported that the effects of the percentage of the O/M and Pu- content in MOX had a minor effect in a comparison with the impact of porosity. The theoretical simulation agreed extremely with the experimental data reported for these nuclear fuels. Because of the close relationship between sound speed and density, this sensor can be utilized to monitor the porosity, O/M, Pu-content, and density of fuel-pellets as a quick and non-destructive evaluation technique in a nuclear fuel fabrication laboratory. This article has proven theoretically that MOX fuel produced from nuclear waste of uranium dioxide and plutonium dioxide gives excellent results compared to other types of nuclear fuels, and this agrees with experimental researches. Thus, it may contribute in preserving the environment from nuclear waste, and this can be considered a novel kind of purification of environmental pollution treatment.

Modeling of distribution parameters for upward steam-water boiling flows in subchannels of a vertical rod bundle

ABSTRACT In simulations of two-phase flow behavior in nuclear reactors, subchannel analysis codes are often used to evaluate the void fraction within a BWR fuel bundle in detail. When solving the momentum conservation equation averaged over a subchannel cross-section for upward two-phase flows, the distribution parameter is required to consider the void fraction and velocity distributions in the subchannel cross-section. In this paper, constitutive equations were developed for the distribution parameters for dispersed two-phase flows applicable to the inner, edge, and corner subchannels, which are typical subchannels in a fuel bundle. The distribution parameters could be calculated by giving the void fraction and the velocity distribution. Therefore, the distribution parameters were evaluated and modeled as a function of the geometrical parameters by assuming the void fraction and velocity distributions with bulk and subcooled boiling for each subchannel type. The developed constitutive equations were evaluated by comparing them with the distribution parameters estimated based on the NUPEC rod bundle void fraction test data. The developed distribution parameter model was implemented into the subchannel analysis code NASCA and compared with the measured cross-sectional average void fraction of the NUPEC rod bundle void fraction test data. In comparison with the original NASCA code, which assumed the distribution parameter to be unity, the improved NASCA with the distribution parameter model decreased the mean error of the measured cross-sectional average void fraction to less than half of the result of the original NASCA code, both in absolute and relative differences.

Introduction of novel characteristic time quantities to describe chemical reactors

The Mean Residence Time is a quantity which is used very often to characterise chemical reactors. This quantity can be calculated using the Residence Time Distribution of a specific, continuous reactor and assuming closed-closed boundary conditions. Many reactors in the industry though, are discontinuous ones and the direct application of these methods is in general not possible. This work presents a new distribution, the Local Residence Time Distribution, which observes each position in a specific reactor. Furthermore, two new, local quantities, the Mean Distribution Time and the Mean Dead Time, are introduced, which can be applied as characteristic times to evaluate the internal behaviour of chemical reactors used for discontinuous processes such as fermentations.

Preliminary analysis focusing on in-vessel thermal hydraulics in loss-of-heat removal systems in a sodium-cooled fast reactor

Accident scenarios for the safety evaluation of sodium-cooled fast reactors (SFRs) are categorized into two groups: Anticipated Transient Without Scram (ATWS) and Loss-of-Heat Removal System (LOHRS). The latter is subdivided into two scenarios—Loss-of-Heat Sink (LOHS) and Loss-of-Reactor Level (LORL)—depending on the initiating event. With the development of SFRs, research on ATWS has been paid much attention to due to its rapid accident progression, which could lead to significant radioactive release. Conversely, the significance of LOHRS has increased in recent decades due to lessons learned from accidents at the Fukushima Daiichi Nuclear Power Plant of Tokyo Electric Power Company. However, compared with ATWS, finding and evaluation tools for LOHRS have been limited. Hence, code development is initiated to enhance the severe accident analysis code AZORES, which the Nuclear Regulation Authority developed for the safety evaluation of SFR accidents. This enhancement is achieved by coupling AZORES with plant dynamics code ADYTUM in order to deal with more detailed thermal hydraulics behavior in reactor cooling system. The paper describes the coupling methodology, focusing on the thermal–hydraulic model as the first step of code development and its application. The coupling has been confirmed to be well achieved through a preliminary analysis of LOHS, comparing the results of coupled and single AZORES. The comparison of the analysis results between LOHS and LORL by the AZORES coupled with ADYTUM has been conducted to identify the key phenomena in accident progression. The comparison shows that the coolant leak area and the pressure drop at the piping of the reactor cooling system affect accident progression in LOHS. Consequently, the coupled AZORES is capable of predicting in-vessel thermal–hydraulic behavior in LOHRS successfully, reflecting accident conditions and plant design appropriately.

Comprehensive reverse flow reactor model for fluid–solid systems

A comprehensive model for a Reverse Flow Reactor was developed and implemented in the present article. The model consists of the combination of mass and energy balance equations written for a fluid–solid packed reactor, where the feed and outlet positions are alternated periodically using dedicated boundary conditions. Several non-idealities were considered, dealing with both the particle and the fluid phases. Different chemical systems were simulated numerically to investigate the behavior of the reverse flow reactor and to determine the optimal operation policy. Intraparticle diffusion resistance allowed to predict concentration gradients in each case, while intraparticle heat transfer was demonstrated to be non-influent. The impact of the main kinetic and heat/mass transfer parameters was checked via a parametric investigation, demonstrating the high flexibility of the model, even predicting harsh operation conditions. The model was tested in the description of data taken from the literature, demonstrating a good descriptive power.

Investigation on Neutronic Behavior of Pebble Bed Reactor for TRU Transmutation

One alternative effort to eliminate the accumulation of transuranic (TRU) elements generated from LWR spent fuel is by utilizing TRU as nuclear fuel in pebble bed reactor. This work was aimed to investigate the neutronic behavior of a pebble bed reactor used for TRU transmutation. The reactor geometry is adopted from the HTR-Modul. MCNP6 multipurpose radiation transport code with ENDF/B-VII.1 neutron library was used to calculate the neutronic aspects. The results indicated that the initial effective multiplication factor (keff) values decrease as the amount of TRU fuel pebbles within the core increases, concurrent with the fuel burnup. Increasing the TRU fuel pebble ratio resulted in weakened Doppler temperature coefficients (DTCs) and improvement in moderator temperature coefficients (MTCs). The calculated total mass of plutonium and minor actinide transmutation demonstrated that a pebble bed reactor with 100% TRU pebble can reduce TRU mass up to 55.47% of its initial mass load.

Overview of multiphysics R&D activities at the CEA/IRESNE institute

The major mission of the IRESNE (Research Institute on Nuclear Systems for low carbon Energy production) institute of the DES (Energy Division) at CEA (French Alternative Energies and Atomic Energy Commission) is to study current and future nuclear reactors integrated into low-carbon energy systems. To fulfill this mission, the IRESNE’s main activities rely on the development and use of advanced physical models, multiphysics tools including efficient coupling schemes, and dedicated experimentation. Approaches to nuclear systems using multiphysics simulations are now required, in order to better understand the complex intrinsic features of the coupled phenomena.Hence, IRESNE addresses eight different multiphysics research fields: fuel fabrication, fuel behavior under irradiation, fuel assembly integrity under fluid–structure interaction, contamination of fluid circuits, sodium technology, behavior of reactors, corium and fission products, and radionuclide transfer to environment. A general overview of IRESNE’s current multiphysics activities in these eight fields is provided in this paper. It focuses on experimentation, modeling, simulation and tool qualification, corresponding to the Verification Validation and Uncertainty Quantification process. Moreover, IRESNE’s key challenges to be met in the next 5 to 10 years, to improve and validate multiphysics Modeling and Simulation, are also discussed in this contribution. The ultimate goal is to get closer to the development of a digital reactor, core and fuel, still progressing on applied mathematics, high-performance computing, and understanding and modeling of physical mechanisms.

Effects of catalyst distribution, particle geometry, and process conditions on the behavior of a water gas shift reactor under moderate pressures: a modeling study

Effects of catalyst distribution, particle geometry, and process conditions on the behavior of a water gas shift reactor under moderate pressures: a modeling study

Evaluation of the relevant mass and heat transfer phenomena in a packed bed membrane reactor for the direct conversion of CO2 to dimethyl ether

This study investigates the relevant heat and mass transfer phenomena occurring at the different scales in a packed bed (membrane) reactor for the direct conversion of CO2 to dimethyl ether (DME) via the implementation of 2D heterogeneous reactor models. Intra-particle diffusion limitations were found to be relevant for particle diameters larger than 1 mm and temperature above 220 ⁰C, such that the catalyst efficiency drops down to 50% and 5% for the Cu/ZnO/Al2O3 and the HZSM-5, respectively, in the most critical conditions (i.e., 270 ⁰C and Dp of 10 mm). A component-specific Thiele modulus-efficiency correlation was developed based on the results of the rigorous particle model to account for pore-diffusion limitations without having to solve a complex heterogeneous reactor model. This correlation shows the typical behavior reported in literature for power law kinetics and accurately predicts the reaction performance with deviation of less than 5% for values of the Thiele modulus lower than 2. In the packed bed membrane reactor (PBMR), the concentration polarization (CP) also showed to affect the reactor performance. The concentration of water at the surface of the membrane selective layer was found to be up to 64% lower than the concentration in the bulk phase, hindering the effectiveness of the membrane separation. To account for this phenomenon via a simplified approach, a Sherwood-type correlation was developed to determine a CP mass transfer coefficient, based on the results obtained via the rigorous 2D PBMR model. Such correlation showed to predict with high accuracy (i.e., errors lower than 5%) the effect of the CP on the PBMR performance. Differently from the pore diffusion and CP phenomena, the intra-particle heat transfer, the particle–fluid mass and heat transfer as well as the axial dispersion were found to have a negligible effect on the reactor behavior. Finally, given the relevant mass/heat transfer phenomena, this study proposes further reactor optimization strategies, such as the reduction of the zeolite loading in the bifunctional catalyst bed by ca. 90% with respect to what is reported in literature.

Design feasibility and failure mode analysis for a HALEU-based Kilopower-class space power reactor

A space fission reactor that will be used on the lunar base in the future, must meet the requirements for a 10 years prolonged power supply depending upon only its inherent response characteristics. Therefore an analysis tool to optimize the design conditions as well as to predict the reactor behavior for self-regulating performance during the mission period is needed. For this purpose, the Lumped Network Analysis and Simulation (LuNAS) code has been developed for a kW-class space fission power reactor. We performed failure mode analyses for a 5 kW HALEU-fueled reactor combined with Stirling converters, heat pipes and a radiator panel, which demonstrated that more than half-power can be guaranteed up to 50% failure of each component.

Transition of the Flow Regime Inside of Monolith Microchannel Reactors Fed with Highly Turbulent Flow

This paper investigates the flow behaviour of monolith microchannels. Specifically, the study characterizes the flow regime within in-series monolith channels where highly turbulent flow approaches them but inside of the channels, the Reynolds number is subcritical. Results from LES and a transitional RANS model are compared to those obtained when directly assuming laminar flow inside of the channels. A space-resolved model of channels placed in series and channel Reynolds numbers ranging from 50 to 300 are considered. The results show that the flow pattern in is almost identical in the two channels and that the frequency of fluctuations tends to increase with the Reynolds number. The flow regime in both channels is unsteady laminar, containing a wide spectrum of frequencies. The tested transitional RANS model (k-kL-ω) is unable to capture the velocity fluctuations predicted by LES. Despite the differences in the velocity field prediction, the pressure drop estimation from all models is practically the same. This study provides insights into the flow behaviour of monolith reactors and is useful for reactor design and optimization.

Kinetic modeling of dynamically operated heterogeneously catalyzed reactions: Microkinetic model reduction and semi-mechanistic approach on the example of the CO[formula omitted] methanation

As the production of green hydrogen from fluctuating resources and the use of Power-to-X technologies become more and more important in the energy sector and the chemical process industries, the interest in dynamically operated reactors is increasing. In order to apply model-based optimization methods for process intensification the dynamic reactor behavior needs to be predicted sufficiently accurate by the underlying model equations, in which the reaction kinetics play a key role. Microkinetic elementary step models are able to describe the dynamic behavior on the catalyst surface. However, these models are laborious to derive, parameterize and require comparably high computational effort to solve. For these reasons, simpler semi-mechanistic kinetic models based on a rate determining step at steady state conditions are much more frequently applied in numerical studies. However, the underlying assumptions of these models are questionable for dynamic operation since sorption processes and possible changes in the reaction mechanisms are not considered. We present a semi-mechanistic kinetic model approach based on so called rate-affecting steps and the dynamic change in surface coverages, which is able to describe the dynamic behavior of the catalyst surface. The applicability of our approach is demonstrated for model reduction of an elementary step mechanism developed for the COx methanation and for kinetic model development with synthetic experimental data. Both methods result in a kinetic expression that can accurately reproduce the dynamic kinetic behavior predicted by the elementary step model, with a significant reduction in computational effort for our new semi-mechanistic model, making this new class of model approach attractive for model-based optimization applications.

Modeling and simulation of trickle bed reactors for the purification of 1-butene

Abstract In this contribution, a mathematical model of an industrial trickle-bed reactor employed in the purification of a C4 cut by selective hydrogenation of acetylenic or dienes compounds to obtain high purity 1-butene is presented. A reaction network of ten reactions is included in the model, with kinetics expressions and parameter estimation obtained from previous experimental studies on a commercial catalyst. Internal mass transfer resistances in the catalyst particles are significant; therefore the reaction-diffusion equations must be solved. External mass transfer resistances in the liquid phase were retained, while those in the vapor phase were negligible. The model was employed to analyze the reactor behavior by varying the inlet molar flow rate of hydrogen, the operating pressure, inlet temperature and the level of activity of the catalyst, taking into account its deactivation. It was demonstrated that the mass transfer resistances, inside and outside the catalyst particles, have a significant impact on the selectivity, but a careful operation of the reactor can improve the selectivity and extent the catalyst life. On the other hand, an alternative system was proposed, with two beds and a distributed input of H2, which led to a significant improvement in the selectivity.