Reactor In Response Research Articles

A reduced-order modeling of a tubular solar reactor for long duration thermochemical energy storage

The storage of solar energy in a solid form, referred to as a “solar fuel”, can be achieved through a process known as endothermic solar thermochemistry. This process transforms the absorbed solar energy into a stable and retrievable form that can be stored for extended periods of time. This paper presents a low–order heat transfer model of a counter–current tubular falling bed reactor designed to produce thermally reduced magnesium manganese oxide pellets for long duration thermochemical energy storage. The energy required for the endothermic reduction was supplied by concentrated solar energy or renewable electricity via indirect heating of the gas and solid reactants flowing in a ceramic tube. The counter-current gas flow enhances the mixing of the solid particles with the heat recuperation zone, allowing the gas and particles to enter and exit the tubular reactor close to room temperature. The reactor was vertically oriented and was heated circumferentially by an adjustable level heat flux along a finite segment of its length. The temperature distribution of the reactor in response to transient changes along the tube was modeled by considering conduction, convection, and radiation heat transfer. Governing equations for the heat transfer model were solved by discretizing the reactor tube into a finite number of control volumes and using an energy balance for the heat exchange between the reactor wall, gas, and particles within the control volume. The energy absorbed during this endothermic reaction was modeled numerically by fitting the data of the chemical conversion rate with the corresponding temperature of particles in the heating zone. The numerical model has been experimentally validated using a reactor prototype made of a 121.92 cm alumina tube heated by a 7 kW electric tube–furnace. The alumina tube receives magnesium manganese oxide pellets of 3.66 ± 0.516 mm in diameter from the top, and a counter–current gas flow from the bottom. The reactor wall temperature was monitored by six thermocouples installed along the reactor tube length. The experimental procedure was numerically simulated, and the temperature variation along the reactor tube was compared with a matrix of experimental runs for a range of particles mass flowrates (0.75–1.25 g/s) and corresponding gas flowrates (36–65 SLPM). The reactor system was heated gradually from room temperature to a steady state temperature of 1673 K, and then cooled down to room temperature. The heating and cooling processes were simulated, and the numerical and experimental results were compared throughout processes. The numerical model showed similar trends to the experimental results, with an error of 0.69 to 7.9 % for the particle inlet and 0.7 to 7.9 % for the gas inlet during steady-state operation. The proposed numerical model can be implemented as a simplified physical model to design a feedback control system to regulate reactor temperature.

Mathematical modeling of point kinetic equations with temperature feedback for reactivity transient analysis in MTR

Abstract The behavior of the nuclear reactor in response to any sudden change in reactivity is very important for reactor control. Positive reactivity insertions causes power excursion and could have a destructive impact on the reactor core. The aim of the study is to investigate the safety features of a material test reactor (MTR) during reactivity transient with emphasis on the capability of the mathematical modeling using programming language. Therefore a mathematical model using Python3.6; high-level programming language is developed to solve the point kinetic equations taking into account Doppler and moderator feedback effects. The model is validated with AIREKMOD_RR; point kinetic computer code for reactivity transient analysis in nuclear research reactors. The results of the Python model demonstrate the inherent safety features of the MTR reactor. Also, there is good agreement between the results of the Python model and AIREKMOD_RR code, illustrating the efficiency of the Python model in simulating the behavior of the reactor core under reactivity transient.

Effects of a change in the design of cooling water injection on risk reduction in a beam tube rupture

In this paper, the difference between the current design and a newly proposed method for the injection of supplementary cooling water into a nuclear research reactor in response to the rupture of a beam tube is investigated. Here, the newly proposed method or design is to maintain the cooling water in the chimney at a certain level solely by gravitational coolant feeding from the emergency water supply system (EWSS) tank to prevent nuclear fuel damage, without the sump recirculation operations described in the current procedure for the beam tube accident. The reduction of risk to the reactor core, quantified in terms of core damage frequency (CDF), as a result of the proposed changes to the design of the emergency water supply in the event of a beam tube rupture is as follows. 1) The overall CDF for the reactor with the newly proposed design for emergency cooling water injection is about 4.16E-06/yr, approximately 8.6% lower than the CDF of the existing design (4.55E-06/yr). 2) The CDF specifically due to a beam tube rupture is 6.33E-08/yr, representing an 85.9% decrease compared to that with the existing design (4.49E-07/yr). In addition to these risk-reduction effects, it is also worth noting that changing the emergency water injection design allows pure coolant to be supplied to the EWSS tank, eliminating the need to decontaminate the reactor after an accident.

Real-Time Behavior of a Microalgae–Bacteria Consortium Treating Wastewater in a Sequencing Batch Reactor in Response to Feeding Time and Agitation Mode

A study of a microalgae–bacteria treatment system was conducted in a sequencing batch reactor (SBR) by combining a precultured native algae Nannochloropsis gaditana L2 with spontaneous municipal wastewater microorganisms. Two types of agitation, air mixing (AI) and mechanical mixing (MIX), were assessed at continuous illumination (L) and photoperiod cycle light/dark (L/D). The obtained consortium, via native microalgae addition, has a better operational efficiency compared to spontaneous control. This allows the removal of 78% and 53% of total Kjeldhal nitrogen (TKN) and chemical oxygen demand (COD), respectively. Under the (L/D) photoperiod, the optimal removal rate (90% of TKN and 75% of COD) was obtained by the consortium at 4 days of hydraulic retention time (HRT) using the AI mode. Moreover, during feeding during dark (D/L) photoperiod, the highest removal rate (83% TKN and 82% COD) was recorded at 4 days HRT using the AI mode. These results bring, at the scale of a bioreactor, new data regarding the mode of aeration and the feeding time. They prove the concept of such a technology, increasing the attraction of microalgae-based wastewater treatment.

Shift of bacterial community and denitrification functional genes in biofilm electrode reactor in response to high salinity

High salinity suppresses denitrification by inhibiting microorganism activities. The shift of microbial community and denitrification functional genes under salinity gradient was systematically investigated in a biofilm electrode reactor (BER) and biofilm reactor (BR) systems. Denitrification efficiency of both BER and BR was not significantly inhibited during the period of low salinity (0–2.0%). As the salinity increased to 2.5%, BER could overcome the impact of high salinity and maintained a relatively stable denitrification performance, and the effluent NO3−-N was lower than 1.5 mg/L. High salinity (>2.5%) impoverished microbial diversity and altered the microbial community in both BER and BR. However, two genera Methylophaga and Methyloexplanations were enriched in BER due to electrochemical stimulation, which can tolerate high salinity (>3.0%). The relative abundance of Methylophaga in BER was almost 10 times as much as in BR. Paracoccus is a hydrogen autotrophic denitrifier, which was obviously inhibited with 1.0% NaCl. The hetertrophic denitrifiers were primarily responsible for the nitrate removal in the BER compared to the autotrophic denitrifiers. The abundance and proportion of denitrifying functional genes confirmed that main denitrifiers shift to salt-tolerant species (nirK-type denitrifiers) to reduce the toxic effects. The napA (2.2 × 108 to 6.5 × 108 copies/g biofilm) and nosZ (2.2 × 107 to 4.4 × 107 copies/g biofilm) genes were more abundant in BER compared to BR's, which was attributed to the enrichment of Methylophaga alcalica and Methyloversatilis universalis FAM5 in the BER. The results proved that BER had greater denitrification potential under high salinity (>2.0%) stress at the molecular level.

Paired Electrocatalytic Oxygenation and Hydrogenation of Organic Substrates with Water as the Oxygen and Hydrogen Source.

The use of water as an oxygen and hydrogen source for the paired oxygenation and hydrogenation of organic substrates to produce valuable chemicals is of utmost importance as a means of establishing green chemical syntheses. Inspired by the active Ni3+ intermediates involved in electrocatalytic water oxidation by nickel‐based materials, we prepared NiBx as a catalyst and used water as the oxygen source for the oxygenation of various organic compounds. NiBx was further employed as both an anode and a cathode in a paired electrosynthesis cell for the respective oxygenation and hydrogenation of organic compounds, with water as both the oxygen and hydrogen source. Conversion efficiency and selectivity of ≥99 % were observed during the oxygenation of 5‐hydroxymethylfurfural to 2,5‐furandicarboxylic acid and the simultaneous hydrogenation of p‐nitrophenol to p‐aminophenol. This paired electrosynthesis cell has also been coupled to a solar cell as a stand‐alone reactor in response to sunlight.

Simulation and analysis of the Loss of Flow Accident (LOFA) scenarios for an open pool type research reactor by using the RELAP5/MOD3.2 code

Abstract In this paper, a 5 MW open pool type research reactor is simulated and analyzed to check and pass a complete and practical safety assessment against various LOFA scenarios. Generally, LOFA may occur in a research reactor when the reactor primary pump or the safety flapper valve fail. The simulation includes the study of a downward core cooling system circulated by the gravity driven force, the performance and failure effects of the safety flapper valve, the flow reversal mode and transition, the natural convection mode, different possible types of LOFA, and impacts of the surrounding empty boxes. First of all, the code nodalization is successfully benchmarked against available experimental data of the reactor operating parameters. Then, all possible DBAs in this field of study are simulated and discussed in detail. Results are completely satisfactory to simulate and analyze a pool-type research reactor in response to LOFA using the RELAP5 code. Furthermore, transients including the natural convection mode and even flow reversal mode are following naturally without any oscillation or source of errors. Finally, TRR is completely safe against the DBA type of LOFA scenarios.

The microbial community structure change of an anaerobic ammonia oxidation reactor in response to decreasing temperatures.

In anaerobic ammonium oxidation (anammox) systems, temperature may regulate the activity of functional bacteria (e.g., anammox bacteria) and the composition of the microbial population, ultimately determining the performance of the anammox reactor. Knowledge of the dynamic changes in nitrogen removal rates and the microbial anammox community at low and/or ambient temperature is still limited. This study explored the response of an anammox sequencing batch reactor (SBR) to a gradient of decreasing temperature (33, 25, 20, 15, 10°C), followed by recovery to 22°C, over 360days. Particularly, the specific anammox activity (SAA) and microbial community were assessed. The anammox reaction in the SBR remained stable and efficient at 20-33°C, with a total nitrogen removal load of 0.4g-NL-1day-1 and an SAA of > 0.32g-Ng-VSS-1day-1; 10°C was the turning point for the anammox bacterial metabolic activity, at which the SAA decreased by 91% compared with that at 33°C. After the temperature was returned to 22°C, the anammox activity recovered to 0.24g-Ng-VSS-1day-1. The apparent activation energy for the anammox reaction was 68.4kJmol-1 at 10-33°C and 152.9kJmol-1 at 10-20°C. High-throughput sequencing results revealed that Kuenenia was the dominant species of anammox bacteria, and Kuenenia had a higher tolerance to low temperature than Candidatus Brocadia and Candidatus Jettenia. This study clearly shows the effectiveness of anammox bioreactors for treatment of wastewater at ambient temperatures of 15-33°C.

System performance and microbial community succession in a partial nitrification biofilm reactor in response to salinity stress

The system performance and microbial community succession in a partial nitrification biofilm reactor in response to salinity stress was conducted. It was found that the NH4+-N removal efficiency decreased from 98.4% to 42.0% after salinity stress increased to 20 g/L. Specific oxygen uptake rates suggested that AOB activity was more sensitive to the stress of salinity than that of NOB. Protein and polysaccharides contents showed an increasing tendency in both LB-EPS and TB-EPS after the salinity exposure. Moreover, EEM results indicated that protein-like substances were the main component in LB-EPS and TB-EPS as self-protection in response to salinity stress. Additionally, humic acid-like substances were identified as the main component in the effluent organic matter (EfOM) of partial nitrification biofilm, whereas fulvic acid-like substances were detected at 20 g/L salinity stress. Microbial community analysis found that Nitrosomonas as representative species of AOB were significantly inhibited under high salinity condition.

Comprehensive assessment of microbial aggregation characteristics of activated sludge bioreactors using fuzzy clustering analysis

Understanding microbial aggregation dynamics in response to the often violent environmental fluctuations is important for activated sludge wastewater biotreatment practice, yet remains poorly understood. We investigated microbial aggregation process of an activated sludge reactor in response to various operating conditions of resource limitations, disinfectant and pH stresses, and quantified aggregation characteristics by employing a fuzzy clustering analysis (FCA) method. The results revealed that the FCA provided a means for comprehensive assessment of microbial aggregation dynamics of the bioreactor relying solely on simple parameter estimation. Proper disinfectant stress (of NaClO 1.00% or 2.00%) is a promising strategy to improve the comprehensive performance of microbial aggregation and sludge settleability. Nitrogen– (of C/N ratio > 40) and dissolved oxygen–limitations (of DO < 0.2 mg/L) had medium influence on the comprehensive performance of the activated sludge system, while little impacts for acidic and alkaline conditions. These quantitative estimations offer insights into the underlying bio–physicochemical processes of an activated sludge bioreactor in response to practical fluctuations that is often beyond typical assessment practice. In addition, it may represent a step towards uncoupling the complex biophysical interactions that is essential for optimized designing and proper engineering practice of biological wastewater treatment reactors.

Measuring residence time distributions of wood chips in a screw conveyor reactor

In rotating screw conveyors both the average and the distribution of the residence time influence the extent and the uniformity of the transformation. Experimenters have applied two distinct experimental approaches to obtain the residence time distribution of granular solids in longitudinal reactors: 1) measuring the mass flow rate of product at the exit from the reactor in response to a step change (either positive or negative) in the mass flow rate of feedstock into the reactor or 2) measuring the appearance of a tracer in the flow exiting the reactor in response to either a pulse or a step change addition of tracer in the inlet. We found that all three methods reveal residence time distributions that are approximately normal (i.e., symmetrical and bell-shaped), but the distribution estimated from the pulse input of tracer exhibited a long trailing tail that was not detectable in either the positive or negative step changes. Second, we demonstrated that a normal probability plot proved valuable in displaying and analyzing the residence time distribution obtained by the pulse addition of tracer. Finally, we observed that all three methods yielded mean residence times that consistently differed from the nominal values. The positive step change averaged 8% shorter, the pulse addition of tracer averaged 7% longer, and the negative step change averaged 60% longer.

Shift in microbial community structure of anaerobic side-stream reactor in response to changes to anaerobic solid retention time and sludge interchange ratio

A laboratory scale nutrient removal activated sludge system coupled with an anaerobic side-stream reactor was operated for 300 days treating real urban wastewater. A significant decrease in sludge production was obtained increasing the anaerobic solid retention time (SRTASSR) and decreasing the sludge interchange ratio (IR). In this study, the microbial community structure was analyzed and compared with the sludge reduction performance. Quantitative polymerase chain reaction analyses encoding 16 ribosomal RNA and functional genes revealed a wide diversity of phylogenetic groups in each experimental period, resulting from long solids retention time and recirculation of sludge under aerobic, anoxic and anaerobic conditions. However, decreasing SRTASSR from 10 to 2.5d and increasing IR from 27 to 100%, an increasing selection of both fermenting bacteria able to release extracellular polymeric substances and hydrolyze organic matter and slow growing bacteria involved in nutrient removal were detected and linked to the sludge reduction mechanisms.

Trichloroethylene removal and bacterial variations in the up-flow anaerobic sludge blanket reactor in response to temperature shifts.

Trichloroethylene (TCE) degradation and the variations of bacteria composition and structure in the up-flow anaerobic sludge blanket (UASB) reactor were investigated by increasing the operating temperature from 20 to 40 °C. The influent was supplemented with 36.5 mg/L of TCE. There was a rise in the chemical oxygen demand (COD) removal efficiency from 20 to 35 °C and a decline when temperature enhanced to 40 °C. It reached maximum at 35 °C. In addition, TCE removal efficiency increased with temperature varying from 20 to 35 °C, and it dropped dramatically to 78.38 % at 40 °C, which presumably because the genus of Dehalobacter, a kind of bacteria with the ability to dechlorinate TCE to the corresponding chlorinated products, was not detected at 40 °C according to sequencing results. The Illumina MiSeq platform was adopted to explore the bacteria composition and structure in response to temperature shifts. The results indicated that temperature impacted greatly on the dominance and presence of specific populations at different taxonomic levels. Importantly, the class Dehalococcoidia was detected from 25 to 40 °C, in which there were many well-known Dehalococcoides sp. strains that were capable of complete dechlorination of TCE to ethene. It also suggested the potential function of the dominant genera (non-dechlorinating bacteria and dechlorinating bacteria) in the reactor.

Control oriented modeling and optimization of one dimensional packed bed model of underground coal gasification

To account for nonlinear nature and huge model uncertainties of underground coal gasification (UCG) process, a robust model based control strategy is to be employed. The available models in the literature do not lend themselves to control applications easily. In this work a control oriented one dimensional (1-D) packed bed model of UCG is developed, which can be used in a closed loop configuration with a robust controller to maintain a desired heating value of the exit gas mixture by manipulating the flow rate of injected gases. The model is also capable of predicting time and space profiles of some important parameters, which include solid temperature, composition of exit gas mixture, rates of different chemical reactions and expected life of the UCG reactor in response to different operating conditions and coal properties. Most of these parameters are either impossible or very expensive to measure. There is uncertainty in some coal properties which is addressed by optimizing few input parameters using sequential quadratic programming (SQP) algorithm, a nonlinear optimization technique. The model results are compared with actual field trials which show a good agreement for the calorific value of exit gas.

An experimental study of basaltic glass–H2O–CO2 interaction at 22 and 50 °C: Implications for subsurface storage of CO2

A novel high pressure column flow reactor was used to investigate the evolution of solute chemistry along a 2.3m flow path during pure water- and CO2-charged water–basaltic glass interaction experiments at 22 and 50°C and 10−5.7 to 22bars partial pressure of CO2. Experimental results and geochemical modelling showed the pH of injected pure water evolved rapidly from 6.7 to 9–9.5 and most of the iron released to the fluid phase was subsequently consumed by secondary minerals, similar to natural meteoric water–basalt systems. In contrast to natural systems, however, the aqueous aluminium concentration remained relatively high along the entire flow path. The aqueous fluid was undersaturated with respect to basaltic glass and carbonate minerals, but supersaturated with respect to zeolites, clays, and Fe hydroxides. As CO2-charged water replaced the alkaline fluid within the column, the fluid briefly became supersaturated with respect to siderite. Basaltic glass dissolution in the column reactor, however, was insufficient to overcome the pH buffer capacity of CO2-charged water. The pH of this CO2-charged water rose from an initial 3.4 to only 4.5 in the column reactor. This acidic reactive fluid was undersaturated with respect to carbonate minerals but supersaturated with respect to clays and Fe hydroxides at 22°C, and with respect to clays and Al hydroxides at 50°C. Basaltic glass dissolution in the CO2-charged water was closer to stoichiometry than in pure water. The mobility and aqueous concentration of several metals increased significantly with the addition of CO2 to the inlet fluid, and some metals, including Mn, Cr, Al, and As exceeded the allowable drinking water limits. Iron became mobile and the aqueous Fe2+/Fe3+ ratio increased along the flow path. Although carbonate minerals did not precipitate in the column reactor in response to CO2-charged water–basaltic glass interaction, once this fluid exited the reactor, carbonates precipitated as the fluid degassed at the outlet. Substantial differences were found between the results of geochemical modelling calculations and the observed chemical evolution of the fluids during the experiments. These differences underscore the need to improve the models before they can be used to predict with confidence the fate and consequences of carbon dioxide injected into the subsurface.

Analysis of Reactivity Induced Accident for Control Rods Ejection with Loss of Cooling

The understanding of the time-dependent behavior of the neutron population in a nuclear reactor in response to either a planned or unplanned change in the reactor conditions is of great importance to the safe and reliable operation of the reactor. In the present work the point kinetic equation is solved numerically using stiffness confinement method (SCM). The solution is applied to the kinetic equation in the presence of different types of reactivities and was compared with different analytical solutions. This method is also used to analyze reactivity induced accidents in two reactors. The first reactor is fueled with uranium and the second is fueled with plutonium. This analysis presents the effect of negative temperature feedback with the addition of positive reactivity of the control rods to overcome the occurrence of the control rod ejection accident and damaging of the reactor. Both the power and the temperature pulse following the reactivity initiated accidents are calculated. The results are compared with previous works and satisfactory agreement is found.

An accurate solution of point reactor neutron kinetics equations of multi-group of delayed neutrons

The understanding of the time-dependent behaviour of the neutron population in a nuclear reactor in response to either a planned or unplanned change in the reactor conditions is of great importance to the safe and reliable operation of the reactor. In this study, an accurate analytical solution of point reactor kinetics equations with multi-group of delayed neutrons for specified reactivity changes is proposed to calculate the change in neutron density. The method is based on formulating a coefficient matrix of the homogenous differential equations of the point reactor kinetics equations and calculating the eigenvalues and the corresponding eigenvectors of the coefficient matrix. A small time interval is chosen within which reactivity relatively stays constant. The analytical method was applied to solve the point reactor kinetics equations with six-groups delayed neutrons for a representative thermal reactor. The problems of step, ramp and temperature feedback reactivities are computed and the results compared with other traditional methods. The comparison shows that the method presented in this study is accurate and efficient for solving the point reactor kinetics equations of multi-group of delayed neutrons.

Mathematical modeling of a three-phase trickle bed reactor

The transient behavior in a three-phase trickle bed reactor system (N2/H2O-KCl/activated carbon, 298 K, 1.01 bar) was evaluated using a dynamic tracer method. The system operated with liquid and gas phases flowing downward with constant gas flow Q G = 2.50 x 10-6 m³ s-1 and the liquid phase flow (Q L) varying in the range from 4.25x10-6 m³ s-1 to 0.50x10-6 m³ s-1. The evolution of the KCl concentration in the aqueous liquid phase was measured at the outlet of the reactor in response to the concentration increase at reactor inlet. A mathematical model was formulated and the solutions of the equations fitted to the measured tracer concentrations. The order of magnitude of the axial dispersion, liquid-solid mass transfer and partial wetting efficiency coefficients were estimated based on a numerical optimization procedure where the initial values of these coefficients, obtained by empirical correlations, were modified by comparing experimental and calculated tracer concentrations. The final optimized values of the coefficients were calculated by the minimization of a quadratic objective function. Three correlations were proposed to estimate the parameters values under the conditions employed. By comparing experimental and predicted tracer concentration step evolutions under different operating conditions the model was validated.

Roadside rest area wastewater treatment system: Performance evaluation and improvement

The performance of a decentralised sewage treatment plant located at a rest area servicing a major freeway was investigated. Long term monitoring and rigorous analyses undertaken in this study revealed several unique and challenging issues associated with such scarcely studied systems. Data collected over a six month period showed that the raw wastewater strength was well above typical household wastewater characteristics, with the average BOD5, COD, TOC, TN and TP values of 880, 4900, 350, 238 and 8 mg/L, respectively. The system performance was considerably lower than that expected of a typical wastewater treatment unit. Several shortcomings in design (e.g., inefficient aeration device and return activated sludge system) and inconsistencies in maintenance practice were identified and some remedial measures were proposed and tested. Of particular interest were the increase of the dissolved oxygen (DO) concentration (from 0.5 to 4 mg/L) and the simultaneous significant improvement of COD and TOC removals in the aerobic reactor in response to the redesigning of the aeration system. The removal of nitrogen, however, remained quite low as expected.

Spatial Reactor Dynamics and Thermo Hydraulic Behavior Simulation of a Large AGR Nuclear Power Reactor in Response to a Reactivity Step Change Disturbance

In this article, two-dimensional partial differential equations with time representation of nuclear power reactor kinetics are considered for spatial reactor dynamics and thermo hydraulic behavior analysis of a large thermal advanced gas cooled reactor (AGR) type used for nuclear power generation. The equations include the neutron flux equation and delayed neutron precursor concentration, together with taking into account the equations to represent the thermo hydraulic behavior of the fuel, coolant and moderator temperatures. These equations are solved numerically using the finite difference method. For time propagation, an implicit method is applied. The desired initial condition for the reactor to stay at stable critical condition is established by finding the correct value of reactivity. The reactivity disturbance effect in the reactor is studied for different cases and presented for high reactivity values. The model was developed for the analysis of a large AGR with 2000 MWe for future power generation. The results show that the model not only behaves stably but also predicts the results physically for all the various parameters.