Once-through Steam Generator Research Articles

Numerical investigation on heat transfer characteristics of liquid metal cross flow over tube bundles

Helical tube Once Through Steam Generator (OTSG) could further improve the compactness of liquid metal cooled reactors. The heat transfer characteristics of liquid metal cross flow over inline tube bundles is important for the design of helical tube OTSG. In order to consider the low molecular Prandtl number (Pr) effects on turbulent heat flux, different turbulent Prandtl number (Prt) models are validated by a Direct Numerical Simulation (DNS) database for convective Poiseuille flow. Applicability of the Prt and turbulence models for convection with flow separation or adverse pressure gradients is further assessed by DNS results of backward-facing step convection with a fluid Prandtl number of 0.025. Heat transfer of liquid metal cross flow over tube bundle is simulated with k-ω SST turbulence model and the selected Prt model. Heat transfer performances of cross flow over tube bundles with different Pr, Reynolds number (Re) and geometry parameters are simulated. The very large molecular thermal diffusivity (a) of liquid metal results in different temperature distributions and heat transfer characteristics in tube bundles compared with conventional fluids (Pr around and higher than 1). The normalized heat transfer coefficients of liquid metal are enhanced on the upwind side of the tubes and are weaken on the rear side of the tubes compared with conventional fluids. The increments of Pr and Re both weaken the low Pr effect of liquid metal and lead to circumferential heat transfer characteristic more similar to that of conventional fluids. Geometry parameters have little impact on the time and area averaged Nu of liquid metal cross flow over tube bundles. A new Nusselt (Nu) number correlation with wide applicable ranges is proposed based on the numerical results.

An improved-MPGA and its application in OTSG load reduction characteristics optimization

Multi-population genetic algorithm (MPGA) perfectly inherits the advantages of standard genetic algorithm (SGA), and improves the global search performance of SGA by introducing multiple populations. However, its optimization results depend on the initial solution and the search ability is weakened in the later calculation process. To overcome these disadvantages, an initial population generation method based on the idea of cluster analysis was improved to improve the diversity of the population. To balance global exploration ability and local search ability, different crossover operation values and different mutation operation values were assigned to different populations. The algorithm is tested by using classical functions of different dimensions and the results show that the improved algorithm has better robustness. What’s more, the Improved-MPGA is introduced to optimize parameters set in the reactor design control scheme. Parameters that can meet the requirements of reactor power overshoot and dimensionless evaluation index are obtained. And the results show that the load reduction characteristics of the Once-through Steam Generator (OTSG) under the optimal scheme are well improved compared with the design scheme.

A load following control strategy for Chinese Modular High-Temperature Gas-Cooled Reactor HTR-PM

Chinese Modular High-Temperature Gas-Cooled Reactor HTR-PM is a promising energy source for comprehensive utilization. The original coordinated control strategy of HTR-PM is a “turbine following reactor” strategy, which is not able to perform load following operation. To have more flexible operation capability, this paper presents a load following control strategy for HTR-PM. A balance of plant (BOP) model of HTR-PM is first developed for dynamic simulation, including the once through steam generator (OTSG), turbine generator, condenser, feed water heater, deaerator, feed water pump, valves and piping system. Then the original coordinated control strategy and the proposed load following control strategy are clarified, and model validation, simulation results are documented respectively. For turbine trip without reactor shutdown from 100%PFP transient, the steam dump strategy is improved to meet the safety requirements. Results demonstrate that the proposed load following strategy can meet the requirements of safety operation, which is desirable for participation in peak load regulating of power system.

Thermal-hydraulic stability investigations of once through helically coiled steam generators for SMR applications

Once-through steam generators with helically-coiled tubes (OTSG-HC) are susceptible to density wave instability phenomena. In order to investigate the stability performance of the CAREM-25 OTSG-HC, a new stability model is presented. Such a model being linear and strictly non-diffusive is able to represent arbitrary heat flux profiles, which is essential to study realistic cases. The OTSG-HC tubes are modeled by three different regions representing the liquid zone, the boiling zone, and the superheated vapor zone. Moreover, the coolant from the primary side is coupled with a model representing the phenomena present in CAREM-25. A convenient parameter space for stability representation is found and used to characterize the CAREM-25 OTSG-HC stability performance.In this work, special attention is paid to the calculation of the friction pressure losses in the coolant flowing through the helically coiled tubes. For this reason, a best estimate correlation is implemented and used in the model together with a sophisticated axial power profile model which allows representing realistic boundary conditions. As a result, it is confirmed the crucial importance of correctly representing the axial power profile transferred in the helically coiled tubes. In addition, it is found the commonly used uniform axial power profile leads to non-conservative stability predictions particularly at low power conditions. The latter is explained by the underestimation of the boiling and superheating lengths, which is proved to have a destabilizing effect.Stability maps are presented in terms of the transferred power and the primary coolant operational pressure, i.e. QSG-PPri plane. It is observed that when modeling realistic power profiles, the less stable operational condition of the OTSG-HC is found at very low powers, for all analyzed range of primary pressures. In addition, a weak stability dependence of the OTSG-HC with the pressure of the primary side is observed. Finally, the system is stabilized in the region of interest and its stability performance quantified.

Carbon intensity of in-situ oil sands operations with direct contact steam generation lower than that of once-through steam generation

The oil sands resource in Western Canada are a considerable energy asset for Canada, but the energy and emissions intensities of producing this petroleum resource are substantially worse than that of conventional petroleum resources. With requirements for carbon-intensity reduction, new, less emissive recovery processes must be developed to enable continued production and cash flow from this important resource. The largest source of carbon emissions on in-situ in oil sands recovery processes, such as Steam-Assisted Gravity Drainage (SAGD), is steam generation, where natural gas is combusted. An alternative to conventional steam generation is Direct Contact Steam Generation (DCSG), where steam is generated in the combustion flame, and with rich oxygen combustion, the steam plus carbon dioxide mixture is injected into the reservoir. We explore the carbon intensity of DCSG and compare it to existing once-through steam generation using life cycle analysis. The results reveal considerable environmental benefits with nearly 30% emission reduction when applying a DCSG in SAGD instead of an OTSG. The results suggest that oil sands operators could improve their emissions intensity significantly if they adopted DCSGs in their operations.

Numerical Investigation of Hot Helium Flow Homogenizer on Inter-Unit Flow Rate Uniformity of HTGR Once Through Steam Generator

High working temperature is a major feature of the high temperature gas-cooled reactor (HTGR). The helical tube once through steam generator (OTSG) should maintain appropriate temperature uniformity. The temperature non-uniformity of the HTGR OTSG includes the in-unit and inter-unit temperature non-uniformity, while the latter is mainly induced by the inter-unit flow rate non-uniformity of primary-side hot helium, which is significantly affected by the inlet structure. In this work, a new inlet structure with a hot helium flow homogenizer is designed, and its flow distribution characteristics are numerically investigated. Accordingly, the optimal geometrical parameters are determined, such as the circular hole diameter on the end wall, the square hole size, and arrangement on the cylinder wall. Increasing the resistance of the flow homogenizer with non-uniformly arranged square holes (NUASHs) can improve inter-unit flow rate uniformity because it decreases the effect of static pressure difference caused by dynamic pressure. Two design parameters (resistance coefficient and flow area ratio of the square hole on both sides) are introduced to evaluate the structure effect of the hot helium flow homogenizer on inter-unit flow rate distribution. They are recommended within the ranges of (7.81–22.42) and (0.53–1.64), respectively. In these recommended ranges, the suction phenomenon near the hot helium inlet can be effectively suppressed, with the critical resistance coefficient of 7.63. By coupling with 19 heat exchange units, the overall performance of the hot helium flow homogenizer is better than that of the current inlet structure with a baffle, with the maximum inter-unit flow rate deviation decreased from 2.97% to 0.30%. This one-magnitude enhancement indicates that the hot helium flow homogenizer with NUASHs is a promising solution to improve inter-unit flow rate uniformity of the HTGR OTSG.

Efficient Water Softening in Heavy Oil Steamflood Operations

Summary Pigging of once through steam generators (OTSGs) has indicated various types of scales, the most predominant of these being silicates of hardness causing ions. It was noted that scaling propensity can potentially go up with higher steam quality (SQ) as the reject stream gets concentrated with ions. However, models suggested that there are benefits of higher SQ in enhancing fuel savings (8%) and electricity savings (2%) when SQ was increased by 20%. The challenges of higher SQ were noted in terms of increased scaling tendency and therefore the need for improved softening. In Field D, the service cycle, the backwash cycle, and the brining cycle were optimized leading to a gain in throughput and reduction in salt consumption. Service cycle improvement gained 30 to 130% in throughput between two regenerations, backwash cycle improvement by fluidizing the bed to nearly 35% helped gain 10% in throughput, and reduction of brining cycle from 75 to 48 minutes helped reduce salt consumption by 56% without impacting the throughput. In Field B, a 6-month pilot revealed that shallow shell resins where ion exchange is more efficient due to inert core (better intraparticle diffusion control) can enhance the throughput by 30 to 80% and simultaneously reduce the number of regenerations by 15 to 30%. Resin fouling is still a major challenge to contend with as oil can foul the resin and throughput can decline by 0.5 to 3 folds. In a plant operation, where there are multiple softener and brine vessels, there is a need to optimize them as a system. Reliability, availability, and maintainability (RAM) models are used in Field C to (a) address equipment configuration optimization with impact on capital capacity expansion project scope, (b) understand how net soft water delivery capacity was affected by increases in inlet hardness, and (c) assess through a comparison scenario if the large cost of addressing the valve issue in an upstream nutshell filter was worth the lost production opportunity related to unplanned downtime.

Pressure control of Once-through steam generator using Proximal policy optimization algorithm

Due to the strong coupling characteristics of the once-through steam generator(OTSG), the outlet pressure control is difficult. The control system using the Proximal Policy Optimization(PPO) algorithm is designed to control the outlet steam pressure of OTSG. The double-layer controller is designed in two layers, the upper layer is the agent using the PPO algorithm to optimize the parameters of the PID in real-time to obtain better control performance. The bottom layer is the PID controller, which receives the commands from the upper layer to directly regulate the feed water valve of OTSG. In the training process of the controller agent, by adopting deep neural network approximation as the approximator of the critic network and actor-new network, good generalization performance is obtained. Compared with the PID controller, the simulation experiment result shows that the method not only has a good tracking ability but also has a good anti-interference ability.

Control system design for the once-through steam generator of lead–bismuth cooled reactor based on classical control theory

The performance of once-through steam generator (OTSG) steam pressure control is directly related to the safe and stable operation of the lead–bismuth cooled reactor system. In this paper, the OTSG of a small lead–bismuth cooled reactor is investigated and its feedwater controller is designed based on classical control theory. A nonlinear OTSG model is first established based on the movable boundary approach, and a nonlinear feedwater system model is developed based on law of similarity and momentum conservation. Then the corresponding linearized models are obtained. On this basis, two control strategies are proposed for regulating the feedwater valve and adjusting the steam valve, and the controller is designed based on the linearized model using the analytical method. Finally, the controller is simulated using a high-order nonlinear model, which shows that both proposed control strategies can effectively control the steam pressure.

Design of Control System of Once-Through Steam Generator Based on Proximal Policy Optimization Algorithm

Because of the characteristics of the small water volume of OTSG, it is hard to control the outlet steam pressure when the load is changed or disturbed. This study is devoted to the control of the once-through steam generator (OTSG). A double-layer controller based on the PPO algorithm is proposed to control the outlet steam pressure of OTSG. The bottom layer is the PID controller; it directly regulates the OTSG feed water valve and then controls the steam pressure. The top layer of the controller is the agent based on the PPO algorithm, which is responsible for optimizing the parameters of the PID in real time to obtain better control performance. The agent chooses PID parameters as actions to the environment, and then, the reward value is obtained through the reward function of the environment which enables online learning of the agent. Compared with the PID controller, the simulation experiment result shows that the method not only has a good control performance but also has a good anti-interference ability.

Fault-tolerant control system for once-through steam generator based on reinforcement learning algorithm

Based on the Deep Q-Network(DQN) algorithm of reinforcement learning, an active fault-tolerance method with incremental action is proposed for the control system with sensor faults of the once-through steam generator(OTSG). In this paper, we first establish the OTSG model as the interaction environment for the agent of reinforcement learning. The reinforcement learning agent chooses an action according to the system state obtained by the pressure sensor, the incremental action can gradually approach the optimal strategy for the current fault, and then the agent updates the network by different rewards obtained in the interaction process. In this way, we can transform the active fault tolerant control process of the OTSG to the reinforcement learning agent's decision-making process. The comparison experiments compared with the traditional reinforcement learning algorithm(RL) with fixed strategies show that the active fault-tolerant controller designed in this paper can accurately and rapidly control under sensor faults so that the pressure of the OTSG can be stabilized near the set-point value, and the OTSG can run normally and stably.

Evaluation of the thermal-hydraulic performances of a once-through steam generator in nuclear fusion applications

After decades of operation in nuclear power plants, Once-Through Steam Generators (OTSGs) were recently proposed for nuclear fusion applications. In particular, they are supposed to be installed in the primary cooling systems of the European Union Demonstration fusion power plant (EU-DEMO). One of the key reactor components is the Breeding blanket (BB). Among the BB concepts that are currently under study, Water-Cooled Lithium-Lead (WCLL) option was considered for this work. The WCLL blanket is divided in two main subsystems, the breeder zone (BZ) and the first wall (FW), each one provided with an independent cooling circuit, named Primary Heat Transfer System (PHTS). Thermal power removed from BB by BZ and FW PHTS is driven to the Power Conversion System (PCS) to be converted into electricity. The thermal coupling is ensured by two OTSGs per system. At the Department of Astronautical, Electrical and Energy Engineering (DIAEE) of Sapienza University of Rome, a simulation activity was carried out to understand the component thermal-hydraulic behavior during DEMO normal operations. For calculation purposes, a full model of the steam generator was prepared by using a modified version of RELAP5/MOD3.3 system code. The computational activity performed allows to preliminary characterize the OTSG thermal-hydraulic performances during both pulse and dwell phases.

Origin of Steam Contaminants and Degradation of Solid-Oxide Electrolysis Stacks

Two once-through steam generators and a combination of a steam generator and a gas preheater for supplying feed gases to solid-oxide electrolysis stacks were evaluated for their carryover characteristics of contaminants from the feed-water into the steam phase. The concentrations of various trace impurities in the steam were determined by sampling the steam condensates and screening them with inductively coupled plasma–mass spectrometry for 19 elements and liquid ion chromatography and continuous flow analysis for chloride and ammonium. Steam-soluble species such as boric acid undergo complete volatilization and transfer into the steam phase. During unstable evaporation in the steam generators an extensive physical carryover of alloying metal species was observed. At realistic operation conditions for steam electrolysis, the gas preheater caused a considerable release of silicon into the steam phase. Two stack experiments were performed with common preheater temperatures and showed largely increased cell voltage degradation at higher operation temperatures. The post-test chemical analysis of cell samples revealed significant concentrations of silicon in the samples that are regarded as primary cause for increased degradation. These findings could partially explain the wide spread of degradation rates reported for solid-oxide steam electrolysis experiments.

Three-Dimensional CFD Model Development and Validation for Once-Through Steam Generator (OTSG): Coupling Combustion, Heat Transfer and Steam Generation

The current research studies the coupled combustion inside the furnace and the steam generation inside the radiant and convection tubes through a typical Once-Through Steam Generator (OTSG). A 3-D CFD model coupling the combustion and the two-phase flow was developed to model the entire system of OTSG. Once the combustion simulation was converged, the results were compared to field data showing a convincing agreement. The CFD analysis provides the detailed flow behavior inside the combustion chamber and the stack, as well as the two-phase flow steam generation process in the radiant and convective sections. The flame shape and orientation, the velocity, the species, and the temperature distribution at the various parts of the furnace, as well as the steam generation and the steam distribution inside the pipes were investigated using the developed CFD model

Experimental investigation on the heat removal capacity of secondary side passive residual heat removal system for an integrated reactor

An experimental facility named Intermediate Heat Exchange Loop (IHEL) is built to simulate the secondary side passive residual heat removal system. The IHEL mainly includes a helically coiled Once-Through Steam Generator (OTSG), a condenser, a cooling water tank, and associated piping and valves. The steam is produced by the water-to-water heat exchange method in the SG secondary side. The heat from the SG primary side is carried to the cooling water tank by the IHEL through a two-phase natural circulation method. The natural circulation characteristics of the IHEL under the low pressure (nearly atmospheric pressure) and low water inventory conditions are investigated. The result shows that the heat removal capacity of the IHEL is good at these conditions, and it is determined by the parameters of the SG primary side and IHEL together. In the present test conditions, for the parameters of the SG primary side, the heat removal rate will increase with the increase of mass flow rate or heat source temperature. For the parameters of the IHEL, the heat removal rate is not sensitive to the initial non-condensable gas content. The heat removal rate is not sensitive to the resistance level when the heat source temperature is low, while it will decrease with the increase of resistance level when the heat source temperature is high. A very meaningful conclusion is that the natural circulation has been established successfully in the very lower water inventory condition (L = 14.8% in the present study), and the heat removal rate almost remains constant with the increase of the water inventory, which indicates the IHEL is robust and is beneficial for the system design.

Study on startup characteristics of prototype once-through steam generator for China fast reactor

Once-through steam generator (OTSG) is a vital heat transfer equipment of China Fast Reactor-600 (CFR-600), and its startup characteristics are of great importance to the safe operation of nuclear power plant. In this paper, first of all, the Thermal-hydraulic analysis Code Of Sodium heated once-through Steam generator (TCOSS) has been further optimized, and the code was introduced in the text. Secondly, the Steam Generator Test Facility (SGTF) and India Commercial Fast Breeder Reactor (CFBR) were selected for steady-state verification, and the feedwater flow rate increase condition of the prototype OTSG experiment was selected for transient verification. All the verifications were in good compliance, which proved the reliability of the TCOSS code for subsequent calculation of startup conditions. Finally, taking the prototype OTSG of CFR-600 as the research object, and the transient characteristics of cold startup and hot startup have been predicted and analyzed. The results show that both processes of cold startup and hot startup for prototype OTSG can be realized stably. The simulation results of startup processes showed a good agreement with expectations, which can provide support for the development of CFR-600 steam generator.

Design of digital nuclear power small reactor once-through steam generator control system

The once-through steam generator used in the small modular reactor needs to consider the stability of the outlet steam pressure and steam superheat of the secondary circuit to achieve better operating efficiency. For this reason, this paper designs a controllable operation scheme for the steam pressure and superheat of the small reactor once-through steam generator. On this basis, designs a variable universe fuzzy controller, first, design the fuzzy control rules to make the controller adjust the PI controller parameters according to the change of the error; secondly, use the domain adjustment factor to further subdivide the input and output domain of the fuzzy controller according to the change of the error, to improve the system control performance. The simulation results show that the operation scheme proposed in this paper have better system performance than the original scheme of the small reactor system, and controller proposed in this paper have better control performance than traditional PI controller and fuzzy PI controller, what's more, the designed control system also showed better anti-disturbance performance in lifting experiment between 100% and 80% working conditions. Finally, the experimental platform formed by connecting the digital small reactor with Matlab/Simulink through OPC(OLE for Process Control) communication technology also verified the feasibility of the proposed scheme.

Research on the feedwater control of a multimodule once through steam generator parallel system

• A complete multimodule parallel OTSG system model is established. • A coordinated control scheme for parallel feedwater of multimodule OTSG is proposed. • Feedforward can improve the response of system in flowrate disturbance. • Wrong control logic may have good control performance, but it hides great danger. Nuclear power plants mostly use the multimodule once-through steam generator (OTSG) parallel system as heat exchange equipment, but the current research mostly focuses on a single OTSG. The huge system brings great challenges to modeling and control system design. In this paper, the ODE method is used to model the main equipment of a multimodule parallel system based on mechanism. The results show that the maximum error is 3.77%. Based on the characteristics of parallel system, a new control strategy is proposed and compared with other control strategies by introducing four typical disturbances. The results show that the proposed feedwater control scheme can effectively solve the coordination between loop valve and module valve of multimodule parallel OTSG system, and the coordination between main feedwater pump and loop valve. Compared with Method 1, the response time of Method 2 in sodium temperature disturbance is shortened by 319 s and the temperature fluctuation is reduced by 1.67 times, but Method 2 has potential safety hazards. Method 1 can improve the system response speed when the sodium flowrate and water flowrate change. Compared with Method 3, the outlet sodium temperature fluctuation of Method 1 is reduced by 10 times and the adjustment time of Method 1 shortens 141.9 s in the case of feedwater flow disturbance.

Investigation on Thermal Hydraulic Characteristics of Helical Coil Once-Through Steam Generator Under Ocean Conditions

Investigation on Thermal Hydraulic Characteristics of Helical Coil Once-Through Steam Generator Under Ocean Conditions

Control of steam bottoming cycles using nonlinear input and output transformations for feedforward disturbance rejection

In this paper we analyze the control problem for a steam cycle that produces power by recovering waste-heat from gas turbines on an offshore installation. The waste-heat recovery unit is based on once-through steam generator technology. The main disturbances are large variations of the gas turbine exhaust gas flowrate and temperature. We analyze the effect of these disturbances on the operation of the steam cycle, specifically for the superheated steam pressure and temperature. We compare the performance of different decentralized control strategies based on standard PID-controllers and nonlinear feedforward. We consider floating pressure and constant pressure operation strategies. For steam temperature control we implement feedback, and feedback in combination with nonlinear input and output transformations for feedforward disturbance rejection. These transformations are based on the steady-state energy balance on the waste heat recovery unit, while for simulation purposes we use a high-fidelity dynamic model of the bottoming cycle designed to minimize the weight and volume. The outcome from this work can be used to propose control strategies for coordinating a combined cycle (gas turbine with a steam bottoming cycle) that can operate with large and rapid changes in power demand.