Nuclear Turbine Research Articles

Modeling method of steam turbine generator set based on cascade feedforward PID coordinated control system

Objective In order to quickly and accurately evaluate the reliability of a steam turbine generator set under actual working conditions, it is necessary to construct a visual model of the set. Method Based on an analysis of a mathematical model of a traditional steam turbine generator set, a cascade PID method is proposed and a feedforward link added. Then, based on a Matlab Simulink environment, a visual model of the optimized control of a steam turbine generator set is built, and the steady state and fault state of the set are simulated and analyzed with the help of the simulated fault module. Results The simulation results show that the output speed of the steam turbine generator set based on cascade feedforward PID coordinated control is higher than the given speed for a short time when starting under load, but the stable value is quickly restored under the action of the control feedback. In the case of failure, the rotation speed can also fluctuate within a given range. Conclusion This modeling method of a steam turbine generator set based on a cascade feedforward PID coordinated control system can be popularized in the modeling of nuclear turbine propulsion systems, providing an analytical means of achieving the visual analysis of the unit's operation state, and saving both cost and time.

A Guiding Framework for Vetting the Internet of Things

Like any emerging and disruptive technology, multiple obstacles are slowing down the Internet of Things (IoT) expansion for instance, multiplicity of things’ standards, users’ reluctance and sometimes rejection due to privacy invasion, and limited IoT platform interoperability. IoT expansion is also accompanied by the widespread use of mobile apps supporting anywhere, anytime service provisioning to users. By analogy to vetting mobile apps, this paper addresses the lack of principles and techniques for vetting IoT devices (things) in preparation for their integration into mission-critical systems. Things have got vulnerabilities that should be discovered and assessed through proper device vetting. Unfortunately, this is not happening. Rather than sensing a nuclear turbines steam level, a thing could collect some sensitive data about the turbine without the knowledge of users and leak these data to third parties. This paper presents a guiding framework that defines the concepts of, principles of, and techniques for thing vetting as a pro-active response to potential things vulnerabilities.

Remaining Useful Strength (RUS) Prediction of SiCf-SiCm Composite Materials Using Deep Learning and Acoustic Emission

Prognosis techniques for prediction of remaining useful life (RUL) are of crucial importance to the management of complex systems for they can lead to appropriate maintenance interventions and improvements in reliability. While various data-driven methods have been introduced to predict the remaining useful life (RUL) of machinery systems or batteries, no research has been reported on the remaining useful strength (RUS) prediction of silicon carbide fiber reinforced silicon carbide matrix (SiCf-SiCm) materials with pivotal role in its potential usage as a structural material in nuclear reactors and turbine engines. Knowledge of its degradation process is of the utmost importance to the manufacturers. For this purpose, two approaches based on the machine-learning techniques of random-forest (RF) and convolutional neural network (CNN) are proposed to predict the RUS of SiCf-SiCm using only acoustic emission (AE) signals generated during the material’s stress applying process. Experimental results show that the CNN models achieved better predictive performance than the RF models but the latter with expert-engineered features achieves better prediction for AE signals in the early stage of degradation. Additionally, our results demonstrate that both models can correctly predict the SiCf-SiCm RUS as evaluated by our robust testing method from which the best average root mean square error (RMSE) and Pearson correlation coefficient of 3.55 ksi units and 0.85 were obtained.

Predictive study of Inconel718 mechanical properties at sub-zero temperatures

ABSTRACT Inconel has applications in nuclear, chemical industry, aircraft and gas turbines due to their excellent mechanical properties in static and dynamic conditions. This study characterised Inconel 718 and predicted mechanical properties at sub-zero temperature. Experiments are conducted on the tensile specimens at different process parameters like temperature ranging from room temperature to −70°C, strain rate of 0.01/s and orientations in Rolling, 45° and transverse direction, and studied the fracture surface of the specimens to understand the mechanical behaviour of materials using a Scanning Electron Microscope. This study mainly focuses on calculation of the properties from tensile stress–strain graph at different temperatures. Using these properties trained an Artificial Neural Network (ANN) model to predict mechanical properties of the material at different temperatures, and validated the ANN output data with experimental data. Predicted values have very good correlation with actual values.

Effect of welding residual stress on operating stress of nuclear turbine low pressure rotor

The purpose of this study is to investigate the effect of welding residual stress on operating stress in designing a nuclear turbine welded rotor. A two-dimensional axisymmetric finite element model is employed to calculate the residual stress before and after post weld heat treatment (PWHT), and then the superposition of residual stress after PWHT and operating stress at normal speed and overspeed were discussed. The investigated results show that operating stress can be affected significantly by welding residual stress, and the distribution trend of superposition stress at the weld area is mainly determined by welding residual stress. The superposition of residual stress and operating stress is linear superposition, and the hoop stress distribution of superposition stress is similar with the distribution of residual stress. With the increasing overspeed, the distribution pattern of the hoop superimposed stress remains almost unchanged, while the stress level increases.

Bayesian Long Short-Term Memory Model for Fault Early Warning of Nuclear Power Turbine

Fault early warning of equipment in nuclear power plant can effectively reduce unplanned forced shutdown and avoid significant safety accidents. This paper presents a Bayesian Long Short-Term Memory (LSTM) neural network method for fault early warning method of nuclear power turbine. The Long Short-Term Memory neural network prediction model is developed to address data uncertainty while taking into account complicated situation of the equipment operation. Quantitative reliability validation method is established based on Bayesian inference. A wavelet packet multi-scale time-frequency analysis is employed for data denoising. A Probabilistic Principal Component Analysis (PPCA) method combined with key factor analysis is proposed for dimension reduction and dealing with the data uncertainty. The principal component inverse search method is developed to identify the critical factors mainly contributing to the turbine fault. Numerical results indicate that the proposed novel model is validated with Bayesian confidence of 92% by using the real-world steam turbine data and the model can provide accurate warning in the early creep stage of the fault.

Dry friction damping blade structural vibration analysis of ocean nuclear power platform steam turbine

The ocean nuclear platform turbine blade is studied. Based on the nonlinear finite element method, the contact pressure load on the friction contact surface is analyzed. The three-dimensional finite element vibration characteristic numerical model is established. The spring damping element is applied to describe the friction damping characteristics between the shroud and snubber contact surfaces of adjacent blades. The equivalent stiffness and damping coefficients of the shroud and snubber contact surfaces are calculated based on the experiment results of dry friction dynamic characteristics with blade material. The structural vibration response under different contact pressure load is analyzed by harmonic balance method. Furthermore, the simulation results are compared with the experiment results. The results show that the blade structural vibration response decreases significantly as a result of the damping between friction contact surfaces. The relative error of the resonance frequency for free blade is only 0.53%. The relative errors of modal damping ratio for the free blade and constrained blade are less than 2.65% and 20% respectively. The relative error of the constrained blade resonance amplitude is less than 15%. The reliability of the numerical analysis method is verified by comparison with experimental data.

Multi-physical fields of rotor windings with axial-radial ventilation system for 1100 MW nuclear half-speed turbine generator

Due to super capacity, nuclear turbine generator is designed with high electromagnetic and thermal load, it easily leads to overheat of rotor windings. Thus, rotor is cooled by hydrogen in complicated ventilation system to protect generator from overheat. It is very difficult to determine the rotor temperature distribution by accounting the influence of complicated ventilation system and rotor rotating. In light of this situation, taking a 1100 MW nuclear turbine generator as an example, the synergistic coupling method is proposed to investigate the multi-physical fields. Firstly, the wind resistance network considering complicated ventilation circuit and rotor rotation is established, and the fluid pressure and velocity distributions are solved. The accuracy is validated by comparing with experimental results. Secondly, three-dimensional fluid flowing and heat transfer coupling mathematical model of the rotor is established to solve multi-physical fields based on the calculation results of the wind resistance network above. The rotor winding and the fluid variables are investigated and the rotor windings temperature is measured to verify the accuracy of the calculation model. Lastly, the gradient of velocity, heat transfer coefficient, fluid velocity components and resultant velocity are analyzed. The contents can provide theoretical basis for design of super capacity turbine generator.

Pitting behavior of welded joint and the role of carbon ring in improving corrosion resistance

Pitting behavior and its transition process to cracking were investigated after immersion at 180 °C in pure water and 3.5% NaCl solution for NiCrMoV welded joint manufactured by narrow gap submerged arc welding technology. Dislocation outcrops in the interior of the grains formed small anodes, which mainly attributed to the pit growth for weld metal (WM) and heat-affected zone (HAZ). The serious pitting in WM was induced by strong strain concentration generating from welding thermal stress. Competition behavior between pitting and cracking for WM in pure water and chloride solution was also proposed. Chloride accelerating anodic dissolution caused serious pitting that predominated over cracking in chloride solution, while cracking preferentially formed and replaced mild pitting in pure water. In addition, carbon rings around pits decreased pitting susceptibility for HAZ. The formation of carbon rings generated from carbon diffusion was driven by strain aging and selective corrosion. Moreover, micro-Raman manifested the carbon ring was covered by protective oxide layer that inhibited pit growth and improved pitting resistance for HAZ. This work contributes to the understanding of pit growth and crack initiation for welded joint used in nuclear turbine.

Stability Prediction of the Nuclear Turbine Blades During Wet Steam Nonequilibrium Condensation Process

Stability of the nuclear turbine blades is difficult to be accurately predicted because the wet steam load (WSL) as well as its induced equivalent damping and stiffness during nonequilibrium condensation process (NECP) is hard to be directly calculated. Generally, in design, NECP is assumed as equilibrium condensation process (ECP), of which the two-phase temperature difference (PTD) between gaseous and liquid is ignored. In this paper, a novel method to calculate the WSL-induced equivalent damping and equivalent stiffness during NECP based on the combined microperturbation method (MPM) and computational fluid dynamics method (CFDM) was proposed. Once the WSL-induced equivalent damping and equivalent stiffness are determined, the stability of the blade-WSL system, of which the blade was modeled by a pretwisted airfoil cantilever beam, can then be predicted based on the Lyapunov's first method. Besides, to estimate the effects of PTD, comparisons between the WSL-induced equivalent damping and equivalent stiffness as well as the unstable area during NECP and ECP were presented. Results show that the WSL-induced equivalent damping and equivalent stiffness during NECP are more sensitive to the inlet boundary due to the irreversible heat transfer caused by PTD during NECP. Accordingly, the unstable area during NECP is about three times larger than during ECP.

Stress corrosion crack growth rate of welded joint used for low-pressure rotor of nuclear turbine in oxygenated pure water at 180 °C

Stress corrosion cracking (SCC) growth behavior of NiCrMoV steels welded joint of nuclear steam turbine was systematically investigated, which was conducted in pure water with 8 ppm oxygen at 180 °C. Crack growth rate (CGR) was used to evaluate the susceptibility of SCC for WM with fine bainites and HAZ with coarse prior austenite grains, simultaneously the cracking mechanisms for both zones were explored. WM has lower CGR than HAZ under the similar stress intensity factor, showing it had higher resistance to SCC. Oxygen promoted the nucleation and propagation for cracks. The oxygen diffusion zone (ODZ) formed in front of inner oxide layer through continually diffusion of oxygen in grain, and caused the embrittlement and transgranular cracking for WM. However, fine grains in WM could bear crack driving force and reduce stress concentration to increase the resistance for transgranular crack propagation. As for HAZ, the chromium-rich carbides along the prior austenite grain boundaries (PAGBs) accelerated the intergranular oxidation and facilitated the formation of intergranular oxide zone (IOZ). Thus, the crack could easily propagate in PAGB, and lead to higher CGR for the HAZ.

Analysis of the thermal-hydraulic parameters of VVER-1200 due to loss of coolant accident concurrent with loss of offsite power

This paper aims to evaluate severe accident in VVER-1200. The consequences of severe accident depend greatly on containment safety features and its performance in retaining radioactive material. The specific type of LOCA includes hot leg or cold leg break in the primary coolant circuit. For both simulations a loss of offsite power is also initiated after 500 seconds. It shows that the Reactor Cooling System (RCS) pressure drops from 155 bars to below 5 bars in both the cases to reduce RCS leakage. Reactor Core Thermal Power, Nuclear Flux Power and Turbine Load fall significantly after 23.5 seconds because of the occurrence of Reactor Trip. Both HA-1 and HPI are activated to prevent Core damage. The flow of HPI drops to zero after the loss of offsite power. The temperature of the fuel falls below 200 ºC within 150-200 seconds of the malfunction in both the cases. The corresponding pressure, temperature, void fraction of reactor etc. are analysed in this paper. The results obtained satisfactorily meet all PSAR (Preliminary Safety Assessment Report) data regarding Loss of Coolant Accident (LOCA). The results provide useful information about safety features of VVER-1200 in case of Loss of Coolant Accident (LOCA) concurrent with loss of offsite power.

Energy Analysis of the S-CO2 Brayton Cycle with Improved Heat Regeneration

Supercritical carbon dioxide (S-CO2) Brayton cycles (BC) are promising alternatives for power generation. Many variants of S-CO2 BC have already been studied to make this technology economically more viable and efficient. In comparison to other BC and Rankine cycles, S-CO2 BC is less complex and more compact, which may reduce the overall plant size, maintenance, and the cost of operation and installation. In this paper, we consider one of the configurations of S-CO2 BC called the recompression Brayton cycle with partial cooling (RBC-PC) to which some modifications are suggested with an aim to improve the overall cycle’s thermal efficiency. The type of heat source is not considered in this study; thus, any heat source may be considered that is capable of supplying temperature to the S-CO2 in the range from 500 °C to 850 °C, like solar heaters, or nuclear and gas turbine waste heat. The commercial software Aspen HYSYS V9 (Aspen Technology, Inc., Bedford, MA, USA) is used for simulations. RBC-PC serves as a base cycle in this study; thus, the simulation results for RBC-PC are compared with the already published data in the literature. Energy analysis is done for both layouts and an efficiency comparison is made for a range of turbine operating temperatures (from 500 °C to 850 °C). The heat exchanger effectiveness and its influence on both layouts are also discussed.

Wet Steam Nonequilibrium Condensation Flow-Induced Vibrations of a Nuclear Turbine Blade

Condensing flow induced vibration (CFIV) of the rotor blade is a tough problem for designers of nuclear turbines because nonequilibrium condensing flow excitation (NECFE) is hard to be directly modeled. Generally, in design, NECFE is assumed as equilibrium condensing flow excitation (ECFE), of which the pressure fluctuations caused by phase temperature difference (PTD) between gaseous and liquid are ignored. In this paper, a novel method to calculate the equivalent load of NECFE based on the principle of virtual work was proposed. This method could consider the effects of PTD-induced pressure fluctuations by simulating nonequilibrium condensation with ANSYS cfx, and improve computational efficiency. Once the equivalent NECFE load is determined, CFIV of the rotor blade, which was modeled as a pretwisted asymmetric cantilever beam, can then be predicted by the finite element method (FEM). Additionally, to estimate the effects of PTD-induced pressure fluctuations, comparisons between NECFE and ECFE as well as their induced vibrations were presented. Results show that PTD in nucleation area could change the position and type of shock waves, restructure the pressure distribution, as well as enhance the pressure fluctuations. Compared with ECFE, the frequency ingredients and amplitude of the equivalent NECFE load and its induced vibrations are increased. Specifically, the amplitude of the equivalent NECFE load is increased by 9.38%, 15.34%, and 7.43% in the tangential component, axial component, and torsion moment. The blade vibration responses induced by NECFE are increased by 11.66% and 19.94% in tangential and axial.

Improving the dynamic performance of grid connected wind farm using new SMES switching technique

Due to the uncertain nature of wind energy, the effective operation of grid connected wind farms is quite a challenge for grid operators. The stipulated grid requirements for frequency and voltage control must be satisfied to avoid the shutdown of grid networks during periods of dynamic grid disturbance. This paper tends to improve the dynamic performance of a grid network consisting of nuclear and thermal turbines that are tied to a wind farm that is made up of fixed speed squirrel cage induction generator wind turbine type. A supermagnetic energy storage (SMES) system is connected to the terminal of the wind farm in order to achieve smoothing of the distorted wind turbine variables so that steady power supply could be maintained during periods of grid disturbance. A new control strategy to improve the performance of the traditional SMES is proposed by integrating a parallel DC-DC chopper in the circuitry of the voltage source converter. The responses of the proposed control strategy for SMES were compared with the traditional SMES scheme during grid disturbance under the same conditions of wind speed. The simulation results in power system computer aided design and electromagnetic transient including the DC platform show the improved performance of the grid variables considering the proposed scheme for SMES.

A framework for reconciliating data clusters from a fleet of nuclear power plants turbines for fault diagnosis

When a fleet of similar Systems, Structures and Components (SSCs) is available, the use of all the available information collected on the different SSCs is expected to be beneficial for the diagnosis purpose. Although different SSCs experience different behaviours in different environmental and operational conditions, they maybe informative for the other (even if different) SSCs. In the present work, the objective is to build a fault diagnostic tool aimed at capitalizing the available data (vibration, environmental and operational conditions) and knowledge of a heterogeneous fleet of P Nuclear Power Plants (NPPs) turbines. To this aim, a framework for incrementally learning different clusterings independently obtained for the individual turbines is here proposed. The basic idea is to reconciliate the most similar clusters across the different plants. The data of shut-down transients acquired from the past operation of the P NPPs turbines are summarized into a final, reconciliated consensus clustering of the turbines behaviors under different environmental and operational conditions. Eventually, one can distinguish, among the groups, those of anomalous behavior and relate them to specific root causes. The proposed framework is applied on the shut-down transients of two different NPPs. Three alternative approaches for learning data are applied to the case study and their results are compared to those obtained by the proposed framework: results show that the proposed approach is superior to the other approaches with respect to the goodness of the final consensus clustering, computational demand, data requirements, and fault diagnosis effectiveness.

Automatic Load Frequency Control of Six Areas’ Hybrid Multi-Generation Power Systems Using Neuro-Fuzzy Intelligent Controller

This manuscript deals with the implementation of adaptive neuro-fuzzy inference system (ANFIS) approach to automatic load frequency control (ALFC) of six unequal areas’ multi-generation hybrid interconnected power system. ANFIS controller has the advantages of fuzzy as well as neural network and stands for adaptive neuro-fuzzy inference system. Areas 1 and 2 comprise thermal and reheat thermal plants, area 3 consists of hydro plant with hydraulic governor system whereas areas 4, 5 and 6 comprise nuclear, diesel and gas turbine power plants, respectively. The hybrid model of interconnected power plant is developed with intelligent ANN (artificial neural network) and hybrid neuro-fuzzy controllers, i.e. proposed ANFIS and existing proportional and integral (PI) controller separately. The simulation work was carried out and the performance evaluation is checked by the proposed control approaches with similar step load change in each areas. Sliding surface is introduced with each controller to enhance the performance of controllers. Evaluation of ANN, ANFIS and PI-based approaches exhibits the advantage of the proposed ANFIS controller over conventional PI controller and ANN controller for tie-line power and frequency deviation in all six areas of interconnected hybrid power system.

Hot Deformation Behavior of 1Cr12Ni3Mo2VN Martensitic Stainless Steel

1Cr12Ni3Mo2VN is a new type of martensitic stainless steel for the last-stage blades of large-capacity nuclear and thermal power turbines. The deformation behavior of this steel was studied by thermal compression experiments that performed on a Gleeble-3500 thermal simulator at a temperature range of 850°C to 1200°C and a strain rate of 0.01s-1 to 20s-1. When the deformation was performed at high temperature and low strain rate, a necklace type of microstructures was observed, the plastic deformation mechanism is grain boundary slip and migration, when at low temperature and lower strain rate, the slip bands were observed, the mechanism is intracrystalline slips, and when at strain rate of 20s-1, twins were observed, the mechanism are slips and twins. The Arrhenius equation was applied to describe the constitutive equation of the flow stress. The accuracy of the equation was verified by using the experimental data and the correlation coefficient R2 = 0.9786, and the equation can provide reasonable data for the design and numerical simulation of the forging process.

Creep Parameters and Dislocation Substructure in AISI 316 Austenitic Stainless Steel From 600ºC to 800ºC

Stainless steels are well known by their corrosion resistance. The austenitic types, in particular, are also applied as structural components in engineering systems operating at high temperatures such as nuclear reactors, petrochemical furnaces and turbines. For these applications operational temperatures may go up to 800oC. Under constant load applications the main mechanism of failure, which would limit the material's life, is creep. In the present work creep parameters were evaluated in the high temperature interval of 600 to 800oC for an AISI 316 austenitic stainless steel. Dislocation substructures were observed by transmission electron microscopy in creep ruptured specimens. Two distinct mechanisms of dynamic strain aging and dynamic recovery associated with different values for the power law exponent n and the Arrhenius activation energy Q for creep were verified below and above 700oC, respectively.

Thermodynamic and economic analysis of nuclear power unit operating in partial cogeneration mode to produce electricity and district heat

This paper presents the methodology of techno-economic analysis for a nuclear unit operating in partial cogeneration mode and its application for the case study: a nuclear power plant planned in Poland. The research objectives were: to propose EPR, AP1000 and ESBWR nuclear condensing-extraction turbine systems modifications required for operation in cogeneration, to determine optimal heat production and heat transport line (HTL) parameters, to evaluate the technological feasibility of proposed solutions, to analyze profitability and competitiveness of the system versus coal-fired technologies. To adapt nuclear turbine to operation in partial cogeneration mode, the steam must be extracted from low-pressure (LP) section of the turbine and crossover pipe connecting high-pressure (HP) or intermediate-pressure (IP) section with LP section. Thermodynamic analysis proved that the operation of nuclear power plant at peak thermal load up to 250 MW neither requires to change primary cycle arrangements of considered nuclear units nor thermal capacities of nuclear reactors. Total annual costs of nuclear power plant operating in partial cogeneration were the lowest of all considered heat and power options, with all types of reactors, for the emission allowance price of 27 EUR/t CO2-eq. The specific cost of heat from nuclear cogeneration option was 10.3–12.7 EUR/GJ.