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Author: Elias Amselem Abecasis
Publisher:
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Languages : en
Pages :
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Book Description
It is well known that boiling water reactors can experience inadvertent power oscillations. When such instability occurs the core can oscillate in two different modes (in phase mode and out of phase mode). In the late 90's a stability benchmark was created using the stability data obtained from the experiments at the Swedish nuclear power plant of Ringhals-1. Data was collected from the cycles 14, 15 , 16 and 17. Later on, this data was used to validate the various models and codes with the aim of predicting the instability behavior of the core and understand the triggers of such oscillations. The current trend of increasing reactor power density and relying on natural circulation for core cooling may have consequences for the stability of modern BWR's designs. The objective of this work is to find the most important parameters affecting the stability of the BWRs and propose alternative stability maps. For this purpose a TRACE/PARCS model of the Ringhals-1 NPP will be used. Afterwards a selection of possible parameters and dimensionless numbers will be made to study its effect on stability. Once those parameters are found they will be included in the stability maps to make them more accurate.
Author: Elias Amselem Abecasis
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
It is well known that boiling water reactors can experience inadvertent power oscillations. When such instability occurs the core can oscillate in two different modes (in phase mode and out of phase mode). In the late 90's a stability benchmark was created using the stability data obtained from the experiments at the Swedish nuclear power plant of Ringhals-1. Data was collected from the cycles 14, 15 , 16 and 17. Later on, this data was used to validate the various models and codes with the aim of predicting the instability behavior of the core and understand the triggers of such oscillations. The current trend of increasing reactor power density and relying on natural circulation for core cooling may have consequences for the stability of modern BWR's designs. The objective of this work is to find the most important parameters affecting the stability of the BWRs and propose alternative stability maps. For this purpose a TRACE/PARCS model of the Ringhals-1 NPP will be used. Afterwards a selection of possible parameters and dimensionless numbers will be made to study its effect on stability. Once those parameters are found they will be included in the stability maps to make them more accurate.
Author: Christian Pablo Marcel
Publisher: IOS Press
ISBN: 1586038036
Category : Technology & Engineering
Languages : en
Pages : 160
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Book Description
In the design of novel nuclear reactors active systems are replaced by passive ones in order to reduce the risk of failure. For that reason natural circulation is being considered as the primary cooling mechanism in next generation nuclear reactor designs such as the natural circulation boiling water reactor (BWR). In such a reactor, however, the flow is not a controlled parameter but is dependent on the power. As a result, the dynamical behavior significantly differs from that in conventional forced circulation BWRs. For that reason, predicting the stability characteristics of these reactors has to be carefully studied. In this work, a number of open issues are investigated regarding the stability of natural circulation BWRs (e.g. margins to instabilities at rated conditions, interaction between the thermal-hydraulics and the neutronics, and the occurrence of flashing induced instabilities) with a strong emphasis on experimental evidence.
Author: Hiroshi Nishihara
Publisher:
ISBN:
Category : Boiling water reactors
Languages : en
Pages : 20
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Author: Rui Hu (Ph. D.)
Publisher:
ISBN:
Category :
Languages : en
Pages : 348
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Book Description
Density Wave Oscillations (DWOs) are known to be possible when a coolant undergoes considerable density reduction while passing through a heated channel. In the development of boiling water reactors (BWRs), there has been considerable concern about the effects of such oscillations when coupled with neutronic feedback. The current trend of increasing reactor power density and relying more extensively on natural circulation for core cooling may have consequences for the stability characteristics of new BWR designs. This work addresses a wide range of issues associated with the BWR stability: 1) flashing-induced instability and natural circulation BWR startup; 2) stability of the BWRs with advanced designs involving high power :densities; 3) modeling assumptions in stability analysis methods; and 4) the fuel clad performance during power and flow oscillations. To capture the effect of flashing on density wave oscillations during low pressure startup conditions, a code named FISTAB has been developed in the frequency domain. The code is based on a single channel thermal-hydraulic model of the balance of the water/steam circulation loop, and incorporates the pressure dependent water/steam thermodynamic properties, from which the evaporation due to flashing is captured. The functionality of the FISTAB code is confirmed by testing the experimental results at SIRIUS-N facility. Both stationary and perturbation results agree well with the experimental results. The proposed ESBWR start-up procedure under natural convection conditions has been examined by the FISTAB code. It is confirmed that the examined operating points along the ESBWR start-up trajectory from TRACG simulation will be stable. To avoid the instability resulting from the transition from single-phase natural circulation to two-phase circulation, a simple criterion is proposed for the natural convection BWR start-up when the steam dome pressure is still low. Using the frequency domain code STAB developed at MIT, stability analyses of some proposed advanced BWRs have been conducted, including the high power density BWR core designs using the Large Assembly with Small Pins (LASP) or Cross Shape Twisted (CST) fuel designs developed at MIT, and the Hitachi's RBWR cores utilizing a hard neutron spectrum and even higher power density cores. The STAB code is the predecessor of the FISTAB code, and thermodynamic properties of the coolant are only dependent on system pressure in STAB. It is concluded that good stability performance of the LASP core and the CST core can be maintained at nominal conditions, even though they have 20% higher reactor thermal power than the reference core. Power uprate does not seem to have significant effects on thermal-hydraulic stability performance when the power-to-flow ratio is maintained. Also, both the RBWR-AC and RBWR-TB2 designs are found viable from a stability performance point of view, even though the core exit qualities are almost 3 times those of a traditional BWR. The stability of the RBWRs is enhanced through the fast transient response of the shorter core, more flat power and power-to-flow ratio distributions, less negative void feedback coefficient, and the core inlet orifice design. To examine the capability of coupled 3D thermal-hydraulics and neutronics codes for stability analysis, USNRC's latest system analysis code, TRACE, is chosen in this work. Its validation for stability analysis and comparison with the frequency domain approach, have been performed against the Ringhals 1 stability tests. Comprehensive assessment of modeling choices on TRACE stability analysis has been made, including effects of timespatial discretization, numerical schemes, thermal-hydraulic channel grouping, neutronics modeling, and control system modeling. The predictions from both the TRACE and STAB codes are found in reasonably good agreement with the Ringhals 1 test results. The biases for the predicted global decay ratio are about 0.07 in TRACE results, and -0.04 in STAB results. However, the standard deviations of decay ratios are both large, around 0.1, indicating large uncertainties in both analyses. Although the TRACE code uses more sophisticated neutronic and thermal hydraulic models, the modeling uncertainty is not less than that of the STAB code. The benchmark results of both codes for the Ringhals stability test are at the same level of accuracy. The fuel cladding integrity during power oscillations without reactor scram is examined by using the FRAPTRAN code, with consideration of both the stress-strain criterion and thermal fatigue. Under the assumed power oscillation conditions for high burn-up fuel, the cladding can satisfy the stress-strain criteria in the ASME Code. Also, the equivalent alternating stress is below the fatigue threshold stress, thus the fatigue limit is not violated. It can be concluded that under a large amount of the undamped power oscillation cycles, the cladding would not fail, and the fuel integrity is not compromised.
Author: Alfonso Prieto Guerrero
Publisher: Woodhead Publishing
ISBN: 0081024460
Category : Business & Economics
Languages : en
Pages : 474
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Book Description
Linear and Non-Linear Stability Analysis in Boiling Water Reactors: The Design of Real-Time Stability Monitors presents a thorough analysis of the most innovative BWR reactors and stability phenomena in one accessible resource. The book presents a summary of existing literature on BWRs to give early career engineers and researchers a solid background in the field, as well as the latest research on stability phenomena (propagation phenomena in BWRs), nuclear power monitors, and advanced computer systems used to for the prediction of stability. It also emphasizes the importance of BWR technology and embedded neutron monitoring systems (APRMs and LPRMs), and introduces non-linear stability parameters that can be used for the onset detection of instabilities in BWRs. Additionally, the book details the scope, advantages, and disadvantages of multiple advanced linear and non linear signal processing methods, and includes analytical case studies of existing plants. This combination makes Linear and Non-Linear Stability Analysis in Boiling Water Reactors a valuable resource for nuclear engineering students focusing on linear and non-linear analysis, as well as for those working and researching in a nuclear power capacity looking to implement stability methods and estimate decay ratios using non-linear techniques. Explores the nuclear stability of Boiling Water Reactors based on linear and non-linear models Evaluates linear signal processing methods such as autoregressive models, Fourier-based methods, and wavelets to calculate decay ratios Proposes novel non-linear signal analysis techniques linked to non-linear stability indicators Includes case studies of various existing nuclear power plants as well as mathematical models and simulations
Author: Róbert Zboray
Publisher: IOS Press
ISBN:
Category : Technology & Engineering
Languages : en
Pages : 178
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Book Description
Contents of this Doctoral Dissertation include: Understanding the linear stability characteristics of BWRs, Experiments on the stability of the Desire facility, Applications of the reducer-order model, Numerical analysis of the nonlinear dynamics of BWRs, Experiments on the nonlinear dynamics of natural-circulation two-phase flows, Experiments on the neutronic-thermalhydraulic stability, Conclusions and Discussion
Author: J. M. Healzer
Publisher:
ISBN:
Category : Boiling water reactors
Languages : en
Pages : 76
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Author: Luv Sharma
Publisher:
ISBN:
Category : Boiling water reactors
Languages : en
Pages : 262
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Abstract: A study of a Boiling Water Reactor (BWR) dynamics is presented with the objective of determining the attractors, domains of attraction and change in system stability with fluctuations in the system parameters. A reduced order model of the system was used for the investigations. The cell to cell mapping technique (CCMT) is used to determine the attractors and the domains of attraction for the system. The CCMT is a numerical technique for the global analysis of non-linear dynamics of systems and models system evolution as a Markov chain in time. The probabilistic modeling of the system dynamics and no differentiability requirements on the governing equations makes CCMT naturally suited for analysis of systems with stochastic parameters. This method, however, runs into performance problems with increasing number of state variables (degrees of freedom) of the system under investigation. Two methods are proposed to improve the performance of the CCMT for higher order systems. One of them is based on choosing different mapping time steps for different initial conditions for the system. The second method restricts the source cell region and gives a conditional probability of the system location. MATLAB v7.0 was used to set up the system and the simulations were run using the Ohio Supercomputing Center's distributed computing facilities. It is shown that these proposed methods lead to significant reduction in computational time compared to conventional CCMT. The results obtained are also generally better compared to the conventional approach. Using these proposed techniques it is determined that the BWR system has only one attractor and the only way to keep the reactor stable is to control the values of the system parameters. The parametric analysis was conducted using Taguchi methods, which are based on design of experiment techniques and the application of the signal-to-noise ratios. The experiment design was done using MINITAB v14. The experiments were used to determine the effects of parameter variation on the stability of the system as well as the interactions amongst the parameters. The parametric studies reveal that the heat transfer coefficient (k), the heat generation coefficient (Q), the Doppler's coefficient of reactivity ([gamma]1), delayed neutron precursor concentration ([beta]) and fitting parameters a1 and a2 have a notably larger effect on the response. Out of these parameters k, a, Q and [gamma]1 tend to destabilize the system at higher values while [beta], a1 and a2 have a stabilizing effect at higher values. Further, cross effects between a, k and Q were also found to be negligible compared to the main effects of the control factors.
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Languages : en
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We have studied the nuclear-coupled thermal-hydraulic stability of boiling water reactors (BWRs) using a model that includes: space-time modal neutron kinetics based on spatial w-modes; single- and two-phase flow in parallel boiling channels; fuel rod heat conduction dynamics; and a simple model of the recirculation loop. The BR model is represented by a set of time-dependent nonlinear ordinary differential equations, and is studied as a dynamical system using the modern bifurcation theory and nonlinear dynamical systems analysis. We first determine the stability boundary (SB) - or Hopf bifurcation set- in the most relevant parameter plane, the inlet-subcooling-number/external-pressure-drop plane, for a fixed control rod induced external reactivity equal to the 100% rod line value; then we transform the SB to the practical power-flow map used by BWR operating engineers and regulatory agencies. Using this SB, we show that the normal operating point at 100% power is very stable, that stability of points on the 100% rod line decreases as the flow rate is reduced, and that operating points in the low-flow/high-power region are least stable. We also determine the SB that results when the modal kinetics is replaced by simple point reactor kinetics, and we thereby show that the first harmonic mode does not have a significant effect on the SB. However, we later show that it nevertheless has a significant effect on stability because it affects the basin of attraction of stable operating points. Using numerical simulations we show that, in the important low-flow/high-power region, the Hopf bifurcation that occurs as the SB is crossed is subcritical; hence, growing oscillations can result following small finite perturbations of stable steady-states on the 100% rod line at points in the low-flow/high-power region. Numerical simulations are also performed to calculate the decay ratios (DRs) and frequencies of oscillations for various points on the 100% rod line. It is determined that the U.S. NRC requirement of DR is not rigorously satisfied in the low-flow/high-power region; hence, this region should be avoided during normal startup and shutdown operations. The frequency of oscillation is shown to decrease as the flow rate is reduced. Moreover, the simulation frequency of 0.5Hz determined in the low-flow/high-power region is consistent with those observed during actual instability incidents. Additional numerical simulations show that in the low-flow/high-power region, for the same initial conditions, the use of point kinetics leads to damped oscillations, whereas the model that includes the modal neutron kinetics equations results in growing nonlinear oscillations.
Author:
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Languages : en
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This thesis is concerned with nonlinear analyses of BWR stability behaviour, contributing to a deeper understanding in this field. Despite negative feedback-coefficients of a BWR, there are operational points (OP) at which oscillatory instabilities occur. So far, a comprehensive and an in-depth understanding of the nonlinear BWR stability behaviour are missing, even though the impact of the significant physical parameters is well known. In particular, this concerns parameter regions in which linear stability indicators, like the asymptotic decay ratio, lose their meaning. Nonlinear stability analyses are usually carried out using integral (system) codes, describing the dynamical system by a system of nonlinear partial differential equations (PDE). One aspect of nonlinear BWR stability analyses is to get an overview about different types of nonlinear stability behaviour and to examine the conditions of their occurrence. For these studies the application of system codes alone is inappropriate. Hence, in the context of this thesis, a novel approach to nonlinear BWR stability analyses, called RAM-ROM method, is developed. In the framework of this approach, system codes and reduced order models (ROM) are used as complementary tools to examine the stability characteristics of fixed points and periodic solutions of the system of nonlinear differential equations, describing the stability behaviour of a BWR loop. The main advantage of a ROM, which is a system of ordinary differential equations (ODE), is the possible coupling with specific methods of the nonlinear dynamics. This method reveals nonlinear phenomena in certain regions of system parameters without the need for solving the system of ROM equations. The stability properties of limit cycles generated in Hopf bifurcation points and the conditions of their occurrence are of particular interest. Finally, the nonlinear phenomena predicted by the ROM will be analysed in more details by the system code. Hence, the thesis i.
