Natural Circulation Boiling Water Reactor Research Articles

Startup transient simulation for natural circulation boiling water reactors in PUMA facility

In view of the importance of instabilities that may occur at low-pressure and -flow conditions during the startup of natural circulation boiling water reactors, startup simulation experiments were performed in the Purdue University Multi-Dimensional Integral Test Assembly (PUMA) facility. The simulations used pressure scaling and followed the startup procedure of a typical natural circulation boiling water reactor. Two simulation experiments were performed for the reactor dome pressures ranging from 55 kPa to 1 MPa, where the instabilities may occur. The experimental results show the signature of condensation-induced oscillations during the single-phase-to-two-phase natural circulation transition. The results also suggest that a rational startup procedure is needed to overcome the startup instabilities in natural circulation boiling water reactor designs.

Flashing-induced density wave oscillations in a natural circulation BWR—mechanism of instability and stability map

Experiments were conducted to investigate two-phase flow instabilities due to flashing in a boiling natural circulation loop with a chimney at low pressure. The SIRIUS-N facility was designed to have non-dimensional values nearly equal to those of typical natural circulation boiling water reactor (BWR). The observed instability is suggested to be flashing-induced density wave oscillations, since the oscillation period correlated well with the passing time of single-phase liquid in the chimney section regardless of system pressure, heat flux, and inlet subcooling. Stability maps were obtained in reference to the inlet subcooling and the heat flux at the system pressures of 0.1, 0.2, 0.35, and 0.5 MPa. The flow became stable below a certain heat flux regardless of the channel inlet subcooling. The stable region enlarged with increasing system pressure. Thus, the stability margin becomes larger in a startup process of a reactor by pressurizing the reactor sufficiently before heating according to the stability map.

Modeling of flashing-induced instabilities in the start-up phase of natural-circulation BWRs using the two-phase flow code FLOCAL

This paper reports on the modeling and simulation of flashing-induced instabilities in natural-circulation systems, with special emphasis on natural-circulation boiling water reactors (BWRs). For the modeling the 4-equation two-phase model FLOCAL [Rohde, U., 1986. Ein teoretisches Modell fur Zweiphasen-stromungen in wassergekulthen Kernreaktoren und seine Anwendung zur Analyse des Naturumlaufs im Heizreaktor AST-500. Ph.D. dissertation, Akademie der Wissenschaften der DDR, Dresden], developed at the Forschungszentrum Rossendorf (FZR, Germany), has been used. The model allows for the liquid and vapor to be in thermal non-equilibrium and, via drift-flux models, to have different velocities. The phenomenology of the instability has been studied and the dominating physical effects have been determined. The results of the simulations have been compared qualitatively and quantitatively with experiments [Manera, A., van der Hagen, T.H.J.J., 2003. Stability of natural-circulation-cooled boiling water reactor during start up: experimental results. Nuc. Technol., 143] that have been carried out within the framework of a European project (NACUSP) on the CIRCUS facility. The facility, built at the Delft University of Technology in The Netherlands, is a water/steam 1:1 height-scaled loop of a typical natural-circulation-cooled BWR.

Multi Parameters Effect on Thermohydraulic Instability in Natural Circulation Boiling Water Reactor during Startup

The purpose of the study is to experimentally investigate driving mechanism of major instabilities simulated in a natural circulation experimental loop, under a predetermined range of system operating pressure and inlet subcoolings. Pressure range of 0.1 up to 0.7 MPa, input heat flux range of 0 up to 577 kW/m 2 , and inlet subcoolings of 5, 10 and 15 K respectively, are applied in the experiments. The objective of the study is to formulate a rational startup procedure, in which major thermohydraulic instabilities can be detected and prevented. The study clarifies that four (4) kinds of thermohydraulic instability might occur even up to a higher pressure of 0.7 MPa. The instabilities' sequence is as follows: (1) geysering induced by condensation accompanied by flashing, (2) oscillation induced by hydrostatic head fluctuation, (3) density wave oscillations, and (4) flashing accompanying those instabilities. The experiments confirmed that the geysering region gets narrower and suppressed with the increased system pressure. With chimneys, natural circulation can be achieved reliably and more easily. However, the flashing in the chimney cannot be avoided at low system pressure. Stable two-phase natural circulation can be established if the system pressure is increased beyond 0.7 MPa, after the high frequency density eave oscillation thoroughly suppressed. The experiments were analyzed based on frequency domain of each instability phenomenon.

Transport Mechanism of Thermohydraulic Instability in Natural Circulation Boiling Water Reactors during Startup

This paper presents experimental study on transport mechanism of thermohydraulic instability, which may occur in natural circulation boiling water reactor during startup. The research was carried out using a natural circulation experimental loop featuring twin parallel boiling channels with chimney assembly. The experiments were performed with the pressure range of 0.1 to 0.7MPa and maximum heat flux of 577kW/m2. The objective of the study is to formulate thermohydraulic stability maps required for determining rational startup procedure of the reactor, in which the instability could be prevented. The study clarified that the flow modes during startup consist of the following sequence: (1) single-phase flow, (2) geysering, (3) oscillation due to hydrostatic head fluctuation, (4) density wave oscillation, (5) transition oscillation, and (6) stable two-phase flow. The main findings of the experiments are as follows: First, low amplitude geysering still occurs at 0.7 MPa under lower heat flux and high inlet subcooling. Second, stable two-phase natural circulation is achieved with system pressure as low as 0.2 MPa, under medium heat flux, and subcooling lower than 5 K. Third, oscillation due to hydrostatic head fluctuation only occurs under atmospheric condition. Finally, thermohydraulic stability maps and rational startup procedure are formulated.

Stability of Natural-Circulation-Cooled Boiling Water Reactors during Startup: Experimental Results

The characteristics of flashing-induced instabilities, which are of importance during the startup phase of natural-circulation boiling water reactors, are studied. Experiments at typical startup conditions (low power and low pressure) are carried out on a steam/water natural-circulation loop. The flashing and the mechanism of flashing-induced instability are analyzed. The effect of system pressure and steam volume in the steam dome is investigated as well.The instability region is found as soon as the operational boundary between single-phase and two-phase operation is crossed. Increasing pressure has a stabilizing effect, reducing the operational region in which instabilities occur. Nonequilibrium between phases and enthalpy transport are found to play an important role in the instability process. In contrast with results reported in the literature, instabilities can occur independently of the position of the flashing boundary in the adiabatic section of the loop. The period of the oscillation is found to be about twice the fluid transit time in the system.

Stabilising boiling water reactors by optimising the position of the feedwater sparger

The influence of changing the position of the feedwater sparger on the thermal-hydraulic stability of natural-circulation boiling water reactors (BWRs) is investigated. Frequency-domain, linear stability analysis is applied using a reduced-order dynamic model. It is found that the phase with which the temperature oscillations, created at the feedwater sparger, reach the core inlet has a significant influence on the stability. This phase is influenced by the position of the sparger. It is shown that moving the sparger to the core inlet has a stabilising influence. Similar result is found using the thermal-hydraulic code MONA.

Transport Mechanism of Thermohydraulic Instability in Natural Circulation Boiling Water Reactors during Startup

Transport Mechanism of Thermohydraulic Instability in Natural Circulation Boiling Water Reactors during Startup

ICONE11-36077 MULTI PARAMETERS EFFECT ON THERMOHYDRAULIC INSTABILITY IN NATURAL CIRCULATION BOILING WATER REACTOR DURING STARTUP

Feasibility study on the future light water reactor has been carried out. Small- and medium-sized natural circulation boiling water reactor (BWR) has been one of the concepts. One of the indispensable topics is thermohydraulic instability during startup. Occurrence of the instability during the startup negatively affects safety, reliability and operability, as it would cause complexity in raising and maneuvering reactor power. The purpose of the study is to experimentally investigate driving mechanism of major instabilities simulated in a natural circulation experimental loop, under a predetermined range of system operating pressure and inlet subcoolings. Pressure range of 0.1 up to 0.7 MPa, input heat flux range of 0 up to 577kW/m^2,and inlet subcoolings of 5,10 and 15K respectively, were applied in the experiments. The objective of this paper is to offer new experimental data as the basis to propose rational startup procedure, in which various thermohydraulic instabilities can be prevented. The study clarifies that four (4) kinds of thermohydraulic instability might occur even up to a higher pressure of 0.7 MPa. The clarified sequence instabilities are as follows : (1) geysering induced by condensation accompanied by flashing, (2) oscillation induced by hydrostatic head fluctuation, (3) density wave oscillations, and (4) flashing accompanying those instabilities. The experiments confirmed that the geysering region gets narrower and suppressed with the increased system pressure. With chimneys, natural circulation can be achieved reliably and more easily. However, the flashing in the chimney cannot be avoided at low system pressure. Stable two-phase natural circulation can be established if the system pressure is increased beyond 0.7 MPa, after the high frequency density eave oscillation thoroughly suppressed. The experiments were analyzed based on frequency domain of each instability phenomenon.

Analytical modeling of flashing-induced instabilities in a natural circulation cooled boiling water reactor

A dynamic model for natural circulation boiling water reactors (BWRs) under low-pressure conditions is developed. The motivation for this theoretical research is the concern about the stability of natural circulation BWRs during the low-pressure reactor start-up phase. There is experimental and theoretical evidence for the occurrence of void flashing in the unheated riser under these conditions. This flashing effect is included in the differential (homogeneous equilibrium) equations for two-phase flow. The differential equations were integrated over axial two-phase nodes, to derive a nodal time-domain model. The dynamic behavior of the interface between the one and two-phase regions is approximated with a linearized model. All model equations are presented in a dimensionless form. As an example the stability characteristics of the Dutch Dodewaard reactor at low pressure are determined.

Investigating the Stability Characteristics of Natural-Circulation Boiling Water Reactors Using Root Loci of a Reduced-Order Model

Linear stability analysis of a natural-circulation boiling water reactor (BWR) and the underlying thermal-hydraulic subsystem is performed using a reduced-order BWR dynamic model. The root-locus method is used to examine the stability of the system. The relation between the poles of the system and the physical processes causing the instabilities is investigated. For a natural-circulation thermal-hydraulic system, the two types of instabilities (type-I and type-II oscillations) can clearly be attributed to the dynamics of different types of pressure drops. However, it is not possible to associate these instability types with certain poles of the system.The root loci of a reactor with weak void reactivity feedback and those of the thermal-hydraulic system behave similarly: The same pole pair remains the least stable one as the operating conditions move from the type-I instability region to the type-II region. In the case of a reactor with strong void reactivity feedback, an exchange in the stability of two pole pairs is found: The least stable pole pair in the type-II region is not the same as in the type-I region.

画像処理技術を用いたスラグ気泡流動特性の計測技術 第１報 スラグ気泡の上昇速度と液膜厚計測技術

Geysering may be induced during start-up in natural circulation boiling water reactors. It is necessary for simulation of this flow instability to model the coalescence mechanism and condensation process of slug bubbles. In this work, image data processing technique using a CCD digital camera has been developed to measure slug flow characteristics. The developed measurement technique of rising velocity of slug bubbles and liquid film thickness surrounding a bubble and its measurement accuracy are discussed in this paper.

画像処理技術を用いたスラグ気泡流動特性の計測技術 第２報 スラグ気泡流のＷａｋｅの流れ場と液膜流速計測技術

Geysering may be induced during start-up in natural circulation boiling water reactors. It is necessary for simulation of this flow instability to model the coalescence mechanism and condensation process of slug bubbles. In this work, image data processing technique using a digital video camera has been developed to measure slug flow characteristics. In this technique, ultraviolet lighting with high frequency power source, laser sheet and stroboscope whose lighting frequency can be regulated are used as lighting and small nylon particles mixed into water are adopted for tracers. The developed measurement technique are applied for slug bubbles rising in stagnant water, and coalescence process of slug bubbles, liquid film velocity surrounding a slug bubble and velocity field in the wake region are measured. From these results, the capability of the developed measurement technique and the measurement accuracy are discussed.

Nonlinear Analysis of a Natural Circulation Boiling Water Reactor

A dynamic model of natural circulation boiling water reactors (BWRs) is analyzed using a bifurcation code and numerical simulations. The two fundamental bifurcation types relevant to BWRs, the supercritical and the subcritical Hopf bifurcations, are first studied in natural circulation systems without nuclear feedback. The effect of nodalization approximation in the riser on stability and bifurcation characteristics of the system is determined. The strong effect of the nuclear-thermohydraulic interaction on the nonlinear characteristics of a natural circulation BWR is then explored in a parametric study. Supercritical bifurcations become dominant in the (high-power) Type-II region for small values of the subcooling number and a strong nuclear-thermohydraulic coupling. A cascade of period-doubling pitchfork bifurcations (deep in the unstable region) is also predicted by the model under these conditions. Subcritical bifurcations in the Type-II instability region were found for larger values of the subcooling number. Both Hopf-bifurcation modes were also encountered in the Type-I instability region (low power or high power/high subcooling). Finally, the nonlinear reactor model was validated successfully compared with nonlinear power oscillations measured in a natural circulation BWR.

327 画像処理技術を用いたスラグ気泡流動特性の計測技術

Geysering may be induced during start-up in natural circulation boiling water reactors. It is necessary for simulation of this flow instability to model the coalescence mechanism and condensation process of slug bubbles. In this work, image data processing technique using a CCD digital camera has been developed to measure slug flow characteristics. The developed measurement technique of rising velocity of slug bubbles and liquid film thickness surrounding a bubble and its measurement accuracy are discussed in this paper.

Stability of Natural Circulation Boiling Water Reactors: Part I—Description Stability Model and Theoretical Analysis in Terms of Dimensionless Groups

A theoretical model describing coupled neutronic-thermohydraulic power oscillations in natural circulation boiling water reactors (BWRs) is developed. The governing equations for the thermohydraulic subsystem are transformed to a dimensionless basis, to eliminate all explicit pressure dependence in the model. It is proved that all necessary information about the operating conditions is incorporated in only two dimensionless numbers: the Zuber and the subcooling number. The density ratio number cancels in the dimensionless equations because a homogeneous flow model is applied. The Froude number is also shown to be redundant in a natural circulation system, as it can be expressed in the other dimensionless groups.The stability boundary of the complete coupled neutronic-thermohydraulic reactor system in the dimensionless Zuber-subcooling plane is estimated to be rather insensitive to the system pressure as well. Therefore the usage of dimensionless stability maps, instead of the traditional power-flow maps, is strongly recommended as an efficient method to determine the dynamic characteristics of natural circulation BWRs.

Stability of Natural Circulation Boiling Water Reactors: Part II—Parametric Study of Coupled Neutronic-Thermohydraulic Stability

A parametric study of coupled neutronic-thermohydraulic stability of natural circulation boiling water reactors (BWRs) is performed. As an example, the stability characteristics of the Dutch Dodewaard reactor, which was cooled by natural circulation, are determined. The Dodewaard reactor can be considered as the prototype of next generation natural circulation BWRs. The stability issues that are identified for this prototype reactor are therefore important in the design of new natural circulation BWRs.Without a riser section installed, only one region of thermohydraulic instability exists in the stability plane. The significant gravitational pressure drop in a riser section, installed to enhance the natural circulation flow, gives rise to the emergence of an additional region of instability. The oscillations in this zone become especially important during low-power/low-pressure (reactor startup) conditions. Significant damping of these oscillations occurs in a reactor, due to the nuclear void reactivity feedback.A comparison between natural circulation in-phase and out-of-phase reactor stability is made, in particular important for large reactor cores but also yielding unexpected results for small reactors. The impact of downcomer inertia on the stability of the in-phase mode is investigated in detail. Typical trajectories in the dimensionless stability plane are calculated as a function of changing operating conditions, to investigate their influence on reactor dynamics.

Stability of Natural Circulation Boiling Water Reactors: Part I - Description Stability Model and Theoretical Analysis in Terms of Dimensionless Groups

Stability of Natural Circulation Boiling Water Reactors: Part I - Description Stability Model and Theoretical Analysis in Terms of Dimensionless Groups

Critical Heat Flux in a Vertical Annulus under Low Upward Flow and near Atmospheric Pressure

As future boiling water reactors (BWR), concepts of evolutional ABWR (ABWR-IER) and natural circulation BWR (JSBWR) have been investigated in order to reduce their construction cost and simplify their maintenance and inspection procedures. One of the promised features of the design of the evolutional ABWR is to reduce the number of internal pumps and to remove the Motor Generation (MG) sets. These design changes may induce boiling transition in the fuel rods of reactor core during a pump trip transient due to the more rapid flow coastdown characteristics than these of the present design. In addition, the understanding of critical heat flux (CHF) is one important subject to grasp safety margin during the start-up for the natural circulation BWR and to establish the rational start-up procedure in which thermo-hydraulic instabilities can be suppressed.The present study is to clarify CHF characteristics under low velocity conditions. CHF measurements were conducted in a vertical upward annulus channel composed of an inner heated rod and an outer tube made of glass. CHF data were obtained repeatedly under the condition of stable inlet flow to examine statistically their reproducibility. The flow regime was investigated from flow observation and measurement of differential pressure fluctuation. The CHF data are correlated with the flow regime transition. It was clear from the obtained flow pattern and the CHF data that the CHF behavior could be classified into specified regions by the mass flux and inlet subcooling conditions. A CHF correlation was developed and agreed with other researchers' data within acceptable error.

Exploring the dodewaard type-I and type-II stability; from start-up to shut-down, from stable to unstable

Stability measurements on the Dodewaard natural circulation boiling water reactor have been performed for five years, covering the entire operational map. A data base of stability characteristics for testing of analytical models and for computer-code benchmarking has been set up. Type-I natural-circulation-flow stability and Type-II reactor stability have been investigated. Flashing turned out to be an important phenomenon. At a Type-II instability, the oscillating reactor power showed a peculiar modulation before the divergence to the scram level. The decay ratio was found to be an unpractical and unsafe stability indicator for reactors with a decay ratio larger than 0.6.