Flashing-induced Instabilities Research Articles

Simulation investigation on flashing-induced instabilities in a natural circulation system

Flashing-driven natural circulation techniques have been widespreadly applied in advanced reactor designs due to its prominent advantages in the aspects of high reliability, cost-efficiency and inherent safety. However, flashing-induced instability probably occurs under certain conditions in a flashing-driven natural circulation system, which deserves comprehensive illustration. By using RELAP5 code, the behavior of a two-phase natural circulation system is investigated based on the experiment benchmark. Three instability flow modes formed by two typical flow instability mechanisms, along with stable single-phase flow and stable two-phase flow are identified and analyzed in this work. The effects of typical thermal boundaries and structural parameters on the instability thresholds are revealed as well.

Validation of RELAP5/MOD3.2 Code for Flashing-Induced Instabilities in a Single Channel

This paper reports on the modeling and simulation of flashing-induced instabilities in naturalcirculation systems, with special emphasis on simplified boiling water reactors (SBWRs). In this work, flashing-induced oscillations have been studied by using an experimental test facility (SIRIUS-N) and RELAP5/MOD3.2 thermal hydraulic code. The behavior of the test facility is investigated for different values of core inlet temperature value. The results of the simulations have been compared qualitatively and quantitatively with experiments. In general, deviations are found between the numerical and experimental results, in spite of the close similarity between the SIRIUS-N facility and the definition of the system in the RELAP code. This result indicates that predictions regarding experimental facility, based on modeled system, should be carefully considered.

Validation of the RELAP5 code for the modeling of flashing-induced instabilities under natural-circulation conditions using experimental data from the CIRCUS test facility

Abstract This paper reports on the use of the RELAP5 code for the simulation of flashing-induced instabilities in natural circulation systems. The RELAP 5 code is intended to be used for the simulation of transient processes in the Russian RUTA reactor concept operating at atmospheric pressure with forced convection of coolant. However, during transient processes, natural circulation with flashing-induced instabilities might occur. The RELAP5 code is validated against measurement data from natural circulation experiments performed 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. It was shown that the RELAP5 code is able to model all relevant phenomena related to flashing induced instabilities. The magnitude and frequency of the oscillations were reproduced in a good agreement with the measurement data. The close correspondence to the experiments was reached by detailed modeling of all components of the CIRCUS facility including the heat exchanger, the buffer vessel and the steam dome at the top of the facility.

Experimental investigations on flashing-induced instabilities in one and two-parallel channels: A comparative study

Abstract In this investigation, experiments conducted in a natural circulation test facility at low power and low pressure conditions, in the one single and two-parallel channels configuration are presented and discussed in detail. The novel manner of visualizing the results allowed characterizing the facility at any time and position which helped to thoroughly understand the instability mechanisms. Different modes were observed for each configuration. In the case of having two-parallel channels, four different behaviors have been observed: stable flow circulation, periodic high subcooling oscillations, a-periodical oscillations and out-of-phase periodical oscillations. In addition, stability maps were constructed in order to clarify the region in which each mode is dominant. The results obtained from both the one and two-parallel channels configurations are thus analyzed and compared. As a result, some similarities have been observed between the intermittent flow oscillations found in the single channel experiments and the high subcooling oscillations found in the two-parallel channels experiments. Moreover, similarities have also been found between the sinusoidal flow oscillations existing in the single channel experiments and the out-of-phase oscillations from the two-parallel channels experiments. The experiments presented in this work can be used to benchmark numerical codes and modeling techniques developed to study the start-up of natural circulation BWRs.

Experimental and numerical investigations on flashing-induced instabilities in a single channel

During the start-up phase, natural circulation BWRs (NC-BWRs) need to be operated at low pressure conditions. Such conditions favor flashing-induced instabilities due to the large hydrostatic pressure drop induced by the tall chimney. Moreover, in novel NC-BWR designs the steam separation is performed in the steam separators which create large pressure drops at the chimney outlet, which effect on stability has not been investigated yet. In this work, flashing-induced oscillations occurring in a tall, bottom heated channel are numerically investigated by using a simple linear model with three regions and an accurate implementation for estimating the water properties. The model is used to investigate flashing-induced instabilities in a channel for different values of the core inlet friction value. The results are compared with experiments obtained by using the CIRCUS facility at the same conditions, showing a good agreement. In addition, the experiments on flashing-induced instabilities are presented in a novel manner allowing visualizing new details of the phenomenon numerical stability investigations on the effect of the friction distribution are also done. It is found that by increasing the total restriction in the channel the system is destabilized. In addition, the chimney outlet restriction has a stronger destabilizing effect than the core inlet restriction. A stable two-phase region is observed prior to the instabilities in the experiments and the numerical simulations which may help to pressurize the vessel of NC-BWRs and thus reducing the effects of flashing instabilities during start-up.

Multifractal Analysis of Chaotic Flashing-Induced Instabilities in Boiling Channels in the Natural-Circulation CIRCUS Facility

In this paper, two-phase-flow oscillations at the natural-circulation CIRCUS test facility are investigated in a two-riser configuration. These oscillations are driven by flashing (and to some extent by geysering). For a given range of operating conditions of the facility, the oscillations exhibit erratic behavior. This study demonstrates that this behavior can be attributed to deterministic chaos. This is proven by performing a continuous wavelet transform of the measured time series. Any hidden self-similarity in the measurement is seen in the corresponding scale-space plane. The novelty of the present investigation lies with the multifractal approach used for characterizing the chaos. Both nonlinear time series analysis and wavelet-based analysis methods show that the dynamics of the flow oscillations has a multifractal structure. For the former, both Higuchi’s method and detrended fluctuation analysis (DFA) were used, whereas for the latter, the wavelet-transform modulus-maxima method was used. The strange attractor corresponding to the dynamics of the system can thus be described as a set of interwoven monofractal objects. The global singular properties of the measured time series is then fully characterized by a spectrum of singularities f(α), which is the Hausdorff dimension of the set of points where the multifractal object has singularities of strength (or Hölder exponents of) α. Whereas Higuchi’s method and DFA allow easily determining whether the deterministic chaos has a monofractal or multifractal hierarchy, the wavelet-transform modulus-maxima has the advantage of giving a quantitative estimation of the fractal spectrum. The time-modeling of such behavior of the facility is therefore difficult since there is sensitive dependence on initial conditions. From a regulatory point of view, such behavior of natural-circulation systems in a multiple-riser configuration has thus to be avoided.

Investigation of flashing-induced instabilities at CIRCUS test facility with the code ATHLET

The test facility CIRCUS (CIRculation Under Start-up) was built to study the start-up phase of a natural-circulation BWR. During the start-up,so-called flashing-induced instabilities can arise. These instabilities are induced by flashing (i.e., steam production in adiabatic conditions) of the coolant in the long riser section, which is placed above the core to enhance the flow rate. The flashing that occurs in the riser causes an imbalance between driving force and pressure losses in the natural-circulation loop, giving rise to flow oscillations. Within the European-Union 5th Framework Programme, a project, NACUSP (Natural circulation and stability performance of BWRs), has been started in December 2000, having as one of its main aims the understanding of the physics of the phenomena involved during the start-up phase of natural-circulation-cooled BWRs, providing a large experimental database and validating state-of-the-art thermo-hydraulic codes in the low-pressure, low-power operational region of these reactors. One part of this project deals with the modelling of selected CIRCUS tests using the thermo-hydraulic code ATHLET (Analysis of THermal-hydraulics of LEaks and Transients). This paper gives an overview about experiments and simulations. The code ATHLET is used to investigate the dynamic behaviour of the CIRCUS test facility and the results of the calculations are compared with the experimental data.

06/00093 Modeling of flashing-induced instabilities in the start-up phase of natural-circulation BWRs using the two-phase flow code FLOCAL: Manera, A. et al. Nuclear Engineering and Design, 2005, 235, (14), 1517–1535

The aim of this paper is to address the problem of suppressing unstable dynamics occurring in rectangular natural circulation loops on the base of a reliable model-based controller.The first part of the study is devoted to define a high order model, through the reduction by truncation of the Fourier series expansion of the functions describing the loop geometry of Navier–Stokes infinite-dimensional partial differential equations describing the flow, the heating conditions and the temperature distribution of the fluid. The first three modes were considered so that a closed model of seven ordinary differential equations was obtained. The model was then integrated and simulations compared with experimental data, confirming its ability to address the description of the dynamical behaviour of rectangular natural circulation loops.The satisfactory performances of the model lead to use it for the design and experimental testing of model-based feedback control strategies. In particular, a traditional proportional-derivative control approach has been applied to the model linearised around its equilibrium points. The flow velocity or an opportunely selected temperature difference between given points of the loop were chosen as feedback variables. Accordingly, the target of the control action was to drive the feedback variable to its stationary value, computed by means of the mathematical model. Experimental validation of the proposed model-based strategies satisfactorily demonstrated the capability of the approach in stabilising the system dynamics.

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.

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.

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.