Annalisa Manera

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.

The multipurpose thermalhydraulic test facility TOPFLOW: an overview on experimental capabilities, instrumentation and results

Abstract A new multipurpose thermalhydraulic test facility TOPFLOW (TwO Phase FLOW) was built and put into operation at Forschungszentrum Rossendorf in 2002 and 2003. Since then, it has been mainly used for the investigation of generic and applied steady state and transient two phase flow phenomena and the development and validation of models of Computational Fluid Dynamic (CFD) codes in the frame of the German CFD initiative. The advantage of TOPFLOW consists in the combination of a large scale of the test channels with a wide operational range both of the flow velocities as well as of the system pressures and temperatures plus finally the availability of a special instrumentation that is capable in high spatial and temporal resolving two phase flow phenomena, for example the wire-mesh sensors.

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

Abstract 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.

Planned experimental studies on natural-circulation and stability performance of boiling water reactors in four experimental facilities and first results (NACUSP)

Within the 5th Euratom framework programme the NACUSP project focuses on natural-circulation and stability characteristics of Boiling Water Reactors (BWRs). This paper gives an overview of the research to be performed. Moreover, it shows the first results obtained by one of the four experimental facilities involved. Stability boundaries are given for the low-power low-pressure operating range, measured in the CIRCUS facility. The experiments are meant to serve as a future validation database for thermohydraulic system codes to be applied for the design and operation of BWRs

Parameter Sensitivity Study of Boiling and Two-Phase Flow Models in CFD

This work presents a sensitivity study of boiling and two phase flow models for thermal hydraulics simulations in nuclear reactors. This study quantifies sources of uncertainty and error in these simulations by computing global sensitivities of figures of merit, or output, to model parameters, inputs, and mesh resolution. Results are obtained for the DEBORA benchmark problem of boiling in a channel driven by a heated wall section. Scalar outputs of interest consist of axial pressure drop, average wall temperature in the heated section, average void fraction at the end of the heated section, and the centroid of the radial distribution of the void fraction at the end of the heated test section. Sensitivities to both individual heat fluxes and to the parameters in the models for these heat fluxes are computed.

Experimental Investigation on Bubble Turbulent Diffusion in a Vertical Large-Diameter Pipe by Wire-Mesh Sensors and Correlation Techniques

Experiments with air-water flows have been carried out in a vertical pipe of ~194-mm diameter and 9-m length, and a wide range of superficial liquid and gas velocities has been covered. At a distance of 7.6 m from the air injection, two wire-mesh sensors are installed, located at a distance of 63.3 mm from each other. The wire-mesh sensors measure sequences of instantaneous two-dimensional gas-fraction distributions in the cross section in which they are mounted, with a spatial resolution of 3 mm and a frequency of 2500 Hz. The spatial cross-correlations of the gas-fraction signals have been evaluated, and on their basis turbulent diffusion coefficients have been estimated. It is found that for a given liquid superficial velocity, a sudden increase of the diffusion coefficient takes place when the superficial gas velocity is increased above a certain value. The abrupt increase of the diffusion coefficient occurs in correspondence of the transition from mono- to bimodal bubble size distributions. The experimental diffusion coefficients are compared with the prediction of the Sato model (experimental gas-fraction profiles and bubble size distributions are given as input). Even though this model has been developed for bubbly flow, the general trends are well captured also in the churn-turbulent regime.

Assessment of linear and non-linear autoregressive methods for BWR stability monitoring

A benchmark has been performed to compare the performances of exponential autoregressive (ExpAR) models against linear autoregressive (AR) models with respect to boiling water reactor stability monitoring. The well-known March-Leuba reduced-order model is used to generate the time-series to be analysed, since this model is able to reproduce the most significant non-linear behaviour of boiling water reactors (i.e. converging, diverging and limit-cycle oscillations). In this way the stability characteristics of the signals to be analysed are known a priori. An application to experimental time-traces measured on a thermalhydraulic natural circulation loop is reported as well. All methods perform equally well in determining the stability characteristics of the analysed signals.

The Use of Flashing Drums and Microchannel Heat Exchangers to Generate Steam in Large Integral Light Water Reactors

Abstract Integral pressurized water reactors are innovative reactors in which all of the components typically associated with the nuclear steam supply system of a nuclear power station are located within the reactor pressure vessel. In order to facilitate this modification in large [~1000-MW(electric)] light water reactors (LWRs), compact heat exchangers such as microchannel heat exchangers must be used. Previous attempts at using microchannel heat exchangers were unsuccessful since they are prone to vapor locking and crud blockage when the primary coolant boils. Therefore, the authors propose the use of a flashing drum to facilitate boiling in conjunction with a primary microchannel heat exchanger for a large integral LWR. The integral inherently safe light water reactor (I2S-LWR) is used as a basis for the implementation of this novel concept. The high-temperature, high-pressure secondary water generated in the secondary loop through heating in the microchannel primary heat exchanger of the I2S-LWR is sent to a flashing drum where 99.9% pure vapor is extracted and sent to the turbines. This prevents boiling in the primary heat exchanger that in turn reduces crud deposition, flow instabilities, and the potential for channel blockage or vapor locking in the small channel sizes of microchannel heat exchangers. The benefits and disadvantages of this approach are presented in this paper. Unfortunately, this innovative approach to nuclear steam generation for integral LWRs is challenged by a potential decrease in thermodynamic efficiency. Therefore, a sensitivity study is presented that explores the impact of several design variables on the thermodynamic efficiency of the plant. As part of this study, a simple and a complex Rankine cycle were modeled in order to determine the impact that system design modifications can play in recovering thermodynamic efficiency lost by the steam drum. Both cycles utilize turbines, condensers, and condensate/recirculation pumps, while the complex Rankine cycle utilizes a four-stage turbine with subsequent separation and open feedwater heaters. The optimized efficiencies for the simple and complex Rankine cycles are 31% and 33%, respectively, indicating that additional system enhancements to the power conversion system could compensate for the inclusion of a flashing drum.

Suitability of drift-flux models, void-fraction evolution, and 3-D flow pattern visualization during stationary and transient flashing flow in a vertical pipe

Abstract An assessment of void-fraction correlations and drift-flux models applied to stationary and transient flashing flows in a vertical pipe has been performed. Experiments have been carried out on a steam/water loop that can be operated both in forced- and natural-circulation conditions to provide data for the assessment. The GE-Ramp and Dix models are found to give very good predictions both for forced- and natural-circulation flow conditions, in the whole range of measured void fractions. Advanced instrumentation, namely, wire-mesh sensors, has been used to obtain a detailed picture of the void-fraction development in the system. On the basis of experimental data, a three-dimensional visualization of the transient flow pattern during flashing was achieved. A transition of the flow pattern between bubbly and slug/churn regimes was found.

Effect of liquid density differences on boiling two-phase flow stability

In order to investigate the effect of considering liquid density dependence on local fluid temperature in the thermal-hydraulic stability, a linear stability analysis is performed for a boiling natural circulation loop with an adiabatic riser. Type-I and Type-II instabilities were to investigate according to Fukuda-Koboris classification. Type-I instability is dominant when the flow quality is low, while Type-II instability is relevant at high flow quality. Type-II instability is well known as the typical density wave oscillation. Neglecting liquid density differences yields estimates of Type-II instability margins that are too small, due to both a change in system-dynamics features and in the operational point. On the other hand, neglecting liquid density differences yields estimates of Type-I stability margins that are too large, especially due to a change in the operational point. Neglecting density differences is thus non-conservative in this case. Therefore, it is highly recommended to include liquid density dependence on the fluid subcooling in the stability analysis if a flow loop with an adiabatic riser is operated under the condition of low flow quality.