Issues for Nuclear Power Plants Steam Generators

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HORIZONTAL TWO-PHASE FLOW PATTERN RECOGNITION

In the present work a Wire Mesh Sensor (WMS) has been adopted to characterize the air-water two-phase flow in a test section consisting of a horizontal Plexiglas pipe of internal diameter 19.5 mm and total length of about 6 m. The flow quality ranges from 0 to 0.73 and the superficial velocity ranges from 0.145 to 31.94 m/s for air and from 0.019 to 2.62 m/s for water. The observed flow patterns are stratified-bubble-slug/plug-annular. The WMS consists of two planes of parallel wire grids (16x16) that are placed across the channel at 1.5 mm and span over the measuring cross section. The wires of both planes cross under an angle of 90°, with a diameter Dwire of 70 μm and a pitch equal to 1.3 mm. The void fraction profiles are derived from the sensor data and their evolution in time and space is analyzed and discussed. The dependence of the signals on the measured fluid dynamic quantities is discussed too. The main task is to predict which flow pattern will exist under any set of operating conditions as well as to predict the value of characteristic flow parameter

Tracer use for the protection of water resources in nuclear sites

Natural and artificial tracers can be used for nuclear sites management. Tracer techniques coupled with modelling and environmental monitoring activities are an effective tool to characterize and to foresee the radionuclide dynamic in the environment. In particular, the safeguard of water resources for human purposes must be guaranteed. In this work, the transport of tracers H-3 and I-129 in groundwater and subsoil was evaluated by means of Hydrus 1D and AMBER codes. Information on how radionuclides migrate in the environment were obtained and preliminary hypothesis on how to design the environmental monitoring network of the investigated site were deduced

Use of RELAP50MOD3.3 Code to Get Fluid Dynamic Stability Maps

The analysis that has been carried out in the present paper shows the possibility to predict the onset of density wave oscillations by means of RELAP5/MOD3.3 code for simple geometry systems, whereas the difference between 1D and 3D approximations is not relevant for the purpose of the analysis; the shape of the boundary line in the stability maps is in agreement with the other authors prediction. The approximation used for helically coiled pipes in the multichannel systems seems not to affect the instability onset and the classical shape of the instability maps, that seem a powerful tool for the fluid dynamic instability predictio

Electrical Capacitance Probe Characterization in Vertical Annular Two-Phase Flow

The paper presents the experimental analysis and the characterization of an electrical capacitance probe (ECP) that has beendeveloped at the SIET Italian Company, for the measurement of two-phase flow parameters during the experimental simulation of nuclear accidents, as LOCA.The ECP is used to investigate a vertical air/water flow, characterized by void fraction higher than 95%, with mass flow rates ranging from 0.094 to 0.15 kg/s for air and from0.002 to 0.021 kg/s for water, corresponding to an annular flow pattern. From the ECP signals, the electrode shape functions (i.e., the signals as a function of electrode distances) in single and two-phase flows are obtained. The dependence of the signal on the void fraction is derived and the liquid film thickness and the phase's velocity are evaluated by means of rather simple models. The experimental analysis allows one to characterize the ECP, showing the advantages and the drawbacks of this technique for the two-phase flow characterization at high void fractio

Thermal hydraulic comparison of helical coil and bayonet tube steam generators for Small Modular Reactors

In the present paper a comparative analysis of the thermalhydraulic performance of helical coil and bayonet tube steam generators (SG) is presented. The tool chosen for the analysis is RELAP5-3D/4.0.3. The reference conditions are the ones of the primary and secondary fluids of the SMART Small Modular Reactor. The analysis has been carried out by considering different operating conditions. First, the performance of the SGs in nominal conditions has been compared; subsequently, assuming a power load control at constant average primary temperature, the thermal hydraulic response of the components at different operating conditions has been studied and the helical coil SG results to be the most compact configuration. It’s behavior is characterized by higher frictional pressure losses and lower inner heat transfer coefficient if compared with the bayonet tube SG. To analyze the performance of the components out of the nominal condition a region of the map of operation is studied by varying the inlet temperatures of the fluids. A common behavior is found in the considered region. Differences of approximately 12% for the outlet temperatures and 6% for the power have been found

Helical coil thermal hydraulic model

A model has been developed in Matlab environment for the thermal hydraulic analysis of helical coil and shell steam generators. The model considers the internal flow inside one helix and its associated control volume of water on the external side, both characterized by their inlet thermodynamic conditions and the characteristic geometry data. The model evaluates the behaviour of the thermal-hydraulic parameters of the two fluids, such as temperature, pressure, heat transfer coefficients, flow quality, void fraction and heat flux. The evaluation of the heat transfer coefficients as well as the pressure drops has been performed by means of the most validated literature correlations. The model has been applied to one of the steam generators of the IRIS modular reactor and a comparison has been performed with the RELAP5/Mod.3.3 code applied to an inclined straight pipe that has the same length and the same elevation change between inlet and outlet of the real helix. The predictions of the developed model and RELAP5/Mod.3.3 code are in fairly good agreement before the dryout region, while the dryout front inside the helical pipes is predicted at a lower distance from inlet by the model

Characterization of water-air dispersed two phase flow

The analysis of two phase flow has a great relevance in many industrial sectors such as nuclear and process industry. The study and the measurement of the related phenomena are particularly dicult due to the variety of the parameters that aect the flow (void fraction, flow regime, orientation, etc.). Measurement instrumentation for two phase flow is nowadays very limited even if it would be highly useful in the industrial field. Before approaching the research and development toward the realization of innovative instrumentation for two-phase flow measurement, it is fundamental a precise description of the particular flow regime of interest. Dispersed water droplets in air/gas is a possible flow regime at high gas void fractions; therefore it is important to characterize this particular flow pattern in a detailed way. To reproduce this condition in a laboratory environment it is possible to use nozzles with very small outlet diameters and high pressure water supply. In this paper the characterization of dispersed flow is performed as function of the nozzle characteristic and water inlet pressure. The tests are performed using an experimental setup realized at the Energy Department at Politecnico di Torino. The water jet is observed in a PMMA (PolyMethylMethacrylate) pipe 1.8 meters long with an inner diameter of 78 mm. High pressure water is obtained using a plunger pump and pump inlet water pressure is adjustable in the range 1-4 bar.Water pressure upstream the nozzle and air entrainment flow rate are measured and used as primary parameters of the study. A sensitivity analysis on these two parameters is performed with the purpose to find the conditions that are optimal to reproduce a dispersed flow

Thermal hydraulics of accelerator-driven system windowless targets

The study of the fluid dynamics of the windowless spallation target of an accelerator-driven system (ADS) is presented. Several target mockup configurations have been investigated: the first one was a symmetrical target, which was made by two concentric cylinders and the other configurations are not symmetrical. In the experiments, water has been used as hydraulic equivalent to lead–bismuth eutectic fluid. The experiments have been carried out at room temperature and flow rate up to 24 kg/s. The fluid velocity components have been measured by an ultrasound technique. The velocity field of the liquid within the target region either for the approximately axial–symmetrical configuration or for the not symmetrical ones as a function of the flow rate and the initial liquid level is presented. A comparison of experimental data with the prediction of the finite volume FLUENT code is also presented. Moreover, the results of a 2D–3D numerical analysis that investigates the effect on the steady state thermal and flow fields due to the insertion of guide vanes in the windowless target unit (TU) of the EFIT project ADS nuclear reactor are presented, by analyzing both the cold flow case (absence of power generation) and the hot flow case (nominal power generation inside the TU)

Learning biophysically-motivated parameters for alpha helix prediction

<p>Abstract</p> <p>Background</p> <p>Our goal is to develop a state-of-the-art protein secondary structure predictor, with an intuitive and biophysically-motivated energy model. We treat structure prediction as an optimization problem, using parameterizable cost functions representing biological "pseudo-energies". Machine learning methods are applied to estimate the values of the parameters to correctly predict known protein structures.</p> <p>Results</p> <p>Focusing on the prediction of alpha helices in proteins, we show that a model with 302 parameters can achieve a Q_{<it>α </it>}value of 77.6% and an SOV_{<it>α </it>}value of 73.4%. Such performance numbers are among the best for techniques that do not rely on external databases (such as multiple sequence alignments). Further, it is easier to extract biological significance from a model with so few parameters.</p> <p>Conclusion</p> <p>The method presented shows promise for the prediction of protein secondary structure. Biophysically-motivated elementary free-energies can be learned using SVM techniques to construct an energy cost function whose predictive performance rivals state-of-the-art. This method is general and can be extended beyond the all-alpha case described here.</p