Reflux Condensation Mode Research Articles

The use of a novel gradient heat flux sensor for characterization of reflux condensation

In this paper we present the developments in heat flux measurements using gradient heat flux sensors (GHFS). The GHFS is a sensor made of artificially created material with anisotropic thermo-electrical properties. Its properties including small size, robustness and low response time make it a valuable addition for characterizing heat flux in space-limited geometries as well as harsh environmental conditions. This makes it ideal for investigation of reflux condensation in the steam generator tubes of a pressurized water reactor. The reflux condenser mode of operation during accident as well as maintenance conditions can provide a significant passive cooling, removing the residual decay heat from the reactor core, thus preventing or at least delaying potential core uncovery. With this novel implementation of GHFS deeper characterization of heat flux during reflux condensation was achieved.

A new correlation for CCFL at full scale PWR excluding the Hutze effect in UPTF data

The effect of the Hutze (a secondary ECC injection pipe within the hot leg pipe of a Konvoi-type PWR) upon CCFL was experimentally investigated at COLLIDER test facility of the Technical University Munich (TUM). COLLIDER represents a PWR hot leg with a large dimeter of 190 mm and ~¼ scale. A Hutze pipe was built into COLLIDER’s hot leg according to a German Konvoi-type PWR reactor design data. The effect of the Hutze upon CCFL are shown through comparisons with previous investigations at COLLIDER without a Hutze. Visual observations using high-speed recordings are presented at different flow conditions: supercritical conditions at low air velocities, transition into subcritical flow, and the onset of CCFL with comparison against previous recording without a Hutze. Limits of: the onset of hydraulic jumps, onset of CCFL (flooding), and most importantly CCFL characteristics are also shown and compared. The Hutze was found to induce a large impact upon these limits: two hydraulic jumps are observed prior to the onset of CCFL instead of only one (in case without a Hutze), the limits of the onset of hydraulic jumps and onset of CCFL occur at much lower air velocities, and the hysteresis effect (difference between limits of flooding and deflooding) disappears. CCFL characteristics curve is also shifted toward lower values of gas velocities and less liquid flows into the reactor vessel at same air velocities in the case with a Hutze which means a less cooling potential during the reflux condensation mode. Ohnuki correction of the Hutze effect upon CCFL characteristics of 1988 is tested and it seems to work well at small diameter but it overestimates CCFL characteristics for COLLIDER data. The previous agreement between CCFL characteristics obtained from downscaled experiments with D > 50 mm without a Hutze (including COLLIDER data and other large scale facilities) with UPTF data (at full scale) is found to happen because of the Hutze effect which shifts the CCFL characteristics downwards in Wallis diagram and not because of the correct representation of occurring CCFL phenomena. The actual CCFL characteristics for the 1:1 scale without a Hutze are expected to be higher than those reported from data with Hutze at UPTF facility. This indicates that there is still a scale effect even at a large diameter channel, and that most previous downscaled experiments have underestimated CCFL characteristics at full scale. Based upon obtained results, a correlation of CCFL characteristics at full scale excluding the Hutze effect is given. This correlation is of a special interest for the design of new PWR reactors as most new designs does not include a Hutze.

Air/Water Counter-Current Flow Experiments in a Model of the Hot Leg of a Pressurized Water Reactor

Different scenarios of small break loss of coolant accident for pressurized water reactors (PWRs) lead to the reflux-condenser mode in which steam enters the hot leg from the reactor pressure vessel (RPV) and condenses in the steam generator. A limitation of the condensate backflow toward the RPV by the steam flowing in counter current could affect the core cooling and must be prevented. The simulation of counter-current flow limitation conditions, which is dominated by 3D effects, requires the use of a computational fluid dynamics (CFD) approach. These numerical methods are not yet mature, so dedicated experimental data are needed for validation purposes. In order to investigate the two-phase flow behavior in a complex reactor-typical geometry and to supply suitable data for CFD code validation, the “hot leg model” was built at Forschungszentrum Dresden-Rossendorf (FZD). This setup is devoted to optical measurement techniques, and therefore, a flat test-section design was chosen with a width of 50 mm. The test section outlines represent the hot leg of a German Konvoi PWR at a scale of 1:3 (i.e., 250 mm channel height). The test section is mounted between two separators, one simulating the RPV and the other is connected to the steam generator inlet chamber. The hot leg model is operated under pressure equilibrium in the pressure vessel of the TOPFLOW facility of FZD. The air/water experiments presented in this article focus on the flow structure observed in the region of the riser and of the steam generator inlet chamber at room temperature and pressures up to 3 bar. The performed high-speed observations show the evolution of the stratified interface and the distribution of the two-phase mixture (droplets and bubbles). The counter-current flow limitation was quantified using the variation in the water levels measured in the separators. A confrontation with the images indicates that the initiation of flooding coincides with the reversal of the flow in the horizontal part of the hot leg. Afterward, bigger waves are generated, which develop to slugs. Furthermore, the flooding points obtained from the experiments were compared with empirical correlations available in literature. A good overall agreement was obtained, while the zero penetration was found at lower values of the gaseous Wallis parameter compared with previous work. This deviation can be attributed to the rectangular cross section of the hot leg model.

Analysis of the OECD/NEA ROSA Project experiment simulating a PWR small break LOCA with high-power natural circulation

OECD/NEA ROSA Project experiment with the large scale test facility (LSTF) in JAEA was conducted simulating a PWR 1% cold leg small break LOCA with an assumption of high-power natural circulation due to failure of scram and total failure of high pressure injection system. The core power curve for the LSTF experiment was obtained by PWR LOCA analysis using JAEA-developed coupled three-dimensional kinetics and thermal-hydraulics code SKETCH-INS/TRAC-PF1 with detailed core model. A post-test analysis was performed against the obtained data by using JAEA-modified RELAP5/MOD3.2.1.2 code to validate the code predictability. The JAEA-modified RELAP5/MOD3.2.1.2 code was used by incorporating a break model that employs maximum bounding flow theory with a discharge coefficient of 0.61 for two-phase break flow. In the experiment, flow in hot legs became supercritical during two-phase natural circulation, causing the hot leg liquid level to be quite low. Liquid accumulation in steam generator U-tube upflow-side took place during reflux condensation mode due to high vapor velocity. The RELAP5 code predicted reasonably well the overall thermal-hydraulic phenomena observed in the experiment. The code, however, overpredicted the break flow rate especially during two-phase flow discharge period probably because of the failure in the correct simulation of the cold leg liquid level due to late decrease in the primary loop flow rate.

Condensation of n-Hexane and Isopropanol Mixed with a Noncondensable Gas in a New Plate Heat Exchanger Geometry

This article presents a study of heat transfer during condensation of n-hexane or isopropanol with a noncondensable gas (nitrogen) in a new plate heat exchanger geometry. Three test sections were installed on the test rig devoted to condensation of mixtures, either in reflux or co-current condensation configurations. For both configurations, heat transfer measurements were carried out. In single-phase flow tests, experimental data reduced by a log mean temperature difference were compared with new correlations on the gas side, adapted to the present specific plate geometry. Correlations of the Fanning friction factor were deduced from gas-side pressure drop measurements on the two geometries tested. In two-phase flow tests, the log mean temperature difference method was assumed to be correct to the first-order for the comparison of results in two test sections. The condensation curve method was applied to the present results, and co-current and reflux condensation configurations were then compared in terms of experimental overall heat transfer coefficients. It is shown that for a gas Reynolds number higher than 2,000, heat transfer coefficients in reflux condensation become higher than those for co-current condensation. Flooding phenomena were observed for specific experimental conditions in reflux condensation mode. The flooding experimental data are compared with five existing correlations (i.e., Wallis, modified Wallis, English et al., McQuillan & Whalley, and Zapke & Kröger) to observe fluid property effects for the same ranges of heat duty and mass flow rates in the condenser. The English et al. and Zapke & Kröger correlations show the best agreement with experimental flooding data in the present geometry.

Local heat transfer during reflux condensation mode in a U-tube with and without noncondensible gases

A series of experiments was performed to investigate the local heat transfer phenomenon inside a U-tube in a reflux condensation mode. A total of 477 data (108 for pure steam flow conditions and 369 for steam–air flow conditions) for the local heat transfer coefficients were obtained for various inlet flow rates of steam and air under atmospheric conditions. Based on the steam–air experimental results, a new correlation applicable to the reflux condensation mode is developed using the degradation factor. The new correlation includes the effects of the flow rates of the condensate and the noncondensible gases (air) on the heat transfer coefficient. Most of our data agree with the predicted values with a RMS error of 27.4%.

Analysis of natural circulation phenomena in VVER-1000

In all light water reactors (LWR), natural circulation is an important passive heat removal mechanism. In the present paper, the natural circulation phenomena are studied with reference to step-wise coolant inventory reduction and a small break loss-of-coolant-accident (SBLOCA) in the cold leg of VVER-1000. The natural circulation flow map (NCFM) approach is considered to evaluate the natural circulation performance of the VVER-1000 NPP also comparing VVER-1000 and PWR systems. Three different elevations between heat source (core) and heat sink (steam generators) zones have been considered in order to characterize the buoyancy force in a VVER-1000. The influence of power and the cold legs loop seal upon the natural circulation performance is also evaluated. In the second part, a series of SBLOCA simulations with break area ranging from 0.5 to 11.7% of the cold leg cross sectional area are performed starting with the VVER-1000 system in nominal conditions. The effect of Emergency Core Cooling System (ECCS) including passive and active parts of ECCS are evaluated. The simulations were performed by the help of the system code RELAP5. Within the framework of the qualification of the adopted computational tools, the results are compared with experimental data from Kozloduy NPP unit 6 test and PSB-VVER integral test facility available from the literature. Namely, the qualification of the adopted nodalisation in steady state conditions is achieved by using experimental data. The accuracy of selected results have been estimated in quantitative terms by applying the fast Fourier transform based method (FFTBM). Finally, the relevance and the potential for the occurrence of the reflux condensation mode, i.e., one of the Natural Circulation regimes, for cooling of reactor core in VVER-1000 are discussed.

Experimental and Numerical Investigation of Boron Dilution Transients in Pressurized Water Reactors

Within the pressurized water reactor (PWR) safety analyses, attention has increasingly focused in recent years on boron dilution events that could potentially lead to reactivity transients. Mixing of the low-boron water with the ambient coolant of higher boron content provides an important mitigation mechanism before the low-boron water enters the core.Experimental support is needed to validate the computational tools to be applied to analyze the mixing of the low-boron water. Experiments were performed in the three test facilities—the Upper Plenum Test Facility (UPTF), the Primärkreislauf (PKL), and the Rossendorf coolant mixing model (ROCOM)—in Germany.The relevant PKL and UPTF tests were focused on small-break loss-of-coolant accident (SBLOCA) scenarios with reflux-condenser mode and restart of natural circulation. The two test facilities represent a typical western-type PWR and are/were operated by Siemens/KWU now Framatome ANP in Germany. While the restart of natural circulation was investigated in the PKL system test facility (volume 1:145, height 1:1), the UPTF experiments dealt with the mixing of water flows with different boron concentration in the cold legs, reactor pressure vessel (RPV) downcomer, and the lower plenum (all these components were full-scale models).The results from the PKL test facility demonstrate that in case of a postulated SBLOCA with reflux condensation phase, natural circulation does not start up simultaneously in all loops. This means that slugs of condensate, which might have accumulated in the pump seal during reflux-condenser mode of operation, would reach the RPV at different points in time. The UPTF tests showed an almost ideal mixing of water flows with different boron concentration in the RPV downcomer.The ROCOM test facility has been built in a linear scale of 1:5 for the investigation of coolant mixing phenomena in a wide range of flow conditions in the RPV of the German KONVOI-type PWR. The test results presented are focused on the mixing of a slug of deborated water during the startup of the first reactor coolant pump. Based on experimentally determined pulse responses, a semianalytical model for the description of coolant mixing inside the KONVOI RPV has been developed. Calculations for a presumed boron dilution event during the startup of the first reactor coolant pump have been carried out by means of the semianalytical model and independently by means of the computational fluid dynamics code CFX-4. The semianalytical model is able to describe the time dependent behavior of the deboration front at each fuel element position in a good agreement with the experiment. All main mixing effects, observed in the experiment, are also reproduced by the CFX calculation.

Thermal hydraulics of PWRS with respect to boron dilution phenomena. Experimental results from the test facilities PKL and UPTF

Within the PWR safety analyses, attention has increasingly focused in recent years on boron dilution events which could potentially lead to reactivity transients. In Germany, the current discussion concerning boron dilution events is focused on small break loss of coolant accident (SB-LOCA) scenarios with reflux-condenser mode and restart of natural circulation (NC). The investigation of this topic was the subject of experiments performed in the Primärkreislauf (PKL) test facility and in the upper plenum test facility (UPTF). The two test facilities represent a typical western-type pressurized water reactor (PWR) and are/were operated by Siemens/KWU in Germany. While the restart of NC was investigated in the PKL system test facility (volume 1:145, height 1:1), the UPTF experiments dealt with the mixing of water flows with different boron concentration in the cold legs, reactor pressure vessel (RPV) downcomer and the lower plenum (all these components were full-scale models). The results from the PKL test facility demonstrate that in case of a postulated SB-LOCA (30 cm 2-break in the cold leg; two out of four high pressure safety injection pumps (HP-SIPs) available) NC does not start up simultaneously in all loops. This means, that plugs of non-borated condensate, that might have accumulated in the pump seal during reflux condenser mode of operation, would reach the RPV at different points in time. The UPTF tests showed an almost ideal mixing of water flows with different boron concentration in the RPV downcomer. The most important results from PKL and UPTF tests which concern boron dilution events following the restart of NC are presented in this paper.

Thermal-hydraulic phenomena during reflux condensation cooling in steam generator tubes

The transitions of cooling mechanism in steam generator tubes during reflux condensation are studied. It is found that the transitions are closely related to the occurrence of flooding or counter-current flow limitation phenomena in steam generator tubes. As shown in the previous studies of other researchers, the transition from filmwise reflux condensation into total reflux condensation occurs when the flooding criterion suggested by Wallis is met. In this study, it is suggested that the transition from total reflux condensation to complete carry-over occurs depending on the tube height and cooling conditions. It is also shown that the flooding at SG tubes occurs before the flooding at hot leg when a reflux condensation mode is existing in steam generator. Though the thermal-hydraulic conditions during reflux condensation after a small-break loss-of-coolant accident have enough margin to the transition into carry-over, considerations for the prevention of primary coolant relocation should be provided.

98/03823 Application of nonuniform-line theory to the simulation of electromagnetic transients in power systems

Loss of residual heat removal has occurred in several PWRs during mid-loop operation after plant shutdown, and is now recognized as an accident situation whose relevant physical phenomena require improved understanding. Several cases have been selected for investigation in the frame of the BETHSY program, with the main purpose of providing an experimental basis for the assessment of safety codes which have been so far especially involved with accident transients initiated during full-power operation.The major results from four experiments are presented in this paper, which addresses two main kinds of physical problem: mechanisms for, and rate of, primary mass inventory reduction in the case of manways opened at various locations in the primary coolant system; cooling of the plant when the primary is only half-open through vent paths and one steam generator is available to remove decay heat in reflux condenser mode with the presence of non-condensable gas.

Nitrogen in a steam generator of a PWR under SBLOCA conditions: Experimental investigations in the PKL test facility and comparison with analytical studies

Since 1976, extensive investigations of the thermal hydraulic behavior of a pressurized water reactor (PWR) during various accident scenarios have been carried out by Siemens/KWU at the integral test facility PKL, which simulates a typical four-loop 1300 MWe PWR of KWU design. One major topic of the current investigations includes tests under small break loss of coolant accident (SBLOCA) conditions, postulating additional system failures. During plant cool down in the course of SBLOCAs, besides other measures, the subsystems of the emergency core cooling system are initiated. Below a primary pressure of 26 bar, the accumulators (ACCs) automatically inject water into the primary system. With the postulation of additional system failures in the presented PKL experiment, nitrogen (N 2) can enter the primary system at a primary pressure > 10 bar before the ACCs are shut off. Under these very hypothetical SBLOCA conditions, nitrogen can accumulate in the steam generator U-tubes, whereby the heat removal mechanism changes from natural circulation to reflux-condenser mode. This phenomenon is analyzed in detail; interesting two-phase flow patterns were observed with heat removal under water-steam countercurrent flow with nitrogen present. This PKL experiment shows that a sudden stagnation of natural circulation in all loops caused by nitrogen at considerably reduced inventory does not impede heat removal from the core; it results in the very effective reflux-condenser mode. The system stabilizes itself at a specific pressure and temperature level, and consequently the removal of the core power is ensured for a given period, until further measures are initiated.

Transient analysis of a condensation experiment in the noncondensable gas-filled closed loop using an inverted U-tube

Two-phase transient phenomena in the noncondensable gas-filled closed loop was investigated numerically using the RELAP5/MOD3 code. Analyses for three different modes observed in the inverted U-tube experiment, i.e., reflux condensation, natural circulation, and oscillatory mode between these two modes were performed. Major thermal-hydraulic phenomena were predicted well, however, system pressure and pressure drop at the U-tube riser side were overpredicted than experimental data. Higher frequent flow oscillation than the data was also predicted.

Reflux condenser mode with non-condensible gas: assessment of Cathare against Bethsy test 7.2C

The condensation process in steam generator (SG) U-tubes with the presence of non-condensible gas (nitrogen) has been investigated on the Bethsy integral test facility. Bethsy test 7.2c, dealing with the reflux condenser mode, consists of a succession of steady states where the effects of the amount of nitrogen, core power and secondary pressure on the primary-to-secondary heat transfer have been analysed. Examination of the non-condensible gas distribution in the SG U-tubes leads to the identification of an active zone where condensation takes place and a passive zone with a high concentration of nitrogen where no heat transfer occurs. The experimental data serve as a basis for safety code assessment. The first verification of these situations has been performed with the Cathare code, and results are presented and discussed in the following.

Loss of residual heat removal during mid-loop operation: BETHSY experiments

Loss of residual heat removal has occurred in several PWRs during mid-loop operation after plant shutdown, and is now recognized as an accident situation whose relevant physical phenomena require improved understanding. Several cases have been selected for investigation in the frame of the BETHSY program, with the main purpose of providing an experimental basis for the assessment of safety codes which have been so far especially involved with accident transients initiated during full-power operation. The major results from four experiments are presented in this paper, which addresses two main kinds of physical problem: mechanisms for, and rate of, primary mass inventory reduction in the case of manways opened at various locations in the primary coolant system; cooling of the plant when the primary is only half-open through vent paths and one steam generator is available to remove decay heat in reflux condenser mode with the presence of non-condensable gas.

Filmwise reflux condensation length and transition point to the complete reflux condensation in vertical U-tubes

Filmwise reflux condensation lengths and transition points from the filmwise reflux condensation to the complete reflux condensation mode have been investigated in a single verticle U-tube. A simple analytical expression for the filmwise reflux condensation length was derived based partly on the existing empirical correlation for the heat transfer in laminar film condensation on vertical tubes. Predictions of this model were compared with experimental data for different shell-side coolant temperatures. In addition, experimentally determined transition points from the filmwise reflux condensation to the complete reflux condensation were compared with predictions of an existing model. Agreement between theoretical predictions and experimental data is fairly good and consistent for the ranges of test parameters examined in the present work.