Countercurrent Flow Limitation Research Articles

Analytical model for CHF in narrow gaps with surface orientation effects

The CHF in rectangular narrow gaps has been investigated to develop a reasonable predictive model for CHF, accounting for the surface orientation effects. The model was based on Wallis formulation of the counter current flow limitation (CCFL) for flooding of the flow entrance gap. The results by the predictive model were compared with the experimental data by Kim and Suh (2003), and a good agreement was obtained for gap sizes of 1mm and 2mm with surface inclination angles ranging from 15° to 90°. However, when the surface inclination angle was less than 15° (nearly downward-facing position), the predictive model underestimated the CHF. A modified Katto–Kosho correlation based on Kutateladze approach was further proposed and it could predict the CHF well for inclination angles less than 15°, due to the surface tension effects. For the gap sizes of 5mm and 10mm, the predictive results were far larger than experimental data by Kim and Suh. And the large differences between the predictive results and experimental data were attributed to the fact that the mechanism of CHF in large gap is quite different from a narrow gap. It indicated that the validity of the model based on CCFL was limited to gaps of less than 5mm. The present work is instructive for the safety analysis of the lower head of PWR in case of core meltdown during severe accident.

Comparison of Countercurrent Flow Limitation Experiments Performed in Two Different Models of the Hot Leg of a Pressurized Water Reactor With Rectangular Cross Section

Abstract In order to investigate the two-phase flow behavior during countercurrent flow limitation in the hot leg of a pressurized water reactor, two test models were built: one at the Kobe University and the other at the TOPFLOW test facility of Forschungszentrum Dresden-Rossendorf (FZD). Both test facilities are devoted to optical measurement techniques; therefore, a flat hot leg test section design was chosen. Countercurrent flow limitation (CCFL) experiments were performed, simulating the reflux condenser cooling mode appearing in some accident scenarios. The fluids used were air and water, both at room temperature. The pressure conditions were varied from atmospheric at Kobe to 3.0 bars absolute at TOPFLOW. According to the presented review of literature, very few data are available on flooding in channels with a rectangular cross section, and no experiments were performed in the past in such flat models of a hot leg. Commonly, the macroscopic effects of CCFL are represented in a flooding diagram, where the gas flow rate is plotted versus the discharge water flow rate, using the nondimensional superficial velocity (also known as Wallis parameter) as coordinates. However, the classical definition of the Wallis parameter contains the pipe diameter as characteristic length. In order to be able to perform comparisons with pipe experiments and to extrapolate to the power plant scale, the appropriate characteristic length should be determined. A detailed comparison of the test facilities operated at the Kobe University and at FZD is presented. With respect to the CCFL behavior, it is shown that the essential parts of the two hot leg test sections are very similar. This geometrical analogy allows us to perform meaningful comparisons. However, clear differences in the dimensions of the cross section (H×W=150×10 mm2 in Kobe, 250×50 mm2 at FZD) make it possible to point out the right characteristic length for hot leg models with rectangular cross sections. The hydraulic diameter, the channel height, and the Laplace critical wavelength (leading to the Kutateladze number) were tested. A comparison of our own results with similar experimental data and empirical correlations for pipes available in literature shows that the channel height is the characteristic length to be used in the Wallis parameter for channels with rectangular cross sections. However, some limitations were noticed for narrow channels, where CCFL is reached at lower gas fluxes, as already observed in small scale hot legs with pipe cross sections.

Numerical Simulation of Countercurrent Gas-Liquid Flow in a PWR Hot Leg under Reflux Cooling

In reflux cooling, the steam generated in the reactor core and the water condensed in a steam generator form a countercurrent flow in a hot leg. In order to investigate flow patterns in the hot leg under countercurrent flow conditions, countercurrent air-water tests were previously conducted using a 1/15th scale model of a PWR hot leg. Numerical simulation results for the tests using a three-dimensional twofluid model in FLUENT6.3.26, implemented with an appropriate set of correlations for the gas-liquid interfacial friction, were in good agreement with the measured data. In the present study, further numerical simulations were carried out for a full-scale hot leg under PWR plant conditions to investigate the effects of pipe diameter and fluid properties. The predicted countercurrent flow limitation characteristics were well correlated with the Wallis parameters and agreed well with the measured data from the 1/15th scale air-water tests as well as the full-scale steam-water UPTF tests. The results indicate that the set of correlations for the gas-liquid interfacial friction can be utilized to simulate countercurrent flows in the full-scale PWR hot leg.

Large Break Loss-of-Coolant Accident Analysis of VVER-1000 Reactor Using CATHARE Code

In the safety analysis of nuclear power plants, large break loss-of-coolant accidents (LB LOCAs) continue to be one of the major issues. In this study, the latest version of the French best-estimate computer code CATHARE2 V2.5_1 Mod6.1 was used in order to predict the thermal-hydraulic phenomena in the VVER-1000 reactor during LB LOCA. This type of reactor is in operation and under construction in several countries including Russia, Ukraine, Bulgaria, Czech Republic, China, and India.The paper first presents the CATHARE modeling of a VVER-1000 reactor, including the core, the vessel, the primary and secondary circuits with the pressurizer, the main circulation pumps, the horizontal steam generators, and the steam lines. The emergency core cooling system (ECCS) is presented also with hydroaccumulators and high- and low-pressure safety injection pumps. The break, located in the cold leg and close to the reactor, is represented by the RUPTURE module of CATHARE, which modelizes a double-ended guillotine type of break.In the current calculations, the bottom-up and top-down reflooding play a very important role, compared to some western pressurized water reactors, because of the ECCS injection into the downcomer and upper plenum of the VVER-1000. That is why special attention is paid to the validation of the CATHARE reflooding model, based on the REWET-II experiment devoted to VVER. In CATHARE, a special two-dimensional reflooding module has been developed, which takes into account the radial and axial conduction in the vicinity of the quench front. The assessment based on the REWET-II facility shows the ability of the code to predict the progression of two simultaneous quench fronts.Then, the paper presents the results of the LB LOCA analysis for VVER-1000 (reference case and sensitivity study), describing the blowdown, refill, and reflood phases of the accident. Problems related to the countercurrent flow limitation (CCFL) phenomena at the core outlet are also considered. The results of the calculations show reasonable prediction of the basic parameters important to the safety of the plant, such as quench front progression, fuel-cladding temperatures, break flows, pressure behavior, etc. As expected, the temperatures—despite the conservative character of the modeling—remain below the safety criteria. It should be noted that the calculated fuel wall temperatures are very sensitive to the proper selection of the reflooding parameters and CCFL and need skillful choice of the reflooding conditions.

Countercurrent Gas-Liquid Flow in a PWR Hot Leg under Reflux Cooling (I) Air-Water Tests for 1/15-Scale Model of a PWR Hot Leg

In the case of loss of residual heat removal systems under mid-loop operation during shutdown of a PWR plant, reflux cooling by a steam generator (SG) is expected, and steam generated in the reactor core and water condensed in the SG form a countercurrent flow in a hot leg. The flow is highly complicated because the hot leg consists of a horizontal pipe, an elbow, and an inclined pipe. In this study, a scale model of the PWR hot leg (1/15th of the actual plant size) is used to investigate flow patterns and characteristics of countercurrent flow limitation (CCFL). The following conclusions are obtained. (1) The effects of gas volumetric flux JG and water volumetric flux JG on flow pattern in the hot leg were made clear. Flow patterns in the elbow and inclined section are strongly affected by those in the horizontal section. (2) When the flow rate of the supplied water is constant, the value of JG at the onset of transition from stratified flow to wavy flow obtained by increasing JG is larger than that at the onset of transition from wavy flow to stratified flow obtained by decreasing JG . (3) CCFL characteristics obtained by increasing JG differ from those obtained by decreasing JG . CCFL data obtained by decreasing JG agree well with available data. (4) The boundary between wavy flow and stratified flow in the horizontal section obtained by decreasing JG agree well with the CCFL characteristics.

Countercurrent Gas-Liquid Flow in a Rectangular Channel Simulating a PWR Hot Leg (3)

To examine the effects of fluid properties on countercurrent flow limitation (CCFL) in a hot leg of a pressurized water reactor (PWR), experiments are conducted using glycerol-water solutions and a 1/5 scale hot leg model with a rectangular channel. Numerical simulations are also carried out using a VOF (volume of fluid) method implemented in the CFD software, FLUENT 6.3.26. The increase in the liquid viscosity does not change the intercept coefficient C in the Wallis-type CCFL correlation but increases the proportional coefficient m, which indicates that the liquid viscosity does not affect the interfacial drag but influences the wall friction. CCFL predicted for steam-water flow at 5 MPa, corresponding to a high gas density and low liquid viscosity condition, is greatly mitigated, which indicates that the gas density has a significant effect on the interfacial drag.

Numerical Simulation of Countercurrent Gas-Liquid Flow in a PWR Hot Leg under Reflux Cooling

In reflux cooling, the steam generated in the reactor core and the water condensed in a steam generator form a countercurrent flow in a hot leg. In order to investigate flow patterns in the hot leg under countercurrent flow conditions, countercurrent air-water tests were previously conducted using a 1/15th scale model of a PWR hot leg. Numerical simulation results for the tests using a three-dimensional twofluid model in FLUENT6.3.26, implemented with an appropriate set of correlations for the gas-liquid interfacial friction, were in good agreement with the measured data. In the present study, further numerical simulations were carried out for a full-scale hot leg under PWR plant conditions to investigate the effects of pipe diameter and fluid properties. The predicted countercurrent flow limitation characteristics were well correlated with the Wallis parameters and agreed well with the measured data from the 1/15th scale air-water tests as well as the full-scale steam-water UPTF tests. The results indicate that the set of correlations for the gas-liquid interfacial friction can be utilized to simulate countercurrent flows in the full-scale PWR hot leg.

Countercurrent Gas-Liquid Flow in a PWR Hot Leg under Reflux Cooling (I) Air-Water Tests for 1/15-Scale Model of a PWR Hot Leg

In the case of loss of residual heat removal systems under mid-loop operation during shutdown of a PWR plant, reflux cooling by a steam generator (SG) is expected, and steam generated in the reactor core and water condensed in the SG form a countercurrent flow in a hot leg. The flow is highly complicated because the hot leg consists of a horizontal pipe, an elbow, and an inclined pipe. In this study, a scale model of the PWR hot leg (1/15th of the actual plant size) is used to investigate flow patterns and characteristics of countercurrent flow limitation (CCFL). The following conclusions are obtained. (1) The effects of gas volumetric flux JG and water volumetric flux JG on flow pattern in the hot leg were made clear. Flow patterns in the elbow and inclined section are strongly affected by those in the horizontal section. (2) When the flow rate of the supplied water is constant, the value of JG at the onset of transition from stratified flow to wavy flow obtained by increasing JG is larger than that at the onset of transition from wavy flow to stratified flow obtained by decreasing JG . (3) CCFL characteristics obtained by increasing JG differ from those obtained by decreasing JG . CCFL data obtained by decreasing JG agree well with available data. (4) The boundary between wavy flow and stratified flow in the horizontal section obtained by decreasing JG agree well with the CCFL characteristics.

Experimental CFD grade data for stratified two-phase flows

Stratified two-phase flows were investigated at two test facilities with horizontal test-sections. For both, rectangular channel cross-sections were chosen to provide optimal observation possibilities for the application of optical measurement techniques. In order to show the local flow structure, high-speed video observation was applied, which delivers the high-resolution in space and time needed for CFD code validation. The first investigations were performed in the Horizontal Air/Water Channel (HAWAC), which is made of acrylic glass and allows the investigation of air/water co-current flows at atmospheric pressure and room temperature. At the channel inlet, a special device was designed for well-defined and adjustable inlet boundary conditions. For the quantitative analysis of the optical measurements performed at the HAWAC, an algorithm was developed to recognise the stratified interface in the camera frames. This allows to make statistical treatments for comparison with CFD calculation results. As an example, the unstable wave growth leading to slug flow is shown from the test-section inlet. Moreover, the hydraulic jump as the quasi-stationary discontinuous transition between super- and subcritical flow was investigated in this closed channel. The structure of the hydraulic jump over time is revealed by the calculation of the probability density of the water level. A series of experiments show that the hydraulic jump profile and its position from the inlet vary substantially with the inlet boundary conditions due to the momentum exchange between the phases. The second channel is built in the pressure chamber of the TOPFLOW test facility, which is used to perform air/water and steam/water experiments at pressures of up to 5.0 MPa and temperatures of up to 264 °C, but under pressure equilibrium with the vessel inside. In the present experiment, the test-section represents a flat model of the hot leg of the German Konvoi pressurised water reactor scaled at 1:3. The investigations focus on the flow regimes observed in the region of the elbow and of the steam generator inlet chamber, which are equipped with glass side walls. An overview of the experimental methodology and of the acquired data is given. These cover experiments without water circulation, which can be seen as test cases for CFD development, as well as counter-current flow limitation experiments, representing transient validation cases of a typical nuclear reactor safety issue.

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.

Experimental study on the air/water counter-current flow limitation in a model of the hot leg of a pressurized water reactor

An experimental investigation on the air/water counter-current two-phase flow in a horizontal rectangular channel connected to an inclined riser has been conducted. This test-section representing a model of the hot leg of a pressurized water reactor is mounted between two separators in a pressurized experimental vessel. The cross-section and length of the horizontal part of the test-section are (0.25 m × 0.05 m) and 2.59 m, respectively, whereas the inclination angle of the riser is 50°. The flow was captured by a high-speed camera in the bended region of the hot leg, delivering a detailed view of the stratified interface as well as of dispersed structures like bubbles and droplets. Countercurrent flow limitation (CCFL), or the onset of flooding, was found by analyzing the water levels measured in the separators. The counter-current flow limitation is defined as the maximum air mass flow rate at which the discharged water mass flow rate is equal to the inlet water mass flow rate. From the high-speed observations it was found that the initiation of flooding coincides with the formation of slug flow. Furthermore, a hysteresis was noticed between flooding and deflooding. The CCFL data was compared with similar experiments and empirical correlations available in the literature. Therefore, the Wallis-parameter was calculated for the rectangular cross-sections by using the channel height as length, instead of the diameter. The agreement of the CCFL curve is good, but the zero liquid penetration was found at lower values of the Wallis parameter than in most of the previous work. This deviation can be attributed to the special rectangular geometry of the hot leg model of FZD, since the other investigations were done for pipes.

Counter-current flow limitation velocity measured in annular narrow gaps formed between large diameter concentric pipes

A two-phase flow configuration in which the gas phase flows upwards while the liquid phase flows downwards is referred to as a counter-current flow pattern. This flow configuration cannot be preserved if any flow rate exceeds a criterion known as the counter-current flow limitation (CCFL) or flooding. Since the CCFL is important to chemical engineers, it has long been studied via experimental and analytical approaches. Most of the previous CCFL experiments in annular channels have been carried out with a small diameter annulus and large gap-to-diameter ratio annulus. The present experiment examines the CCFL in narrow annular channels having gap sizes of 1, 2, 3, and 5 mm. The outer diameter of the annular passage is 500 mm. At a gap size of 1 mm, it was visually observed that a CCFL locally occurred in some region of the periphery while the other region remained in a counter-current flow configuration. The region under partial CCFL condition expanded with an increase in the air flow rate, finally reaching a global CCFL. The air flow rate for the global CCFL was roughly 15% larger than that for initiation of a partial CCFL. This difference in air flow rate between the initiation of a partial CCFL and the global CCFL was reduced as the gap size increased. When the gap size was 5 mm, the partial CCFL was not observed, but onset of flooding led to a global CCFL. Because of the existence of a transient period, the CCFL was experimentally defined as the situation where net water accumulation is sustained. The measured CCFL data are presented in the form of a Wallis’ type correlation. Two length scales, hydraulic diameter and average circumference, were examined as the characteristic length scale. The average circumference appeared to better fit the experimental data, including results reported elsewhere. A new correlation using the average circumference as the characteristic length scale is suggested based on the experimental measurements of the present work and previous reports.

ＰＷＲホットレグを模擬した矩形流路での気液対向流（２）（気液対向流制限の解析的評価）

In the previous study, flow patterns under countercurrent air-water flow were observed in a rectangular channel simulating a PWR (pressurized water reactor) hot leg and close interrelations between flow patterns and countercurrent flow limitation (CCFL) were indicated. In this study, simple calculations of CCFL characteristics based on a momentum equation and numerical simulations using the thermal hydraulic calculation code FLUENT6.3.26 were conducted. The CCFL characteristics by the simple calculations agreed with the measured results but the constant m in the Wallis correlation was underestimated in the region of large air flow rates. In the numerical calculations, flow patterns in the simulated hot leg were not reproduced by 2D calculations due to the limitation of capability to predict the flow pattern transition from stratified flow to wavy-mist flow but were successfully reproduced by 3D calculations because of the interrelation between CCFL characteristics at the inlet and outlet of the simulated hot leg. Falling water flow rates predicted by 3D calculations agreed with the data in the region of large air flow rates.

ICONE15-10752 ANALYSIS OF THE EFFECT OF BED INHOMOGENEITY ON DEBRIS COOLABILITY

The present study investigates the potential impact of bed inhomogeneity on coolability of volumetrically heated debris beds which may form in a hypothetical severe accident in a LWR. Specifically, we examine the effect of the debris bed's micro and macro heterogeneity (in term of particle size and pore distributions in space). A vehicle for the analysis is WABE-2D code which simulates two-phase thermal-hydraulics in porous bed. The present analysis is performed for two types of "unit volume" which correspondingly represent the macro-heterogeneous bed and micro-heterogeneous bed. Results of calculation of dryout heat flux with WABE- 2D code show the extent by which macro and micro inhomogeneity can enhance the bed coolability. Phenomenologically, in both cases, inhomogeneity is found to facilitate natural circulation driven coolability (NCDC) which helps overcome the counter-current flow limitation. Implications of the findings on assessment of debris bed coolability and related research are discussed in the paper.

Countercurrent Flow Limitation in a Horizontal Tube Connected to an Inclined Riser

This paper presents the results of an experimental study of the countercurrent flow limitation, or flooding, performed in small reproductions of the PWR hot leg. The experiments were carried out with air and water at room temperature and low air pressure. The effects of the inclined and horizontal lengths, inclination angle of the riser, and diameter of the test section on the phenomenon were investigated. The results with partial delivery of water showed that for a fixed air velocity, an increase in the horizontal length or inclined length of the flow channel leads to an increase of the water carried over by the air. On the other hand, in pipes with larger diameters, the drag of the water is smaller. The experimental results showed small differences in the results for tests when the inclination of the riser is lower than 90°. For an inclination equal to 90°, the water carried by the air tends to be lower than in the other angles for a fixed air velocity. The study led also to a new correlation for the flooding.

Analytical model for CHF in narrows gaps on plates and in hemispherical geometries

This study deals with CHF in narrow gaps and the purpose is to propose a predictive model for the CHF, taking into account the effect of geometry and pressure. The modelling is based on a limitation by flooding of the flow entering the gap (CCFL for counter-current flow limitation). A model has been derived for both vertical plate and hemispherical geometry. The formulation proposed by Kutateladze (Eqs. (12) and (17) with a = 1 and b ∼ 1.7) provides best-fit results for both vertical channels and hemispherical geometry. The comparison with the results obtained by Köhler et al. [Köhler, W., Schmidt, H., Herbst, O., Krätzer, W., 1998. Thermohydraulische Untersuchungen zur Debris/Wand-Wechselwirkung (DEBRIS), Abschlussbericht Project No. 150 1017, November; Köhler, W., Schmidt, H., Herbst, O., Krätzer, W., 1998. Experiments on heat removal in a gap between debris crust and RPV wall. In: OECD/CSNI Workshop on In-Vessel Core Debris Retention and Coolability, Garching, Germany, March 3–6, also in First European-Japanese Two-Phase Flow Group Meeting, 36th European Two-Phase Flow Group Meeting, Portoroz 1–5 June, and Seventh Conference on Nuclear Engineering Tokyo, Japan, April 19–23, 2000 ICONE 7012.] seems to indicate that the validity of models based on CCFL controlled CHF is limited to gaps of less than 3–5 mm. Beyond this gap size, mechanisms other than CCFL might control the CHF. However, the experimental results are too scarce and affected by too large uncertainties to validate a theoretical model. Experimental uncertainties are mainly linked to the positioning of the structure (evolution of the gap with the temperature) and to the criteria that are applied to detect the CHF. The conclusion of applications to reactor situations at reduced pressure is that the corium mass that might be coolable through a gap is certainly much nearer to the mass observed in TMI2 (∼10–20 tonnes) than to the whole mass contained in a core (100 tonnes). The main uncertainty for reactor applications still remains the knowledge of the distribution and configuration of the relocated corium.

Experimental measurement of frictional law under CCFL in porous debris

Counter-current flow limitation (CCFL) is the dominant phenomena for dryout in porous debris, which would be formed during a severe accident of a nuclear power plant. Since flow at CCFL in porous debris is far away from normal two-phase flow in a pipe, it is not clear whether the interfacial friction laws in a pipe can be applied to the CCFL in porous debris. In the present experimental research, the void fraction in porous debris is measured simultaneously with the differential pressure and flow rates for gas and liquid. The combination of these simultaneously measured data makes it possible to estimate the shear stresses and friction factors in a porous debris at CCFL. It results that the wall friction factor estimated is larger than the Ergun equation proposed for single-phase flow in a porous bed. Furthermore, the interfacial friction factor estimated is well correlated by the theoretical correlation derived here from a force balance for a hypothetical flow channel in porous debris at CCFL.

Effect of gas density on countercurrent flow limitation in wetted wall column

Countercurrent wetted wall flow limitation was studied in a 20.6 mm i.d. cylindrical column, with water as the liquid. The gas was either air at up to 0.8 MPa, or argon at up to 1.3 MPa, thus covering a range of gas densities (1.6–21.3 kg/m3) of great practical interest for which very few data are available. The film Reynolds number varied in the range 515–3090. The velocity of gas for which signs of flooding instability first became apparent was typically 5 to 25% below the velocity at which liquid carryover started and this may be a result of the particular design adopted for fluid inlet to and outlet from the test section. Over the range of gas densities tested, the limiting gas velocity decreased with pressure from between 2.5 and 5.0 m/s to between 0.4 and 0.8 m/s, very approximately following the law: (gas density) × (limiting gas velocity)2 = constant. The data collected were used to test the correlation of Wallis, which gave remarkably good prediction of the limiting gas velocities, and a correlation of Alekseev and coworkers modified by McQuillan and Whalley in 1985, which gave poor predictions, particularly with regard to the effect of changes in liquid flow rate. The results of the present work are shown to be useful in interpreting recent data on the effect of gas density on the slug/churn flow transition in cocurrent vertical gas–liquid flow. © 2006 American Institute of Chemical Engineers AIChE J, 2006

COUNTER CURRENT FLOW LIMITATION, CCFL

COUNTER CURRENT FLOW LIMITATION, CCFL

MELCOR Analysis of Steam Generator Tube Creep Rupture in Station Blackout Severe Accident

A pressurized water reactor steam generator tube rupture (SGTR) is of concern because it represents a bypass of the containment for radioactive materials to the environment. In a station blackout accident, tube integrity could be threatened by creep rupture, particularly if cracks are present in the tube walls. Methods are developed herein to improve assessment capabilities for SGTR by using the severe-accident code MELCOR. Best-estimate assumptions based on recent research and computational fluid dynamics calculations are applied in the MELCOR analysis to simulate two-dimensional natural circulation and to determine the relative creep-rupture timing in the reactor coolant pressure boundary components. A new method is developed to estimate the steam generator (SG) hottest tube wall temperature and the tube critical crack size for the SG tubes to fail first. The critical crack size for SG tubes to fail first is estimated to be 20% of the wall thickness larger than by a previous analysis. Sensitivity studies show that the failure sequence would change if some assumptions are modified. In particular, the uncertainty in the countercurrent flow limit model could reverse the failure sequence of the SG tubes and surge line.