Michio Murase

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 PWR Hot Leg under Reflux Cooling (II) Numerical Simulation of 1/15-Scale Air-Water Tests

When reflux cooling is executed in the case of loss of residual heat removal systems under mid-loop operation of PWR, steam generated in the reactor core and water condensed in the steam generator may form a complicated countercurrent flow in the hot leg. Numerical simulations of air-water countercurrent flows in a 1/15-scale model of the hot leg are carried out in this study to examine the capability of predicting two-phase flow patterns and CCFL characteristics in the hot leg. A three-dimensional two-fluid model implemented in FLUENT6.3.26 is used in the simulations. Good agreements between measured and predicted flow patterns and CCFL characteristics are obtained by using an appropriate set of correlations for the interfacial friction coefficient in the momentum equation of the two-fluid model.

Evaporation and condensation heat transfer with a noncondensable gas present

Abstract To evaluate the system pressure of an external water wall type containment vessel, which is one of the passive systems for containment cooling, the evaporation and condensation behavior under a noncondensable gas presence has been experimentally examined. In the system, steam evaporated from the suppression pool surface into the wetwell, filled with noncondensable gas, and condensed on the containment vessel wall. The system pressure was the sum of the noncondensable gas pressure and saturated steam pressure in the wetwell. The wetwell temperature was, however, lower than the supression pool temperature and depended on the thermal resistance on the suppression pool surface. The evaporation and condensation heat transfer coefficients in the presence of air as noncondensable gas were measured and expressed by functions of steam/air mass ratio. The evaporation heat transfer coefficients were one order higher than the condensation heat transfer coefficients because the local noncondensable gas pressure was much lower on the evaporating pool surface than on the condensing liquid surface. Using logal properties of the heat transfer surfaces, there was a similar trend between evaporation and condensation even with a noncondensable gas present.

Development of Simplified Wave-type Vane in BWR Steam Dryer and Assessment of Vane Droplet Removal Characteristics

Air-water experiments were performed for the BWR steam dryer in order to elucidate droplet removal characteristics of the vane. Based on the results, a simplified vane was developed and its droplet removal characteristics were confirmed by air-water experiments using a whole dryer model. Phase-Doppler anemometer was used to measure droplet diameter distributions. In the experiments of the current vane with four-wave stages and 120° bend angle, almost all of the droplets were found to be trapped in the first and second vane stages. For the air velocity of 3.1 m/s, 90% of the inlet droplets were trapped there and 4% were trapped in the third and fourth stages, resulting in 6% being carried over. Sauter mean diameters at the exit were 6 and 5μm while at the inlet they were 71 and 64μm for the respective air velocities of 1 and 3 m/s. Based on the Weber number evaluation, the possible mechanism for the fine droplet generation was considered to be the breakup of droplets due to impingement on the liquid film f...

Conceptual Design and Thermal-Hydraulic Characteristics of Natural Circulation Boiling Water Reactors

A natural circulation boiling water reactor (BWR) with a rated capacity of 600 MW(electric) has been conceptually designed for small- and medium-sized light water reactors. The components and syste...

Correlation for countercurrent flow limitation in a PWR hot leg

Numerical simulations were done to evaluate countercurrent flow limitation (CCFL) characteristics in a pressurized water reactor (PWR) hot leg with the diameter of 750 mm by using a volume of fluid (VOF) method implemented in the CFD software, FLUENT6.3.26. The calculated CCFL characteristics agreed well with known values including the UPTF data at 1.5 MPa. Sensitivity analyses for system pressures up to 8 MPa showed that the calculated CCFL characteristics in the Wallis diagram were slightly mitigated from 0.1 MPa to 1.5 MPa with increasing system pressure, but they did not change from 1.5 MPa to 8MPa. Using the CCFLs calculated in this study and values measured under air–water and steam–water conditions, a CCFL correlation and its uncertainty were derived.

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.

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

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 mm 2 in Kobe, 250 × 50 mm 2 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.

BWR Loss of Coolant Integral Tests with Two Bundle Loop, (I) Thermal-Hydraulic Characteristics in Parallel Channels

Two simulation tests of a BWR loss-of-coolant accident (LOCA) by a postulated guillotine rupture of a recirculation suction line were conducted using the Two Bundle Loop (TBL), which was volumetrically scaled to a BWR/5 plant with 764 fuel bundles. The major objective of the tests was to clarify thermal-hydraulic difference in parallel bundles. In the tests, the failure of a diesel generator for two low pressure coolant injection (LPCI) pumps was assumed, and the initial bundle power combinations were 4.0 and 5.9 MW in the first test, and 5.0 and 4.9 MW in the second. In one of the two bundles, the rods heated up locally in the radial direction. In the other, the rods heated up rather uniformly and later than in the former bundle. Much water fell locally into the former bundle, while ascending steam flow from the lower plenum was larger in the latter. A difference in thermal-hydraulic responses was observed even in the case of nearly identical bundle powers, but the difference was less than in the case of...

Experimental study on heat removal characteristics for water wall type passive containment cooling system

To evaluate the heat removal capability of a water wall type cooling system, which is one passive containment cooling system (PCCS), the thermal hydraulic behavior in the suppression pool (S/P) and the outer pool (O/P, flat plate water wall) have been investigated experimentally. The following results were obtained. (1) A thermal stratification boundary, which separates the pools into the upper high temperature and lower low temperature regions, was formed just below the vent tube outlet. (2) Convection heat transfer characteristics in the S/P and O/P along the primary containment vessel (PCV) wall had no significant differences and were those of natural convection. Correlation of the natural convection heat transfer up to the Ra number of 2×1014 was obtained. (3) Vertical variations of local condensation heat transfer coefficients under a noncondensable gas presence were within ±10% of the average value for the 4.7 m heat transfer length. An experimental correlation for the average condensation heat tran...