Bottom Flooding Research Articles

CFD simulation on hydrodynamic and thermal behavior of elliptical condensing tubes with an improved mass transfer model

Comprehensive two-phase CFD simulations were conducted to investigate the hydrodynamic and thermal performance of the elliptical condensing tubes. An improved mass transfer model was developed to account for the condensation phase change. The model was validated to be in good agreement with the analytical solution as well as the experimental data. The transient drop-off behavior of elliptical tubes was first reported. The drop frequency increased with the eccentricity, except for the case of E-0.8, which configuration showed a unique geometric shape whose frequency was 10% lower than E-0.6, therefore it should be avoided in condenser design. The surface tension affected the local heat transfer more in the bottom flooding regions, but had little effect on the average heat transfer. Both external and internal heat transfer were evaluated for overall thermal performance, suggesting that the elliptical tube with an eccentricity beyond 0.92 is not preferred to be adopted unless the pump power of the internal flow is not a concern. Although increasing eccentricity allows the tube to achieve higher condensation heat transfer, it is still a restrictive factor for the overall performance. Therefore, future work will consider additional improvements such as adding low-fins or 3-D roughness to the smooth elliptical tube.

Chrono-photographic visualization and characterization of the flow regimes in rewetting by bottom flooding

Many engineering applications encounter the highly complicated rewetting phenomenon. The complexity of the process arises as the phenomenon is highly transient and possesses an implicit interplay of several transport processes. In particular, the hydrodynamics of in-conduit rewetting is poorly understood due to lack of visualization. The present study reports an in-depth quantitative representation of the hydrodynamics of different two phase flow regimes associated with rewetting by bottom flooding, perceived from an intelligently fabricated experimental test facility. The technique of chronophotography and image processing based on pixel level information of a fixed region of interest has been utilized to quantitatively characterize different flow regimes like – nucleate boiling, transition boiling, inverted annular film boiling and dispersed flow film boiling. The effects of inlet mass flux and initial wall temperature have been explained based on operating regime maps. The rich hydrodynamics captured in this report provides an in-depth knowledge of the associated heat transfer mechanisms.

Effect of Nozzle Diameter on Quenching Behavior of Simulant Material CaO-Fe2O3 Under Bottom Flooding Conditions

To mitigate severe accidents in nuclear reactors, the present research sheds light on the melt-coolability behavior of corium with hypothetical experiments that have been performed at two different nozzle diameters under bottom flooding conditions. In this research, a simulant material CaO-Fe2O3 powder mixture was melted and poured into the test section that was embedded in the test facility (using a bottom pouring furnace instead of a tiltable furnace). Then, from the bottom of the melt pool, water was flooded through a nozzle at a pressure of 0.70 bar and a water flow rate of 12 liters per minute. Because of the interaction between the water and melt, the melt quenched and converted into fine porous debris, and the temperature history was recorded using 12 K-type thermocouples connected to a data acquisition system. The average quenching time and porosity of the debris were affected by variations in the nozzle diameter. This research will help in understanding real core-melt accidents that generally occur in nuclear power plants.

Experimental investigation of quenching behavior of simulant material CaO-Fe2O3 under bottom flooding with the effect of melt volume

In a severe accident of a nuclear reactor, the termination and stabilization of the melt pool are necessary for the safety of the reactor. To address this issue, melt pool coolability plays a significant role in severe accident scenarios. In the context of this coolability measures, there are three major techniques i.e. top flooding, bottom flooding, and indirect cooling were used in the past and also in the present scenario to analyzed the actual phenomena occurred during a severe accident in a nuclear reactor with numerous simulant material of different composition and melt volume. This is because of extremely difficult to use the prototype material and conditions. Among the various strategy of melt coolability, it has been found that the melt coolability with the bottom flooding technique is very efficient to quench the material within a few minutes. In this regard, the present paper has involved the study of quenching phenomena of melt pool using bottom flooding at a small scale with a simulant material CaO-Fe2O3 at two different melt volume i.e. 1 l and 2 l. The simulant material is melted into an electric furnace at 1420 °C (~200 °C superheat) and poured into the test section where it has been quenched through water induction by a nozzle (12 mm) from the bottom side. Temperature history has been analyzed along the radial and axial direction of melt flow and further particle size and porosity of debris were measured. The size of debris was measured in the range of 0.4 to 40 mm although this debris was so fragile in nature. The average porosity was measured as 56% and the result was found in good agreement with literature.

Leidenfrost Phenomenon and Rewetting of Hot Vertical Tubes by Bottom Flooding Using Nanofluids

Leidenfrost (LF) and rewetting (RWT) phenomenon are both closely related and involved for the cooling of a very hot object utilizing a suitable cooling agent. The present study focusses on the influence of different parameters on both LF and RWT phenomenon of vertical tubes by bottom flooding. The various parameters primarily include the quantity or injection rate of the coolant (CLNT), type of CLNT and initial temperature of the heated substrate. For both the phenomena, a critical comparison of the findings between nanofluids (NFs) and water has been presented. In case of the LF phenomenon, as a small mass of liquid comes into contact with a heated solid, the dynamics of the vapor layer formed in between the solid and the liquid governs the rate of evaporation of the liquid. The vapor layer vibration leads to the formation of different shapes which are orderly random in occurrence and has been investigated in details. In another event, during the RWT phenomenon, as the CLNT enters the heated solid, different flow regimes are observed to form which has been investigated in details. The temperature responses of the vertical tubes have been utilized to construct the boiling curves from which the surface heat flux can be easily estimated.

Cooldown Study of TVS-2M Model FA with Reflooding

An experimental study was performed of the influence of mixing intensifier-grids on the cooling of a 37-rod electrically heated TVS-2M model with top and bottom reflooding. It is shown that the mixing grids bring about a very small reduction of the maximum temperature of the rods and longer cooling time of the assembly. To cool the TVS-2M model, the starting temperature of the fuel-rod simulators and the power input can be significantly lower in top reflooding than in bottom reflooding. The effect of the flooding rate is similar to that observed in bottom flooding – as the flooding rate increases, the maximum temperature of the rods and the cooling time of the assembly decrease. The experiments showed that with reflooding the mixing grids do not greatly affect the cooling efficiency of TVS-2M.

Experimental study of the dryout of a 2D heat-generating model porous medium

Dryout experiments are performed in a quasi two-dimensional heat-generating model porous medium, with bottom flooding. The test section is composed of an array of heating cylinders placed between two ceramic plates. It is held vertically and a liquid (HFE-7000) is injected from bottom at a controlled flow rate, pressure and temperature. Dryout incipient power is investigated by applying an increasing thermal power to a bundle of heating cylinders until a dry zone is detected. The liquid inlet mass fluxes range from 1.0kgm-2s-1 to 10.5kgm-2s-1 and the heat fluxes are between 195kWm-2 and 1086kWm-2. Two kinds of dryout phenomenology are observed. First, at low liquid injection rate (below 4.2kgm-2s-1 inlet mass flux), reaching the dryout power results into a liquid front receding from the top of the test section to the upper limit of the heated zone, while downstream the heated zone, the porous medium is vapour-saturated. Second, at higher flow rate (over 5.2kgm-2s-1 inlet mass flux), the boiling crisis happens at the surface of a single heating element, resulting in a local film boiling, whereas a two-phase flow still go through the whole test section. In both cases, high-speed visualizations allow characterizing 8 boiling/flow regimes, depending on the inlet mass flux, the power released by the heated zone, and the location inside the porous medium. In particular, when reaching the dryout incipient power, the existence of a pulsed flow regime is highlighted. Such a characterization of boiling/flow regimes might be helpful in improving dryout models.

Development of debris bed cooling evaluation code, DPCOOL, based on heating porous media submerged in two-phase pool

ABSTRACT A framework of the modular code system, THERMOS, aiming to evaluate cooling of a debris bed having complex configurations was introduced with a focus on one of the major modules, DPCOOL, which models heating or non-heating porous media of particulate debris in a two-phase pool. In DPCOOL, pool and debris bed regions are discretized in a three-dimensional Cartesian coordinate system. A pool region is formulated based on the two-fluid model. A two-phase flow in the debris bed region is formulated based on the Tung & Dhir model with modifications for smaller particles proposed by Schmidt. In order to synthesize the momentum equations of the two regions, interpolation factors as piecewise linear functions of porosity are introduced. The interfacial friction model was validated based on Chu’s test using a debris bed composed of non-heating SUS spheres in a water pool with air injection from below where the net water flow in the layer became zero so that pressure loss of the layer was governed by interfacial friction. The sum of the two-phase flow friction and the interfacial friction models was validated based on top flooding and bottom flooding tests conducted in IKE’s DEBRIS test facility loaded with mixed steel and alumina spheres that were heated by an induction coil system.

Numerical Modeling and Experimental Validation of Melt Pool Coolability Under Bottom Flooding with Decay Heat Simulation

The nuclear accident at Fukushima has brought an increased research focus on nuclear safety and severe accidents. Radioactivity leakage into the environment has environmental, societal, and political impact, and further robustness of nuclear reactor design is essential. Thus, coolability and stabilization of corium within reactor containment in a severe accident scenario are important issues that need to be resolved. In this context, many new reactors have been envisaged with dedicated core catchers. In ex-vessel core catchers, corium coolability is one of the biggest concerns. Despite several efforts, melt pool coolability still needs to be understood in sufficient detail. Among the various cooling strategies, melt coolability using bottom flooding has been demonstrated to be one of the most efficient techniques so far. This paper presents the numerical and experimental study of melt pool coolability under bottom flooding with decay heat simulation. An experiment has been performed using a test section consisting of two parts: a lower part for melt retention and quenching, and an upper part for steam expansion and its outlet. To simulate the decay heat, ten radiative heaters equivalent to 10 kW were used to heat the lower part of the test section housing the melt. The experimental measurements showed that quenching of about 25 L of melt at 1200°C initial temperature took only a few minutes. The same phenomenon has been modeled using a mechanistic model. The model considers heat transfer in the melt pool, eruption in the melt pool by steam formed when water is injected at the bottom of the melt pool, and heat transfer from porous debris to a steam mixture. The model postulates the formation of crust below the melt pool when water is being inserted from the bottom. The model predicts the failure of this crust due to various stresses, resulting in an inverted cone–shaped melt eruption. The model captures the physics of this eruption cone along with the spatial variation in porosity. The model predictions have been compared with the measurements for the melt pool temperature during the cooling process. The results show that the model is able to capture quite accurately the multidimensional temperature fields in the melt pool during the cooling process.

Assessment of RELAP5/MOD3.3 Against Single Rod Reflooding Experiments

This study presents an assessment of the RELAP5/MOD3.3 using the experimental work upon the rewetting mechanism of bottom flooding of a vertical annular water flow inside a channel enclosing concentrically a heated rod. The experiments have been carried out in the experimental rig 1 of the Nuclear Engineering Department of National Technical University of Athens (NTUA-NED-ER1) inside which the dry out and the rewetting process of a hot vertical rod can be simulated. Experiments have been conducted at atmospheric conditions with liquid coolant flow rate within the range of 0.008 and 0.050 kg·s−1 and two levels of subcooling 25 and 50 K. The initial average surface temperature of the rod for each experiment was set at approximately 823 K. The predicted rod surface temperatures during rewetting of the RELAP5/MOD3.3 calculations were compared against the experimental values. The results presented in this study show that RELAP5/MOD3.3 provides temperature estimations of the reflooding mechanism within acceptable marginal error. However, larger deviations between predicted and experimental values have been observed when subcooled water was used instead of saturated one.

Experimental Investigation of Melt Coolability Under Bottom Injection: Effects of Melt Volume, Melt Composition, Nozzle Diameter, and Inlet Pressure

Melt pool coolability is one of the concerns when addressing severe accident scenarios. Long-term cooling and stabilizing of highly radioactive and reactive molten corium are still issues to be addressed and understood in order to devise a successful handling strategy. Core catchers have been envisaged in present and future advanced reactors to manage this concern. In the ex-vessel core catcher scenario, corium relocates to form a molten pool. Injecting water from the bottom of this melt pool to achieve coolability is found to be an efficient technique so far. However, the numbers of tests with prototypic materials and conditions are very limited and difficult to perform. In view of this, most of the earlier studies have been conducted with simulated melts. There are concerns with regard to scalability of experiments, effects of melt composition, and geometric parameters on melt coolability. To address these issues, series of experiments have been conducted and are presented in this paper. The experiments are performed with borosilicate glass, with two melt volumes, i.e., 3 and 20 L, and are compared. They show that the melt quenching time was more or less the same and suggest that the results can be extrapolated to higher scales. Two different simulants were used in the experiments, i.e., sodium borosilicate glass and CaO-B2O3, to study the effect of melt composition, and it was observed that the coolability behavior remains the same but the melt quenching time varies. The effect of nozzle diameters was studied by conducting experiments with three different nozzle diameters of 8, 12, and 18 mm keeping the same inlet pressure. It was found that the quenching time was higher for the 8-mm-diameter nozzle experiment due to smaller flow rates compared to the others. The experiments were repeated at two inlet water pressures of 0.35 and 0.75 bar(g) for the same nozzle diameter to study their effects on melt coolability. As expected, the quenching time was found to be less for the case of higher inlet pressure. The experimental measurements suggest that the overall average melt pool coolability behavior under bottom flooding was almost the same in all cases. However, each physical parameter affects the melt quenching time required to cool the melt. This average melt quenching time can be optimized using suitable combinations of geometric and physical parameters. The debris sizes and porosities formed during the melt eruption were also measured. The measured porosities ranged between 50% and 60% in all experiments.

Quenching of a Heated Rod: Physical Phenomena, Heat Transfer, and Effect of Nanofluids

The physical phenomena of rewetting and quenching are of prime importance in nuclear reactor safety in the event of a loss of coolant accident (LOCA). Generally, top spray or bottom flooding concepts are used in reactors. Numerical simulation of these processes entails the use of the concept of a rewetting velocity. However, heat transfer just before and after the rewetting front is often assumed in an ad hoc fashion. The present work aims to evaluate the surface heat flux during quenching as a function of surface temperature. The experiments presented herein are primarily applicable to the bottom flooding scenario with high flooding rate. In the experiments, a rod heated above Leidenfrost point is immersed in a pool of water. The surface temperature was recorded using a surface-mounted thermocouple. The surface heat flux was then determined numerically and hence can be related to a particular value of surface temperature. This type of data is useful for numerical simulations of quenching phenomena. In addition to this, high-speed photography was undertaken to visualize the phenomena taking place during the rewetting and quenching. Both subcooled and saturated water pools have been used and compared in the experiments. Surface finish was seen to influence rewetting process by a mechanism which here is termed as “transition boiling enhanced film boiling.” The effect of using nanofluids was also studied. No marked change is observed in the overall quenching time with nanofluids, however, the initial cooling is apparently faster.

A study of different cases of VVER reactor core flooding in a large break loss of coolant accident

The paper covers the results of VVER core reflooding studies in fuel assembly (FA) mockup of 126 fuel rod simulators with axial power peaking. The experiments were performed for two types of flooding. The first type is top flooding of the empty (steamed) FA mockup. The second type is bottom flooding of the FA mockup with level of boiling water. The test parameters are as follows: the range of the supplied power to the bundle is from 40 to 320 kW, the cooling water flow rate is from 0.04 to 1.1 kg/s, the maximum temperature of the fuel rod simulator is 800 °C and the linear heat flux is from 0.1 to 1.0 kW/m. The test results were used for computer code validation.

Rewetting of Vertical Pipes by Bottom Flooding Using Nanofluid as a Coolant

The present investigation reports the rewetting phenomenon by bottom flooding in vertical pipes using both water and nanofluid as coolant. The transient temperature response of rewetted surface indicates that rewetting takes place faster in nanofluids than in water. The effect of several parameters, including the coolant flow rate, distance from the inlet of fluid, concentration of nanoparticle loading on the rewetting characteristics, has been investigated. The rewetting velocity, for both water and nanofluid, is observed to depend strongly on the inlet coolant flow rate and initial wall temperature of the tube. The rewetting velocity is observed to follow the correlation for water proposed in an earlier work. Starting from the logic proposed in that previous report, the authors propose a correlation for predicting the rewetting velocity in nanofluids.

Valley bottom groundwater flooding in Berkshire

Valley bottom groundwater flooding in Berkshire

Experimental investigation on melt coolability under bottom flooding with and without decay heat simulation

Investigations on severe accident phenomena help us in understanding the realistic accidental phenomena for the assessment of associated risk. The societal impact of radiological leakage to the environment has demanded further robustness in the line of defence of nuclear safety. Thus, to ensure the cooling and stabilization of corium within reactor containment in case of severe accident scenarios, many new reactors have been envisaged with core catcher. In this regard, corium coolability still remains an unresolved issue in spite of several efforts being taken towards its understanding. After studying the various cooling strategies, it has been demonstrated that melt coolability using bottom flooding of water is one of the most efficient techniques so far. To study the effect of decay heat on melt pool coolability under bottom flooding condition, two experiments have been performed in this paper; one without the decay heat and the other with decay heat. The test section used for carrying out these experiments consisted of two parts viz. lower part for retaining the melt from furnace, water inlet and melt quenching, and upper part for steam expansion and its outlet. The total height of the test section was 1400mm and was made of 33mm thick carbon steel. Total six stainless steel nozzles of diameter 12mm were used for injecting water at the bottom of the melt pool. The lower part was surrounded by 10 radiative heaters to simulate decay heat of 10kW which corresponds to 0.5MW/m3. The experiments showed that quenching of about 25l of melt at initial temperature of nearly 1200°C took only a few minutes in both the cases. The debris formed in the experiments ranged between 0.5mm fine particles to 50mm porous chunks. The measured porosities were nearly 51% without decay heat and nearly 67% with decay heat. This paper presents the comparative experimental study of melt coolability using bottom flooding without and with decay heat scenarios. The experimental measurements show that the quenching behaviour of melt pool is not affected by decay heat, although stabilization of debris temperatures to saturation temperature takes a bit longer duration as compared to without decay heat scenarios.

Study on coolability of melt pool with different strategies

After the Fukushima accident, there have been a lot of concerns regarding long term core melt stabilization following a severe accident in nuclear reactors. Several strategies have been contemplated for quenching and stabilization of core melt like top flooding, bottom flooding, indirect cooling, etc. However, the effectiveness of these schemes is yet to be determined properly, for which, lot of experiments are needed. Several experiments have been performed for coolability of molten pool under top flooding condition. A few experiments have been performed for study of coolability of melt pool under bottom flooding as well as for indirect cooling. Besides, these tests are very scattered because they involve different simulant materials, initial temperatures and masses of melt, which makes it very difficult to judge the effectiveness of a particular technique and advantage over the other. In the present paper we have carried out different experiments wherein a single simulant material with same mass was cooled with different techniques starting from the same initial temperature. The result showed that, while top flooding and indirect cooling took several hours to cool, bottom flooding took a few minutes to cool the melt which makes it the most effective technique.

Effectiveness of radial flow on rewetting of AHWR fuel cluster

Abstract Rewetting of a hot surface is the process of establishing direct liquid contact with a large portion of the surface whose initial temperature exceeds that required to maintain film boiling for prescribed surface and flow conditions. The Advanced Heavy water Reactor (AHWR) is a natural circulation vertical pressure tube type boiling light water cooled and heavy water moderated reactor. In case of a loss of coolant accident, the clad surface temperature goes up very high and comes down due to coolant injection from the Emergency Core Coolant System (ECCS). The rewetting takes place in Boiling Water Reactors (BWR) due to top flooding and Pressurised Water Reactors (PWR) due to bottom flooding. But in AHWR, the emergency coolant enters into the reactor core in radial direction and after this, cross flow phenomenon takes place from one fuel pin ring to next. The study is being carried out on the effect of cross flow on rewetting of AHWR fuel bundle. This paper will discuss the modeling of the experimental setup having pressure tube, fuel cluster, steam generator, accumulator etc and study the effect of radial flow on rewetting of fuel pins. An analysis of the model, considering with and without cross flow, has been carried out and shows that the pick fuel temperature is sensitive to cross flow. The thermal hydraulic safety analysis code Relap5/3.2 is being used for modeling of experimental setup for rewetting study.

A simple model to understand physics of melt coolability under bottom flooding

Following a severe accident in a nuclear reactor, stabilization and termination of molten pool/debris is necessary to ensure safety of the reactor. In this regard, ex-vessel corium coolability still remains an unresolved issue in spite of several efforts being taken towards its understanding. Extensive research has been carried out in the field of corium coolability phenomenon, but the outcome is far from establishing a certainty of complete coolability of melt under all possible scenarios. Major thrust so far was on top flooding condition, which often results in uncoolable condition due to inability of water ingression deep into the solid crust. The research on coolability under bottom flooding condition started long back, but has gained lot of momentum recently. However, mainly the focus is on development of engineering system to enhance the coolability using bottom flooding. The understanding of the phenomenon is not yet totally brought out. In this paper, we have postulated the mechanism of melt coolability under bottom flooding. We have developed a simplified mechanistic model for prediction of coolability of molten corium under bottom flooding condition based on the postulations. The model has been validated with the experimental data from literature. Comparison of top flooding vis-à-vis bottom flooding has also been presented.

CONTRIBUTIONS OF THE VULCANO EXPERIMENTAL PROGRAMME TO THE UNDERSTANDING OF MCCI PHENOMENA

Molten Core Concrete Interaction (MCCI) is a complex process characterized by concrete ablation and volatile generation; Thermal and solutal convection in a bubble-agitated melt; Physico-chemical evolution of the corium pool with a wide solidification range (of the order of 1000 K). Twelve experiments have been carried out in the VULCANO facility with prototypic corium and sustained heating. The dry oxidic corium tests have contributed to show that silica-rich concrete experience an anisotropic ablation. This unexpected ablation pattern is quite reproducible and can be recalculated, provided an empirical anisotropy factor is assumed. Dry tests with oxide and metal liquid phases have also yielded unexpected results: a larger than expected steel oxidation and unexpected topology of the metallic phase (at the bottom of the cavity and also on the vertical concrete walls). Finally, VULCANO has proved its interest for the study of mitigation solutions such as the COMET bottom flooding core catcher.