PANDA Facility Research Articles

Analyses of Jet Buoyant Flow in a Multicompartment Containment Using an Open-Source Solver

Hydrogen mixing and distribution in a nuclear power plant containment during a postulated severe accident is a phenomenon with high safety relevance for current and advanced light water reactors. The Organisation for Economic Co-operation and Development (OECD)/Nuclear Energy Agency (NEA) SESAR Thermal-Hydraulics (SETH) project was carried out to generate experimental data on basic containment phenomena with the required instrumentation to assess computational fluid dynamics (CFD) and lumped parameter codes. In this paper, we analyze the PANDA SETH Test 16 of the OECD/NEA SETH project using containmentFOAM, which is a tailored open-source CFD code package for containment analysis developed at Forschungszentrum Jülich GmbH based on OpenFOAM.® SETH Test 16 addressed steam-air mixture transport driven by jets and plumes in the multicompartment PANDA facility (two vessels and an interconnecting pipe) at the Paul Scherrer Institut in Switzerland. Steam as a wall plume was injected at 0.040 kg/s and 140°C in vessel 1, which was initially filled with dry air at 108°C. The steam-air mixture was vented out from the top of vessel 2. The system pressure was constant at 1.3 bar for the whole test duration (4000 s). To simulate SETH Test 16, the k-ω shear stress transport turbulence model was adopted with the generalized gradient diffusive hypothesis buoyancy turbulence model. The simulation represented the experimental phenomenology reasonably well. The trends for the calculated temperature distribution were similar to those of the experiments and only yielded a slight discrepancy of about 2.7%. In addition, the distribution of the steam molar fraction was well predicted above the injection tube in vessel 1. However, below the injection tube in vessel 1, some discrepancies between the simulations and the experimental results were observed. These may have been caused by the higher turbulent mass diffusivity in the physical model and the challenge of the mesh resolution in the middle region of vessel 1.

Large-scale PANDA facility – radiation experiments and CFD calculations

Abstract CFD modelling of the thermo-hydraulic phenomena in the containment during the various phases of a severe accident necessarily requires consideration of radiative heat transfer – in the presence of steam. These radiative phenomena include (i) energy transfer within the gas mixture and (ii) between the gas and surrounding structures. Preliminary calculations carried out for these types of experiments within the OECD/NEA HYMERES-2 project with the CFD code containmentFOAM using a Monte Carlo solver for thermal radiation, demonstrated that the radiative heat transfer is significant even for very small amounts of vapour in the range of ≈0.1 % to ≈2 %. For this reason, the test matrix was tailored to the two opposite extremes: either gas compositions with a low vapour/steam content, where radiative heat transfer can be neglected, or gas mixtures with higher vapour contents, so that radiative heat transfer plays a dominant role. For the selected experiments of the H2P2 series and the corresponding CFD calculations, a vessel with a diameter of 4 m and a height of 8 m was preconditioned with different air-vapour mixtures (a) at room temperature and (b) elevated temperatures. A stable helium layer was then built-up in the upper part of the vessel. The gas was then compressed by injecting helium from above which resembles with best efforts a compression with a piston in a cylinder. This results in a height-dependent and transient increase of the gas temperature. These experiments and the associated CFD calculations were developed to isolate the phenomena of thermal radiation as good as possible from convective and diffusive effects – within the always present experimental limitations. For the reference experiment with ‘dry conditions’ corresponding to the lowest experimentally possible humidity of ≈0.1 %, we show that the use of a model without radiation provides the best agreement between the experimental and numerical results. For the much higher steam content of ≈60 %, the statistical narrow band correlated-k model (SNBCK), non-gray gas model, is the best candidate for future calculations – with computationally forgivable additional effort. We also provide with the Filtered Rayleigh Scattering technique (FRS) an outlook for a possible future instrumentation approach to better meet the requirements of the CFD community.

Modeling of the influence of local heat sources on a light gas stratification formation and erosion in a large-scale experimental facility using eddy resolving numerical approach

The main objective of this work is to evaluate the results of application of Implicit Large Eddy Simulation (ILES) turbulence modeling approach to simulate the complex hydrogen safety problem. The paper presents the results of numerical simulation of the OECD/NEA HYMERES (HYdrogen Mitigation Experiments for Reactor Safety) HP2 series tests in the PANDA facility, aimed at studying the thermal effect of passive autocatalytic recombiner (PAR) operation on mixing the containment atmosphere, using CABARET-SC1 code. The code is based on the eddy resolving CABARET technique, which allows implicit modeling of the subgrid turbulence scales without using tuning parameters (ILES approximation). The natural convection flow inside the PAR model was calculated explicitly using a simplified model of the heating section and the housing. Good agreement of calculated transient with experimental data was obtained, which indicates the effectiveness of such approach.

Simulation of experiments on thermal radiation effect studying in a large-scale PANDA facility using the CABARET-SC1 code

The paper presents the results of numerical simulation of OECD/NEA HYMERES-2 H2P2 series experiments in the large-scale PANDA facility aimed at studying the effect of thermal radiation on heat transfer in steam-containing atmosphere, using CABARET-SC1 CFD code. The code was designed for the 3D high-resolution numerical simulations of hydrogen safety problems. It is based on the eddy resolving CABARET technique, which allows implicit modeling of the subgrid turbulence scales without using tuning parameters (Implicit LES approach). To model radiative heat transfer a control volume method for discrete ordinates numerical solution with an artificial speed of light has been developed and implemented in code. Fairly good agreement with the experiment was obtained for the gas temperature, helium concentration, and pressure values. Thus, the calculation results confirm the experimental conclusions about the significant effect of thermal radiation on the gas temperature distribution even in a low steam content atmosphere. The developed radiative heat transfer model can be further improved in terms of angular discretization method and gas radiation absorption model.

CFD analyses of large-scale tests highlighting the effects of thermal radiation in steam-containing atmospheres

In simulations of heat transfer in large gaseous volumes under moderate temperatures, thermal radiation is traditionally neglected to simplify the computations. In recent years however, some small as well as large-scale experiments have demonstrated that thermal radiation may have a strong effect on the temperature field if the atmosphere contains an infrared absorbing gas such as vapor, even in small concentrations. This has in particular been shown in the H2P2 series of tests performed in the large-scale PANDA facility at the Paul Scherrer Institute. For these tests an air-helium-steam mixture was compressed with a helium injection, which caused the formation of a hot bubble. The initial vapor volume fraction varied between 0.1 % and 60%. In this paper, we report the results of CFD simulations conducted for the five H2P2 tests. The turbulence model k-ω SST was used, while thermal radiation was considered using the simple P1 model as a default, in conjunction with the Weighted Sum of Gray Gases Model (WSGGM) to specify steam absorptivity. For comparison purposes we also made use of the more advanced Monte-Carlo model for selected experiments. CFD Best Practice Guidelines were employed, in particular through preliminary grid and time-step sensitivity studies.The temperature and helium concentration fields matched the experimental data quite accurately, except for the test with very low vapor content (H2P2_1_2, 0.1% vol. steam), where the temperature was somewhat over-estimated. This may be due to the inadequacy of the radiation models at these extreme conditions. Two major conclusions can be drawn from this study: 1) ignoring thermal radiation can lead to large over-predictions of temperature, even when the steam content is low (order of 1%) and 2) the simple P1 model provides sufficient accuracy. Use of the more sophisticated Monte-Carlo model yielded similar and slightly more accurate results, albeit at a considerably larger CPU expenditure (5–7 times).

Assessment of the depressurization and mixing effects of two spray systems in the PANDA facility, part I – Experimental results

The spray safety system is one of the most effective solutions to limit the pressure and temperature rise in the containment of a nuclear power plant during a design basis accident. However, its actuation under severe accident conditions may have either a positive effect on the hydrogen risk by mixing the containment atmosphere and avoiding high H2 local concentrations or a negative impact by condensing the steam and de-inerting the atmosphere. The experiments performed during the OECD/NEA HYMERES-2 Project aimed to extend the experimental database dedicated to assessing the capabilities of the codes used to optimise the operation of the safety systems. Specifically, the H2P5 series consisted of two experiments that compared the activation of a central full cone spraying nozzle (H2P5_2) with nine smaller spraying nozzles, eight equally distributed at 0.5 m from the metallic walls of PANDA (H2P5_1). The ninth nozzle was positioned in the vessel vertical axis for the latter configuration. H2P5_1 was the first experiment ever using a multiple nozzle configuration in PANDA, which was aimed to address the conditions expected in a plant containment with droplets interacting with the vessel walls as well as with each other.The measurements indicated a considerable difference between the two configurations regarding the pressure and temperature decay, which signified the presence of non-adiabatic effects. The stronger cooling of the walls measured during H2P5_1 resulted in a slower depressurisation for most of the test duration. This can be explained by an intensified interaction of the injected droplets (by direct droplet-wall interactions or by potential formations of liquid films) and the vessel walls, which was coherent with the use of the spray ring in H2P5_1. It was also shown that the different spray configurations (type of nozzles and nozzles distribution) had a significant impact on the gas mixing. The measurements of the mass spectrometer available during the tests revealed a non-obvious initial faster mixing for the single nozzle experiment, which is approximately five times faster compared with the nozzle array.

Assessment of the depressurisation and mixing effects of two spray systems in the PANDA facility, part II – Simulations using GOTHIC8.3(QA)

The spray safety system is one of the most effective solutions to limit the pressure and temperature rise in the containment of a nuclear power plant during a design basis accident. However, its actuation under severe accident conditions may have either a positive effect on the hydrogen risk by mixing the containment atmosphere and avoiding high H2 local concentrations or a negative impact by condensing the steam and de-inerting the atmosphere. The experiments performed during the OECD/NEA HYMERES-2 Project aimed to extend the experimental database dedicated to assessing the capabilities of the codes used to optimise the operation of the safety systems. Specifically, the H2P5 series consisted of two experiments that compared the activation of a central full cone spraying nozzle (H2P5_2) with nine smaller spraying nozzles, eight equally distributed at 0.5 m from the metallic walls of PANDA (H2P5_1). The ninth nozzle was positioned in the vessel vertical axis for the latter configuration. These experiments created a demanding scenario for the computational codes, which need to consider three different fields: three components of the gas phase (air, helium, and steam), the dispersed liquid (droplets), and the continuous liquid accumulated on the walls and at the bottom of the PANDA vessel during the test. Furthermore, the proximity of the nozzles to the walls in H2P5_1 requires considering the strong interaction of the fluid with these metallic structures.In this work, the GOTHIC 3D models were able to qualitatively represent all the relevant phase change phenomena affecting the cooling effect of the spray and provided a reasonable qualitative representation of the depressurization behaviour observed during the experiments. However, the code overestimated the steam condensation on droplets. To achieve quantitative agreement between the experiments and computer models it was necessary to enhance the spatial distribution of the droplets (to compensate for anomalies due to a conical injection in a relatively coarse cartesian mesh) and to diminish the steam condensation on the droplets by decreasing the spray mass flow rate. Regarding the helium mixing, the simulations revealed a potential explanation for the faster mixing observed in the single nozzle (H2P5_2) experiment. Furthermore, the droplet diameter showed to be the parameter inducing the most significant variations in the mixing.

Experimental assessment of thermal radiation effects on containment atmospheres with varying steam content

The thermal-hydraulics phenomena in a containment during an accident will necessarily include radiative heat transfer (i) within the gas mixture due to the high radiative absorption and emission of steam and (ii) between the gas mixture and the surrounding structures. The analysis of some previous PANDA experiments (PSI, Switzerland) demonstrated the importance of the proper modelling of radiation for the benefit of numerical simulations. These results together with dedicated scoping calculations conducted for the present experiments indicated that the radiative heat transfer is considerable, even for a very low amount of steam (≈2%).The H2P2 series conducted in the large-scale PANDA facility at the Paul-Scherrer-Institut (PSI) in the framework of the OECD/NEA HYMERES-2 project is intended to enhance the understanding of thermal radiation phenomena and to provide a benchmark for corresponding numerical simulations. Thus, the test matrix was tailored around the two opposite extremes: either gas compositions with small steam content such that radiative heat transfer phenomena can be neglected. Or gas mixtures containing larger amounts of steam, so that radiative heat transfer is expected to play a dominant role.The H2P2 series consists of 5 experiments designed to isolate the radiation phenomena from convective and diffusive effects as much as possible. One vessel with a diameter of 4 m and a height of 8 m was preconditioned with different mixtures of air / steam at room and elevated temperatures. This was followed by the build-up of a stable helium stratification at constant pressure in the upper part of the vessel. After that, helium was injected from the top into the vessel which leads to an increase of the vessel pressure and a corresponding elevation-dependent and transient rise of the gas temperature. It is shown that even the addition of small amounts of steam in the initial gas atmosphere considerably impacts the radiative heat transport throughout all phases of the experiments and markedly influences i) the monitored gas peak temperature, ii) the temperature history during the compression and iii) the following relaxation phase after the compression was stopped. These PANDA experiments are the first of its kind conducted in a large scale thermal-hydraulic facility.

Parametric study highlighting cooler and spray activation on the transient mixing of a three-gas composition in the PANDA facility

In postulated accident scenarios with the release of steam and hydrogen in the containment, for some nuclear power plant designs it is foreseen that sprays and coolers are activated to condense steam and mitigate the containment pressure build-up. The operation of the sprays and the coolers induce flow with the effect of mixing a stratified atmosphere which reduces local hydrogen concentration. However, steam condensation could lead to the formation of combustible air-hydrogen mixtures. Therefore, for the management of a severe accident, it is crucial to know the overall effect of spray and cooler activation on the containment pressure history and gas-mixture concentration evolution. In this article, the results of a series of five experiments, identified as HP4 (HYMERES PANDA series 4) series, are presented. The HP4 series was part of the OECD/NEA HYMERES project, and the main objective was to investigate the combined effect of a cooler and a spray in the containment atmosphere. The HP4 series was performed in the large-scale thermal-hydraulic facility PANDA facility (PSI, Switzerland). In the HP4 test matrix, the cooler design (with cooling tube partially or fully enclosed in a cooler case) and the spray design (full cone or hollow cone nozzle) were varied. Moreover, the spray water temperature was an additional parameter. The HP4 series revealed that the containment pressure history and the gas mixture composition during the postulated scenarios are strongly affected by spray and cooler safety systems whereas the details of the spray and cooler design are of minor importance.The containment pressure during the experiments increased from atmospheric up to 2.5 bar and maximum power removed by the cooler was about 100 KW. The spray activation induced steam condensation and the highest de-pressurization induced by the combined spray and cooler operation was about 0.7 bar, i.e. from 2.5 bar to 1.8 bar.

PANDA experiments within the OECD/NEA HYMERES-2 project on containment hydrogen distribution, thermal radiation and suppression pool phenomena

In this article, we summarize the experimental results obtained in the large-scale thermal-hydraulics PANDA facility (PSI, Switzerland) within the OECD/NEA HYMERES2 project. The experiments addressing hydrogen distribution allowed characterizing the flow patterns forming from jet flows impacting plates or grating platforms and the following interaction with a postulated hydrogen cloud in the containment dome. The experiments with activation of safety components allowed to compare the effects of multiple spray nozzles with a single spray nozzle or multiple cooler units with unique cooler and to assess the scalability of the related thermal hydraulics phenomena such as pressure histories, power removed, gas mixture distribution. The experiments related to thermal radiation confirmed that the presence of steam in a containment atmosphere even at very low concentration (in the order of 0.1 %) has a significant impact on the evolution of containment mixture temperature.The experiments studying pressure suppression pool phenomena provided insight on the formation and stability of thermocline under a variety of steam release conditions, e.g. from load reduction rings and from multi-hole sparger; moreover allowed to characterize containment pressure histories as function of sparger submergence in the suppression pool.

Assessment of the Depressurization and Mixing Effects of Two Spray Systems in the Panda Facility, Part I – Experimental Results

Assessment of the Depressurization and Mixing Effects of Two Spray Systems in the Panda Facility, Part I – Experimental Results

Assessment of the Depressurisation and Mixing Effects of Two Spray Systems in the Panda Facility, Part Ii – Simulations Using Gothic8.3(Qa)

Assessment of the Depressurisation and Mixing Effects of Two Spray Systems in the Panda Facility, Part Ii – Simulations Using Gothic8.3(Qa)

Analysis on helium stratification erosion by vertical steam jet using the CUPID code

This paper presents a validation of a computational fluid dynamics (CFD) code (CUPID) for predicting the erosion of helium stratification by a vertical steam jet. For the validation of CUPID, CFD grade experimental data obtained from the PANDA facility in the framework of the OECD/NEA HYMERES-2 project was used. To predict the stratification erosion phenomena appropriately, the turbulence model and radiative heat transfer model need to be employed as major physical models. Therefore, the standard k–ε turbulence model, including the buoyancy term, was applied, and a sensitivity study was carried out to investigate the effect of buoyancy production in the turbulence dissipation rate equation on the predicted helium concentration. Additionally, a radiative heat transfer model (P-1) was implemented by applying the appropriate parameter for the calculation, and the predicted gas temperature distributions with and without radiative heat transfer were compared. First, the physical models were validated for a baseline test case in which the vertical steam jet did not interact with any flow obstacles. Then, a second validation was carried out as a blind benchmark calculation where the experimental data were released after the calculation results had been submitted. The CUPID calculations for the helium concentration and gas temperature were very close to the measured data and showed that the analysis models can be used to accurately predict the gas mixing phenomena in the containment.

The effects of activated cooler power on the transient pressure decay and helium mixing in the PANDA facility

The main outcomes of the experiments H2P6 performed in the thermal-hydraulics large-scale PANDA facility at PSI in the frame of the OECD/NEA HYMERES-2 project are presented in this article.The experiments of the H2P6 series consists of two PANDA tests characterized by the activation of three (H2P6_1) or one (H2P6_2) cooler(s) in an initially stratified and pressurized containment atmosphere. The initial stratification is defined by a helium-rich region located in the upper part of the vessel and a steam/air atmosphere in the lower part. The activation of the cooler(s) results i) in the condensation of the steam in the vicinity of the cooler(s), ii) the corresponding activation of large scale natural circulation currents in the vessel atmosphere, with the result of iii) the re-distribution and mixing of the Helium stratification initially located in the upper half of the vessel and iv) the continuous pressure decay. The initial helium layer represents hydrogen generated in a postulated severe accident.The main question to be answered by the experiments is whether or not the interaction of the different, localized cooler units would be important for the application of numerical methods.The paper describes the initial and boundary conditions and the experimental results of the H2P6 series with the suggestion of simple scaling laws for both experiments in terms of i) the temperature difference(s) across the cooler(s), ii) the transient steam and helium content and iii) the pressure decay in the vessel. The outcomes of this scaling indicate that the interaction between separate, closely localized units does not play a prominent role for the present experiments. It is therefore reasonable to model several units as one large component with equivalent heat transfer area and total water flow rate.

Pre-test analysis for definition of steam injection tests through multi-hole sparger in PANDA facility

Pressure Suppression Pool (PSP) is a passive safety feature in Boiling Water Reactors (BWR) and Advanced Pressurized (AP) reactors. Steam released from the primary coolant system is condensed in a large water pool to prevent containment overpressure. Injected steam induces sources of heat (buoyancy force) and momentum (inertia). The competition between the sources might result in the development of thermal stratification or mixing of the pool. Increased temperature of the top pool layer leads to higher partial pressure of steam in the containment and thus reduces pressure suppression capacity of the pool. Models with predictive capabilities are needed for the analysis of the reactor pool transients. Development and validation of the models require adequate experimental data. In this work we discuss results of the pre-test analysis that was carried out to select conditions for the tests with steam injection through sparger head and Load Reduction Ring (LRR) in a large scale PANDA facility. The aim of the tests was to obtain data on pool thermal stratification and mixing under different regimes of steam injection. Effective Heat Source (EHS) and Effective Momentum Source (EMS) models were implemented in a computational fluid dynamics (CFD) code in order to carry out the analysis. Evolution of the pool temperature and velocity characteristics were analyzed in the scoping analysis to provide suggestions for selection of (i)pool depth, (ii)elevations of the sparger head and LRR, (iii)number of open LRR holes, (iv)layout of instrumentation, and (v)steam injection procedure for each test.

Thermal radiation, its effect on thermocouple measurements in the PANDA facility and how to compensate it

Thermocouples are one of the most widespread used sensors for temperature measurements in several applications, thanks to their robustness, low cost and ease of installation. However, when used in high temperature environments or in the presence of flames or elevated hot sources, the measurement can be affected, for example, by radiative exchanges to and from the ambient. The focus of the present study is to experimentally assess the thermal radiation effect on the thermocouple measurements in a controlled environment. To do so, K-type thermocouples having different diameters (1 mm and 0.2 mm) and radiated homogeneously from all sides are used to measure the air temperature between 40 °C and 145 °C inside a black-body calibrator cavity. The experimental data are then used to implement and discuss an existing radiation correction framework, the reduced radiative error method (RRE). The results suggest to use smaller size thermocouples to reduce the errors associated with thermal radiation and improve the accuracy of the measurement. Additionally, the results show that the effect of thermal radiation is higher for larger size thermocouples, i.e. thermocouples with 1 mm diameter show an error of 1% in the range considered here. This error reduces by a factor of three when using thermocouples with 0.2mm diameter. This benefit needs to be ultimately balanced with engineering considerations – smaller sized thermocouples are more likely to break during positioning or experiments.

Gas mixing caused by interacting heat sources PART I: Experiments

The four experiments presented in this paper investigate the influence of the plumes released by the operation of one or two heat sources located at different elevations, on the erosion of a stratified gas atmosphere in the presence of steam condensation on vertically oriented cooled pipes. The heat sources are located at two different elevations in a vessel with 4 m diameter and 8 m height, and they represent Passive Autocatalytic Recombiners (PARs) in a Light Water Reactor (LWR) containment and the heat PARs would release. The condensation on the cooled tubes represents the condensation which would take place during postulated accident scenarios at the concrete wall of a real containment. The condensation on the tubes contributes to the large-scale convection loops and enhances the gas mixing caused by the rising plumes from the heat sources. The stratification with a helium-rich layer – the helium represents hydrogen in a real containment – was created by a vertical steam and steam-plus-helium jet interacting with a circular disk located 1 m above the jet exit. Temperature, gas concentration and velocity field measured during the experiments showed that the activation of only the upper heater created a homogeneous gas mixture above the level of the heater, while forming a sharp concentration gradient below it. By operating only the lower heater, the emerging plume homogenized the entire vessel at the end of the test, showing that it was more effective in gas mixing. The simultaneous activation of the upper and the lower heaters accelerated the breakup of the helium stratification. The experiments were performed in the PANDA facility in Switzerland in the frame of the OECD/NEA HYMERES project which is focused on the hydrogen safety issues related to Light Water Reactors, specifically to hydrogen distribution in the containment. The experiments presented in the present paper were analyzed with the GOTHIC code which is the subject of Part II of this article.

Gas mixing caused by interacting heat sources. Part ii: Modelling

The analyses of hypothetical accident scenario in various containment designs include the evaluation of the effectiveness of Passive Autocatalytic Recombiners (PARs), which are installed to mitigate the hydrogen risk, or are considered by utilities for retro-fitting the plants to implement revised regulatory requirements motivated by the Fukushima accident. The assessment requires validated computational tools. To build confidence in the codes, their capability must also be assessed against separate effect tests addressing specific phenomena under conditions that are representative of those expected in an accident scenario in a real plant. Under the conditions of a severe accident, the large amount of hydrogen release requires the installation of several units, and therefore the study of their interaction is necessary for the optimal layout. Within the OECD/NEA HYMERES project, four experiments have been performed in the large-scale PANDA facility, addressing the thermal effect of one or two PARs, which was simulated by means of heaters (Part I). The tests were of increasing complexity, to permit a step-by-step validation of the codes. These experiments have been analyzed with the GOTHIC code. In general, the GOTHIC code in conjunction with a coarse mesh could predict the mixing process reasonably well. The only notable discrepancy with the experiments was the overprediction of the mixing rate when the lower heater was activated. The analyses revealed that this was due to the incorrect representation of the interaction of the flow out of the lower heater with the obstruction, and better results could be obtained by refining the mesh around the heaters. It was concluded that a coarse mesh is adequate for representing the flow produced by multiple heat sources in an open geometry, but detailed models are generally required if the flows could interact with structures.

Analyses of Gas Stratification Erosion by a Vertical Jet in Presence of an Obstacle Using the GOTHIC Code

Abstract The GOTHIC code was validated using three experiments carried out in the PANDA facility in the framework of the OECD/NEA HYMERES project. These tests addressed the mixing of an initially stratified atmosphere by means of a vertical jet in the presence of on obstacle (circular plate). This paper reports on the simulations of three experiments, and complementary, quasi-steady-state tests without stratification, where the flow structure above the impingement plate could be observed by means of particle image velocimetry (PIV) velocity measurements in a region larger than that considered in the transient experiments. Moreover, simulations of similar tests without obstacle conducted during the OECD/SETH-2 project are also discussed. The reference, best-estimate model used for the analyses of the three experiments with different flowrates and initial and pressure boundary conditions was built on the base of a multistep approach. This was based on mesh and modeling sensitivity studies mostly performed for the complementary tests, to assess the capability to represent the flow structure produced by the jet–plate interaction with different meshes around the plate. Generally, the results show that the use of a coarse mesh and the standard k–ε turbulence model permits a reasonable representation of the erosion process, but with a systematic over prediction of the mixing time. The results with the reference model were more accurate for two experiments with two flowrates and same initial conditions and all complementary tests. For the third test with different initial and boundary conditions, however, poor results were obtained with the reference model, which could only be improved by further refining the mesh. These results indicate that a model “qualified” for certain conditions could be inadequate for other cases, and sensitivity studies are necessary for the specific conditions considered in the analyses.

Modeling of PANDA isolation condenser experiments with MELCOR and Apros

Modeling methods were developed for passive cooling systems, in which steam condenses in vertical tubes. Isolation condenser experiments performed in the PANDA facility were calculated with two computer codes: the severe accident code MELCOR, and the process simulation software Apros. Both pure steam tests and tests with steam–non-condensable gas mixtures were calculated. Recommendations are given for MELCOR users regarding an appropriate nodalization, flow path definition, and heat transfer parameters. With proper model parameters and nodalization, both MELCOR and Apros codes are able to calculate the operation of the passive cooling system well.