Pressurized Thermal Shock Scenarios Research Articles

Direct numerical simulation of a pressurized thermal shock scenario at higher reynolds number

Direct Numerical Simulation (DNS) is performed for a pressurized thermal shock scenario in a reactor downcomer geometry. In the present work, a simplified representative geometry is adopted where cold fluid injected from a square duct is injected in a planar downcomer. A secondary hot inlet at the top of the downcomer is employed to enforce a downward flow of the injected emergency core cooling water, to mimic the effect of density driven flow. The Reynolds number of the impinging flow, based on bulk velocity and duct height, equals 12,500, while a unity Prandtl number fluid is used. The simulation is performed for two thermal boundary conditions, namely an isotemperature condition and an adiabatic condition, representing the two extreme scenarios of a conjugate heat transfer problem. The instantaneous and mean fields are first analysed in order to study the flow pattern and formation of vortical structures within the downcomer. The impinging flow is deflected radially outwards in the vicinity of the barrel wall, which then interacts with the surrounding hot fluid forming large vortical structures and bringing the cold fluid back to the vessel wall. The mixing of flow and penetration of colder temperatures in the downcomer is observed to be greatly influenced by interaction of these vortical structures. The descending cold plume in the downcomer is observed to form three branches separated by low-speed regions. The maximum values of turbulent kinetic energy are observed only in the vicinity of flow impingement, while high values of its production and dissipation are also observed within the shear layers of the descending plume. The maximum values of temperature fluctuations are found at the interface of the descending cold plume and surrounding hot fluid, while relatively high values are also seen at the locations of large vortical strucutres. Anisotropy in turbulence is also analysed using the componentality contour approach and a modified barycentric colour mapping scheme. Also reported herein is a comparison of mean and statistical profiles with those at lower Reynolds number reported in the literature. At higher Reynolds number, the cold impinging jet is deflected farther outwards in the downcomer. The locations of peak fluctuations in velocity and temperature are also farther away from the impingement. The magnitudes of fluctuating temperature and turbulent kinetic energy are observed to be higher for the present higher Reynolds number case. The peak Nusselt number at the barrel wall is observed to scale with a factor of Re1/2 in the present configuration.

Numerical prediction of a single phase Pressurized Thermal Shock scenario for crack assessment in an Reactor Pressure Vessel wall

A pressurized thermal shock (PTS) in a reactor pressure vessel (RPV) wall of a Pressurized Water Reactor (PWR) could eventually lead to reduced integrity of the RPV. A PTS scenario may be initiated by a Loss-of-Coolant Accident (LOCA) in which the Emergency Core Cooling (ECC) injection causes a thermal shock in the vessel wall while the RPV is still partially pressurized. The traditional Thermal-Hydraulic system codes fail to reliably predict the complex three-dimensional thermal mixing phenomena in the downcomer occurring during the ECC injection. Therefore, a three-dimensional computational fluid dynamics (CFD) simulation of the transient is required to provide an accurate temperature distribution for reliable probabilistic analyses. The assessment of the failure probability due to a single phase PTS initiated by postulated defects in the vessel wall requires the computation of a long transient solution. Since the CFD calculation is computationally expensive, the symmetry of a four-leg PWR geometry is utilized to reduce the computational domain.In the present work, several Unsteady Reynolds Averaged Navier-Stokes (URANS) CFD simulations are performed to select an optimal computational domain that is representative for a typical PWR. The computed temperature distribution is provided as input for the structural analyses to calculate the Stress Intensity Factor (SIF) along several postulated defects. The SIF is then used to evaluate the potential of crack propagation initiation of the defects.

Direct numerical simulation of flow and heat transfer in a simplified pressurized thermal shock scenario

Direct numerical simulation (DNS) is carried out using a high order spectral element method in order to analyse the flow and thermal fields in a Pressurized Thermal Shock (PTS) scenario. The adopted configuration is representative of a simplified reactor pressure vessel. It consists of a square duct that intersects perpendicularly with a rectangular vessel (downcomer). Cold water is injected from the square duct. It impinges against the wall of the downcomer that contains water at higher temperature, and a mixing between the cold and hot water occurs in the downcomer. The fluid properties are assumed to be constant; hence the temperature is treated as a passive scalar. A secondary flow inlet in the upper part of the downcomer is used to mimic the effect of density driven flow. The friction Reynolds number in the duct equals to 180, and a unitary Prandtl number of the fluid is used. The thermal field is solved with two types of boundary conditions, i.e., iso-thermal and adiabatic conditions, which encompass the two extreme scenarios of a conjugate heat transfer problem. A careful meshing strategy is adopted and the mesh resolution is verified a posteriori in order to meet DNS quality. The mean flow and turbulent statistics are analysed first, and a particular attention is devoted to the analysis of the vortical structures at the edges of the duct flow. The instantaneous flow field is then analysed in order to study the turbulent structure and the vorticity field within the impingement region. The mean temperature and the turbulent statistics of the thermal fields are subsequently analysed for both thermal boundary conditions. The case with iso-thermal boundary condition corresponds to the initial phase of the thermal mixing during a PTS scenario. It is characterized by the presence of cold fluid near the impinging wall and pockets of fluid at higher temperature near the exit of the duct. Regions with large temperature fluctuations are observed near the edges of the duct and at the interface between the deflected impinging fluid and the descending fluid in the downcomer. The case with adiabatic boundary condition corresponds to the later stage of the thermal mixing during a PTS scenario in which a thermal equilibrium is reached. For this case, a region with lower temperature is observed in the downcomer within the impinging jet region and in the region below, whereas the fluid at higher temperature remains confined in the remaining part of the downcomer.

Large Eddy Simulation of the injection of cold ECC water into the cold leg of a pressurized water reactor

The aging of the reactor pressure vessel (RPV) can be a limiting factor for the lifetime extension of pressurized water reactors. In case of loss of coolant accidents, cold emergency core cooling (ECC) water will be injected under high-pressure thermal-hydraulic conditions into the cold leg that is filled with hot coolant. The hot RPV wall can be exposed to this cold fluid, suffering important thermal stresses. This process is called pressurized thermal shock (PTS). The efficiency of the PTS depends on the mixing between cold ECC water and hot coolant in the cold leg and the upper downcomer. A single-phase PTS scenario was investigated experimentally in the TOPFLOW facility under representative thermal-hydraulic conditions. A transient of 390 s of this experiment is analyzed here numerically with Large Eddy Simulations and two different hypothesis to account for thermal effects: incompressible fluid and dilatable fluid (low Mach number compressible fluid). Detailed information is given on the temporal development of both thermal stratification and flow behavior in the cold leg. Calculated temperature profiles in the cold leg are compared to detailed temperature measurements. The dilatable approach reproduces well the experimental results and outperforms the incompressible fluid approach.

IAEA CRP benchmark of ROCOM PTS test case for the use of CFD in reactor design using the CFD-Codes ANSYS CFX and TrioCFD

Over the last 15 years, considerable effort has been expended in assembling the available information on the use of CFD in the nuclear reactor safety field. Typical application areas here are heterogeneous mixing and heat transfer in complex geometries, buoyancy-induced natural and mixed convection, etc., with specific reference to Nuclear Reactor Safety (NRS) accident scenarios such as Pressurized Thermal Shock (PTS), boron dilution, hydrogen build-up in containments, thermal fatigue and thermal striping issues, etc. The development, verification and validation of CFD codes in respect to Nuclear Power Plant (NPP) design necessitates further work on the complex physical modelling processes involved, and on the development of efficient numerical schemes needed to solve the basic equations. Therefore, a set of ROCOM CFD-grade test data were made available to set up an International Atomic Energy Agency (IAEA) benchmark, relating to PTS scenarios. The benchmark deals with the injection of the relatively cold Emergency Core Cooling (ECC) water, which can induce buoyancy-driven stratification. Data obtained from the PTS experiment were compared in the study presented here with predictions obtained from the CFD software packages ANSYS CFX and TrioCFD. In addition a test case without buoyancy forces was selected to show the influence of density differences.The results of the two test cases and the numerical calculations show that mixing efficiency is strongly influenced by buoyancy effects. At higher mass flow rates without density differences the injected slug propagates in the circumferential direction around the core barrel. Buoyancy forces reduce this azimuthal propagation. The ECC water falls in an almost vertical path and reaches the lower downcomer sensor below the inlet nozzle. Therefore, density effects play an important role during natural convection with ECC injection in PWRs. Both CFD codes were able to predict the observed flow patterns and mixing phenomena.

Design of a single-phase PTS numerical experiment for a reference Direct Numerical Simulation

The integrity assessment of the Reactor Pressure Vessel (RPV) is considered to be an important issue for lifetime extension of nuclear reactors. A severe transient that can threaten the integrity of the RPV is the existence of a Pressurized Thermal Shock (PTS) during a Loss-of-Coolant Accident (LOCA). A PTS consists of a rapid cooling of the RPV wall under pressurized conditions that may induce the criticality of existing or postulated defects inside the vessel wall. The most severe PTS event has been identified by Emergency Core Cooling (ECC) injection during a LOCA. The traditional one-dimensional system codes fail to reliably predict the complex three-dimensional thermal mixing phenomena in the downcomer occurring during the ECC injection. Hence, CFD can bring real benefits in terms of more realistic and more predictive capabilities. However, to gain trust in the application of CFD modelling for PTS, a comprehensive validation programme is necessary. In the absence of detailed experimental data for the RPV cooling during ECC injection, high fidelity Direct Numerical Simulation (DNS) databases constitute a valid alternative and can serve as a reference.The aim of this work is to design a numerical experiment aimed to generate a high quality reference DNS database for a simplified PTS scenario. This takes into account the turbulent mixing in the downcomer and the evolution of the temperature distribution for both structures and fluid during a single-phase flow PTS scenario. In spite of simplifications, such a DNS analysis represents a very demanding application. A priori, it should be demonstrated that all the relevant turbulent scales will be fully resolved, which requires a huge computational power. A wide range of Reynolds Averaged Navier–Stokes (RANS) calculations are performed in order to determine a realistic PTS computational domain. Furthermore, the resulting numerical experiment is optimized in terms of geometry, boundary conditions, and physical properties.

CFD based approach for modeling direct contact condensation heat transfer in two-phase turbulent stratified flows

This paper describes a CFD based strategy for the modeling of stratified two-phase flows with heat and mass transfer across a moving steam-water interface due to direct contact condensation. Such flows have been of major importance for example in connection with the analysis of nuclear reactor safety systems, in particular during two-phase Pressurized Thermal Shock (PTS) scenarios. The approach is based on the two-fluid phase-average model. The interfacial friction was modeled by using an Algebraic Interfacial Area Density (AIAD) framework where the drag coefficient is a function of the local flow characteristics. To show the impact of the modeling of interfacial friction the simulation with the AIAD model was compared with a simulation where a constant drag coefficient of 0.44 was used in the whole domain. For the modeling of interfacial heat and mass transfer two correlations for the water heat transfer coefficient based on the penetration theory were utilized. The CFD simulations were validated against a steady-state TOPFLOW-PTS steam/water experiment. In the experiment, very detailed temperature measurements were conducted using special thermocouple lances and infrared thermography. Total condensation rate was determined indirectly by using three different methods. The simulations have shown that the results obtained with the AIAD model are considerably closer to the experimental observations than the results obtained with the constant drag coefficient. The condensation models used in the current study predict quite different total condensation rates. That caused significant differences in the temperature field. The simulations of the TOPFLOW-PTS steam/water experiment with condensation have shown that the proposed CFD modeling approach can be successfully applied for the prediction of temperature field and condensation rate during two-phase Pressurized Thermal Shock scenarios. However, the modeling of turbulent interfacial heat transfer should be improved.

Comparison of CFD simulations on two-phase Pressurized Thermal Shock scenarios

Small Break Loss of Coolant Accident (SB LOCA) is one of the most severe transients which may lead to Pressurized Thermal Shock (PTS) on the Reactor Pressure Vessel (RPV) wall. During postulated SB LOCA Emergency Core Cooling (ECC) water is injected into the cold leg, where it mixes with the hot coolant. The mixture of cold and hot coolants flows toward the downcomer. Knowledge of transient temperature distribution in the downcomer is necessary to predict thermal gradients in the structural components of the RPV wall. For the prediction of the temperature fields and heat transfer coefficient between the fluid and wall in the cold leg and the downcomer, reliable computational fluid dynamics (CFD) simulations are needed. To validate CFD models for two-phase PTS scenarios numerical simulations of the TOPFLOW-PTS experiments were performed in the framework of the EU NURISP (NUclear Reactor Integrated Simulation Project) project. The paper presents the post-test CFD simulations of a steady-state TOPFLOW-PTS air/water experiment and the pre-test blind simulations of a steady-state TOPFLOW-PTS steam/water case with condensation. CFD simulations were performed with ANSYS FLUENT, ANSYS CFX and NEPTUNE_CFD. The simulations of the air/water test have shown that correct modeling of the ECC jet behavior is essential for the temperature prediction in the cold leg. For modeling these two-phase flows with rather smooth large free surfaces, Reynolds Averaged Navier–Stokes approach seems to be appropriate. The pre-test simulations of steam/water flow predicted a thermal stratification at the entrance of the downcomer. Finally, the simulations of the TOPFLOW-PTS experiments have depicted considerable differences between the codes and the models.

Experimental and Numerical Modeling of Transition Matrix From Momentum to Buoyancy-Driven Flow in a Pressurized Water Reactor

The influence of density differences on the mixing of the primary loop inventory and the emergency core cooling (ECC) water in the cold leg and downcomer of a pressurized water reactor (PWR) was analyzed at the Rossendorf coolant mixing (ROCOM) test facility. This paper presents a matrix of ROCOM experiments in which water with the same or higher density was injected into a cold leg of the reactor model with already established natural circulation conditions at different low mass flow rates. Wire-mesh sensors measuring the concentration of a tracer in the injected water were installed in the cold leg, upper and lower part of the downcomer. A transition matrix from momentum to buoyancy-driven flow experiments was selected for validation of the computational fluid dynamics software ANSYS CFX. A hybrid mesh with 4×106 elements was used for the calculations. The turbulence models usually applied in such cases assume that turbulence is isotropic, whilst buoyancy actually induces anisotropy. Thus, in this paper, higher order turbulence models have been developed and implemented, which take into account that anisotropy. Buoyancy generated source and dissipation terms were proposed and introduced into the balance equations for the turbulent kinetic energy. The results of the experiments and of the numerical calculations show that mixing strongly depends on buoyancy effects: At higher mass flow rates (close to nominal conditions) the injected slug propagates in the circumferential direction around the core barrel. Buoyancy effects reduce this circumferential propagation with lower mass flow rates and/or higher density differences. The ECC water falls in an almost vertical path and reaches the lower downcomer sensor directly below the inlet nozzle. Therefore, density effects play an important role during natural convection with the ECC injection in PWR and should be also considered in pressurized thermal shock scenarios. ANSYS CFX was able to predict the observed flow patterns and mixing phenomena quite well.

CFD Code Validation against Stratified Air-Water Flow Experimental Data

Pressurized thermal shock (PTS) modelling has been identified as one of the most important industrial needs related to nuclear reactor safety. A severe PTS scenario limiting the reactor pressure vessel (RPV) lifetime is the cold water emergency core cooling (ECC) injection into the cold leg during a loss of coolant accident (LOCA). Since it represents a big challenge for numerical simulations, this scenario was selected within the European Platform for Nuclear Reactor Simulations (NURESIM) Integrated Project as a reference two-phase problem for computational fluid dynamics (CFDs) code validation. This paper presents a CFD analysis of a stratified air-water flow experimental investigation performed at the Institut de Mécanique des Fluides de Toulouse in 1985, which shares some common physical features with the ECC injection in PWR cold leg. Numerical simulations have been carried out with two commercial codes (Fluent and Ansys CFX), and a research code (NEPTUNE CFD). The aim of this work, carried out at the University of Pisa within the NURESIM IP, is to validate the free surface flow model implemented in the codes against experimental data, and to perform code-to-code benchmarking. Obtained results suggest the relevance of three-dimensional effects and stress the importance of a suitable interface drag modelling.

An Empirical Mixing Model for Pressurized Thermal Shock Applications

Empirical correlations are developed for the local temperature and velocity distributions in the pressurized water reactor downcomer for pressurized thermal shock scenarios. The correlation is based on Creare test data and has been validated with Science Applications, Inc. experiments and COMMIX code calculations. It provides good agreement under pump flow and natural circulation conditions and gives a conservative estimate under stagnation conditions.