A Case Study Evaluating the Effects of High Cycle Thermal Loading Within a Pressurised Water Reactor Mixing Tee Using Conjugate CFD/FE Methods

Following the CIVAUX 1 incident of a leak on RHR system, EDF has developed a step by step procedure to screen and analyse similar locations: mixing tees with long duration at high ΔT between the 2 fluids. The paper summarise the procedure, the background and some R&D results that support this procedure. The procedure is based on: • screening criteria without any duration consideration, only ΔT and material; • a simplified and conservative estimation of usage factor; • a detailed analyses of usage factor and crack growth rate, based on specific data collection of operating transients and fabrication parameters. Around that procedure, EDF launched an R&D program on fatigue curves and different fatigue reduction factors; thermal fatigue test on structures and review the agreement of predictions with French RHR system field experience. The major results confirm that, to-day, present Code rules (RCCM or ASME) are not totally in agreement with our results. Proposals and suggestions of future works are discussed in the paper.

Large eddy simulation studies on the influence of turbulent inlet conditions on the flow behavior in a mixing tee

Large-eddy simulations (LES) of temperature fluctuations in a mixing tee with/without a porous medium

In the present work the temperature fluctuations in a mixing tee were simulated on FLUENT platform using the Large-eddy simulation (LES) turbulent flow model with the sub-grid scale (SGS) model of Smagorinsky-Lilly (SL). The temperature and velocity fields, the normalized mean and fluctuating temperatures and velocities were predicted and analyzed with consideration of buoyancy. The normalized mean and fluctuating temperatures were defined to describe the time-averaged temperature and the time-averaged temperature fluctuation intensity. The numerical results of the normalized mean and fluctuating temperatures were compared with those of the experimental ones published in previous literature, which shows that numerical results have good agreement with the experimental data. The temperature fluctuations and power spectrum densities (PSD) at the locations having the strongest temperature fluctuations both in tee junction and on the walls were analyzed to evaluate the potential of thermal fatigue. The LES flow simulation and power spectral analysis are helpful for the Integrity evaluation of the structures such as the tee junction, elbow, piping system to predict the temperature fluctuation and thermal stripping in a tee junction of mixing hot and cold fluids.

Metal fatigue is a well-known phenomenon whereby material characteristics are deteriorated when even a small load is applied repeatedly. Therefore, an accurate fatigue evaluation is very important in terms of component integrity and reliability. In the design stage, the fatigue evaluation of nuclear class 1 components has to be performed in accordance with Sec. III of the ASME Boiler and Pressure Vessel Code. However, operating experience shows that the design transients are very conservative compared to the actual ones in terms of the heating/cooling rates and the number of transient occurrences. Considering that these two factors affect the thermal stress and thereafter the fatigue usage factor (FUF), the actual fatigue damage can differ from the design fatigue evaluation result. In order to evaluate and monitor the FUF exactly, therefore, various methods have been proposed and widely implemented. Among these, the cycle-based approach (CBA) utilizes the stresses for the design transients and reflects only the actual number of transient occurrences. For this reason, the CBA provides a conservative FUF, although it is very simple and easy to implement. Therefore, a simple and accurate fatigue monitoring method is still needed.The purpose of this paper is to develop a new approach for effective fatigue damage monitoring. To do this, a thorough review is conducted for the design transients and actual transients for the Westinghouse-type pressurized water reactors in Korea. In addition, a wide range of finite element analyses are carried out varying the heating/cooling rates and the pattern of the transients. Based on this result, a new CBA is proposed incorporating the simple correction factors for both the heating/cooling rates and the transient patterns. A case study is also carried out for the reactor pressure vessel outlet nozzle to verify the validity and applicability of the proposed method. The result indicates that the proposed method can provide a realistic FUF, and more importantly, it is very easy to implement. From these, it is anticipated that the new approach can be widely used in practical fatigue monitoring of nuclear components and piping.

Large-eddy simulation of thermal fatigue in a mixing tee

In the present work the temperature fluctuations in a mixing tee were simulated on FLUENT platform using the large-eddy simulation (LES) turbulent flow model with three kinds of sub-grid scale (SGS) models such as Smagorinsky-Lilly (SL) model, Wall-adapted Local Eddy-viscosity (WALE) model, and Kinetic-energy transport (KET) model. The normalized mean and root mean square temperatures were predicted and analyzed with consideration of buoyancy. The numerical results showed that buoyancy greatly influences the mixing flow and the thermal striping phenomena were quite obvious. These three SGS models have somewhat similar accuracies for prediction of the temperature fluctuation and thermal stripping in a tee of mixing hot and cold fluids.

Real-time estimation of the fatigue usage factor of nuclear reactor components are helpful tools for on-demand assessment of component structural integrity. Real-time measurements of field variables, such as stress and strain, along with the use of historic/available stress/strain versus fatigue life data and a Bayesian-probabilistic framework can not only help to estimate the fatigue usage factor of reactor components in real time but also can help to estimate the associated confidence bounds. In this paper, a Gaussian Process based Bayesian-probabilistic framework is proposed for real-time estimation of mean fatigue usages factor and associated probabilistic bounds. The preliminary proof of concept was demonstrated through fatigue experiments with 316 stainless steel specimens under different conditions: a) under 300 °C and pressurized water reactor (PWR) water chemistry, and b) room temperature and in-air condition. At present the proposed probabilistic monitoring approach is not part of any code requirements, however the proposed framework is output of a futuristic basic research work, which requires more advancement. Also the results discussed in the paper are part of very preliminary basic research and requires advance validation such as under realistic nuclear reactor conditions.

Prediction of the mixing of multi-component fluids is important in many chemical process applications. Although laminar mixing is a complicated process per se (involving multi-component diffusion coefficients, for example), there is a far greater challenge in predicting mixing in turbulent flows because of their intrinsic, chaotic nature. In turbulent flows, large-scale eddies with coherent structures are mainly responsible for the mixing of passive scalars. The large-scale eddies embody themselves in the form of identifiable and organized distributions of vorticity. In addition, the mixing process involves all mechanisms typically found in vortex dynamics, such as stretching, break-up, concatenation, and self-induction of vortices. Experimental work suggests that large-scale, time-dependent structures, with periods much longer than the time of an impeller revolution, are involved in many of the fundamental hydrodynamic processes in stirred vessels. For example, local velocity data histograms may be bi-modal or tri-modal, even though they are being analyzed as having only one mode in most Laser-Doppler experiments. Digital particle image velocimetry experiments have shown that large-scale asymmetries with periods up to several minutes exist in stirred vessels equipped with axial flow impellers. These complex phenomena are not limited to single-phase systems. Many industrial vessels are operated with a multiphase flow. In such systems, the gas holdup distribution may be asymmetric and oscillating. In solids suspension processes, solids can be swept from one side of the vessel to the other in an oscillating pattern, even in dilute suspensions. The numerical modeling of these complicated mixing processes is a daunting task. Direct numerical simulation (DNS) provides the most exact approach in which the mechanism involved in turbulent mixing can be accurately represented. DNS requires resolving the smallest eddies, which makes the approach prohibitively expensive, even with the most powerful computers of the present day and foreseeable future as well. On the other hand, the popular approaches based on Reynolds-Averaged Navier-Stokes (RANS) equations amount to averaging out the large eddies that are primarily responsible for mixing. One is left to model the effects of large eddies by relying on empirical data and phenomenological reasoning and hypotheses, which are often questionable. The advantage of Large Eddy Simulation (LES) is that it explicitly resolves the large eddies, which are responsible for much of the mass, energy, and momentum transport, and only small eddies are modeled with a sub-grid model. In this lecture we will first briefly review the fundamentals of turbulent flows in stirred vessels, and how modeling these has evolved during the past decade. The focus will be on those aspects of turbulence that are relevant to mixing processes and the modeling thereof. We will continue with a discussion of the applicability of various turbulence models. For single-phase systems, we will then discuss the application of LES to the prediction of large-scale chaotic flow structures in stirred tanks. The focus of those studies is on systems with unsteady flows that are especially difficult to model with eddy-viscosity style models, namely those with strong swirl such as glass-lined mixing vessels (which usually have one baffle) and multiple impeller systems with strong interaction between the impeller flows. For multiphase systems, turbulence modeling is an even greater challenge. Interesting developments in this field include the use of LES models coupled with discrete particle simulations. More recently, full Reynolds stress models for use on unstructured finite volume meshes for Eulerian-Eulerian multiphase flow models have become available. Recent results with these models and expected future developments will be discussed.

Nowadays, it is well known that the low cycle fatigue (LCF) life of austenitic stainless steels can be affected in specific conditions of temperature, strain rate, strain amplitude or dissolved oxygen concentration by the effect of Pressurized Water Reactor (PWR) primary coolant environment. Nevertheless, questions remain about the best methodology that must be used to consider environmental effects for nuclear power plant licensing and for operating lifetime extensions. These environmental effects are most commonly evaluated from a mean fatigue curve based on tests conducted in air at room temperature. However, it is well established that air is not a neutral environment for metallic alloys and its effect can be highly dependent on the temperature level. Thus, in order to evaluate the intrinsic fatigue resistance of a 304L austenitic stainless steel at 300°C and the importance of complex fatigue – environment interactions in air or in PWR water, LCF tests were performed in both environments and specifically designed ones were conducted in secondary vacuum. Tests were performed on 304L cylindrical specimens at 20 or 300°C in vacuum or in air and only at 300°C in PWR water, under total axial strain control using a triangular waveform at strain amplitudes of ±0.3 or ±0.6% and strain rates of 4 × 10−3, 1 × 10−4 or 1 × 10−5 s−1. It was found that compared with vacuum, air is responsible for a strong decrease in fatigue lifetime in this steel, especially at 300°C and low strain amplitude. The PWR water coolant environment is still more active than air and leads mainly to increased damage kinetics, with slight effects on initiation sites or propagation modes. More precisely, the decreased fatigue life in PWR water is essentially attributed to an enhancement of both crack initiation and “short crack” micropropagation stages. Furthermore, a detrimental influence of low strain rates on the fatigue lifetime at 300°C was observed in PWR water environment or in air, but also in vacuum without environmental effects, and was in the last case exclusively attributed to the occurrence of the dynamic strain aging (DSA) phenomenon. So, the use of data obtained in a neutral environment as a reference allows the evaluation of the intrinsic effect of each environmental or loading condition. Moreover, in an active environment such as air or PWR primary water, damage evolutions as well as fatigue lives cannot be predicted by a simple multiplication of each parameter effect taken separately because they are the result of numerous interactions. The last conclusion is supported by complementary results showing that the PWR water environment effect as well as the ground surface finish effect can be attenuated when LCF tests are performed with a more representative loading signal shape.

The lifetime extension of nuclear power stations is considered an energy challenge worldwide. That is why the risk analysis and the study of various effects of different factors that could potentially prevent safe long-term operation are necessary. These structures, often of great dimensions, are subjected during their life to complex loading combining varying multiaxial mechanical loads with non-zero mean values associated with temperature fluctuations under a PWR (pressure water reactor) environment. Based on more recent fatigue data (including tests at 300 °C in air and a PWR environment, etc.), some international codes (RCC-M, ASME, and others) have proposed and suggested a modification of the austenitic stainless steels fatigue curve combined with a calculation of an environmental penalty factor, namely Fen, which has to be multiplied by the usual fatigue usage factor. The determination of the field of validation of the application of this penalty factor requires obtaining experimental data. The aim of this paper is to present a new device, “FABIME2e” developed in the LISN (Laboratory of Integrity of Structures and Normalization) in collaboration with EDF (Electricity of France) and Framatome. These new tests allow the effect of a PWR environment on a disk specimen to be quantified. This new device combines structural effects such as equibiaxiality and mean strain and the environmental penalty effect with the use of a PWR environment during fatigue tests.

Mixing, scalar boundedness, and numerical dissipation in large-eddy simulations

Qinshan Phase III is the first commercial pressurized heavy water reactor (PHWR) NPP in China, and it uses CANDU-6 design developed by AECL. Based on plant design and operation experience, the event tree analysis model has been developed for both small break LOCA (SB-LOCA) and large break LOCA (LB-LOCA), which is an important aspect of operational Probabilistic Safety Assessment (PSA). Both SB-LOCA and LB-LOCA event tree analysis have been performed for Qinshan Phase III CANDU-6 PHWR NPP (TQNPC). And the event sequence development and plant damage status (PDS) were provided in the analysis. It reflects actual plant configuration and response under a certain event, and various break type and locations were also considered in the event tree analysis, e.g. Pressure Tube Rupture, Pressure Tube and Calandria Tube Rupture, Feeder Breaks, Pressurizer Relief/Steam Bleed Valves Fail Open, Liquid Relief Valves Fail Open, etc.

Numerical simulation is carried out to study the combined effects of rotation and induced swirl on the fully developed turbulent pipe flow at Reynolds number Reb = 4900, based on the bulk velocity and the pipe diameter. The swirl is induced by a body force in the tangential momentum equation, which produces a tangential velocity field in the near wall region. Results from a single swirl along with five different strengths of the rotation are considered. The effects of rotation and swirl on turbulent structures are investigated in detail. Also instantaneous axial velocity fluctuations are provided to visualize the effects on the turbulent structures. Introduction Turbulent flows of liquids or gases through long straight pipes occur in a variety of different industrial applications. Such flows have received considerable attention throughout the years and are fairly well understood today, although some uncertainties still prevails at very high Reynolds numbers; see e.g. Hultmark et al. (2012). Under certain circumstances, however, the streamlines are helical rather than straight lines and the mean flow becomes twocomponential rather than one-componential. This happens if a swirling motion arises or if the pipe is subjected to axial rotation. Swirl may result from a swirl generator or an upstream elbow, whereas axially rotating pipes are found in turbo machinery cooling systems. In both cases, a circumferential componentUθ of the mean velocity vector coexists with the axial mean velocity componentUz. The presence of a circumferential mean velocity component tends to orient the coherent near-wall structures with the local mean flow direction. Besides the tilting of the near-wall structures, the structures may be strengthened or weakened in a two-componential mean flow. Fully developed turbulent flow in axially rotating pipes has been studied experimentally by Murakami & Kikuyama (1980) and Imao et al. (1996) and by means of large-eddy simulations (LES) by Eggels & Nieuwstadt (1993) and direct numerical simulations (DNS) by Eggels et al. (1994) and Orlandi & Fatica (1997). It is observed that rotation results in drag reduction. Recent DNS studies of swirling pipe flow by Nygard & Andersson (2010) showed the same influence of the induced swirl on the axial mean velocity as axial rotation. However, the presence of swirl turned out to have less clear-cut effects on the turbulence field. In the cases with stronger swirl, even drag reduction was reported whereas weak swirl gave rise to excess drag. Swirl and axial rotation both give rise to helical streamlines and it is therefore not unexpected that similarities between these two circumstances can be found. The aim of the present study is to examine how an originally swirling pipe flow reacts to axial rotation. For a given swirl number, five different rotation rates (N = −1,−0.5,0,0.5,1) with both senses of rotation will be studied. To this end, the full Navier-Stokes equations are solved in three-dimensional space and in time on a computational mesh sufficiently fine to resolve the energetic large-eddy structures. Governing equations The governing equations are solved in cylindrical coordinates θ , r, and z. For practical reasons, the variables qθ = ruθ , qr = rur and qz = uz are introduced. Here uθ , ur, and uz are velocity components in the respective coordinate directions. All variables in the governing equations are non-dimensionalized with the centerline velocity of the Poiseuille profile, Up and the pipe radius, R. To simplify, the non-dimensionalized total pressure, ptotal is divided in to three parts as follows: ptotal = P(θ)+ P(z)+ p(θ ,r,z, t). (1) The first part, P(θ), is the artificial transverse pressure component. In order to introduce a swirl in the pipe flow, the azimuthal pressure gradient, dP/dθ , is introduced. The second part of Eq. (1) is the mean axial pressure, P(z) and finally, p(θ ,r,z, t) represents the remaining part of the total pressure. The Navier-Stokes equations in terms of the new variables, in a reference frame rotating with the pipe wall

Improved design fatigue curves were developed in the Subcommittee on Design Fatigue Curve in the Atomic Energy Research Committee in the Japan Welding Engineering Society (JWES). Working Group on Design Fatigue Curves (WG DFC) in the JSME has studied the validity and the applicability of the improved design fatigue curves developed in the JWES to incorporate into the JSME Environmental Fatigue Evaluation Method. The authors propose a fatigue analysis method using the design fatigue curves developed in the JWES that are applied revised factors to optimize the environmental fatigue analysis. Also, the Japanese pressurized water reactor (PWR) utility group developed equations of environmental fatigue factors (Fen) for austenitic stainless steel base metal, weld metal and cast stainless steel in PWR environment. The WG DFC has investigated the Fen equations and concluded that the Fen equation of austenitic stainless steel base metal is the most conservative among the three equations and close to NUREG/CR-6909 Rev.1 [24]. The authors propose to use the Fen equation for base metal for austenitic stainless steels for all of the base metal, weld metal and cast stainless steel. In addition, the authors have confirmed that the employment of the proposed Fen equation to the proposed design fatigue curves of austenitic stainless steels accurately predicts the existing environmental fatigue test data of austenitic stainless steels, which were used in the development of the current Fen equation of austenitic stainless steels in PWR environments in the JSME Environmental Fatigue Evaluation Method. Therefore, the proposed Fen equation can be applied to environmental fatigue evaluation for austenitic stainless steels.