The Development of a Three Field Two Phase Rewetting Numerical Model for a Boiling Water Reactor

During normal operation in a Boiling Water Reactor (BWR), a constant contact between a liquid film and the fuel rod surface is maintained thus guaranteeing efficient cooling of the cladding surface. During a Loss-Of-Coolant Accident (LOCA) or an Anticipated Transient Without Scram-Instability (ATWS-I), the established liquid film on the fuel rods can be vaporized due to coolant inventory loss. This inventory loss can be local (ATWS-I) or at a larger scale (LOCA). When the film is vaporized, optimal heat transfer at the cladding surface is lost and fuel bundles begin heating-up rapidly. To avoid high surface temperatures that could compromise the integrity of the cladding, the film of the coolant needs to be re-established on the surface. This phenomenon is called rewetting and its modeling is very important for determining the thermal behavior of the cladding. In order to predict the cladding temperature behavior and the rewetting of the fuel rods, Nuclear Safety Analysis (NSA) calculations need to be performed. Traditionally, for Appendix K compliant methods [14], LOCA transients have used conservative thermal-hydraulic models. The simplified computer models and often excessive conservatism of these analytical methods do not allow to fully capture the details of the physical phenomena during a LOCA transient. In addition to unrealistic modeling, this conservatism leads to unnecessary large thermal margins that may impact plant economics and operation. While ATWS-I is a beyond design basis event and best-estimate conditions can be used for its analysis, both LOCA and ATWS-I rely on the numerical prediction of the rewetting. Therefore, accurately predicting flow regimes and heat transfer regimes is critical to the safety analysis of nuclear reactors and rewetting in particular. The predictions of the rewetting phenomenon requires a variety of constitutive relationships to describe the mass, momentum, and energy exchange that occurs between the flow fields (steam, droplets and liquid film) and provide closure to the set of momentum equations. Rewetting is characterized as a localized physical phenomenon where sharp temperature, void fraction and mass flux gradients are observed within a small distance. Often, these characteristics are not well represented by Appendix K compliant methods or overly conservative analyses in general. The optimal utilization of modern high performance fuel designs in combination with the industrial application of best-estimate methodologies generated interest to depart from these conservative approaches. However, most thermal-hydraulic system codes used throughout the nuclear industry for accident analysis employ a one-dimensional two-field two-phase model that tracks single homogeneous liquid and vapor phases and relies on simplified models or correlations to predict the complex heat transfer and flow phenomena during quenching. Several best-estimate models used in NSA codes have a fine mesh option for the heat conduction solution (two-phase two-field codes such as TRACE, RELAP and two-phase three-field subchannel codes such as COBRA)but the very small mesh sizes required to characterize the temperature gradients at the quench fronts makes such method impractical for system calculations. Consequently, the multiscale characteristics of the quench front could be lost and important aspects of the physics of this phenomenon may not be captured. Improving the predictive capabilities of ECCS models is crucial for safety analysis since ECCS capacity may dictate core operating limits on local power production. The originality of the present study lies in that it has examined the predictive capabilities of two-field and three-field transient analysis codes focusing on the rewetting region for BWR at LOCA and ATWS-I conditions. This analysis of the rewetting characteristics yielded the development of a quenching model that includes: 1) A dynamic axial re-nodalization scheme of the local quench front. A coupled T-H re-nodalization model with a fine moving mesh dynamically refining the Eulerian hydraulic mesh of an NSA computer code was developed to better model quenching heat transfer. This refined Eulerian hydraulic mesh is in turn coupled with a Lagrangian non-uniform nodalization in order to compute the temperature distribution within the fuel rod and calculate the heat transfer to the coolant. The Lagrangian-Eulerian coupling and dynamic re-nodalization logic for three-field solvers is a new approach that has been developed for this thesis. 2) The development of a heat transfer logic and the use of pr-established heat transfer correlations to properly calculate temperature gradients at locations where sharp thermal-hydraulic property changes are experienced, 3) The development of a 2D-Conduction Controlled model that solves the temperature distribution in the fuel rod and computes film front velocities, heat transfer from the rod surface to the fluid and couples these results to the considered NSA computer code original T/H solution. The three-field GEH proprietary code COBRAG code was used for this work because of its capability to model steam, droplets and film which provides additional resolution in modeling the rewetting phenomenon. Its spatial resolution capabilities were increased with including the dynamic re-nodalization scheme and the implementation of the quenching model. In order to incorporate the models discussed previously, COBRAG was modified to include constitutive relations to compute the flow regimes and heat transfer, mass and energy distribution between fields at the rewetting front.. The models that are incorporated into the proposed three-field rewetting package are developed uniquely for the problem at hand. The developed rewetting modeling package for COBRAG includes: 1) The implementation of a functional relationship modeling the liquid phase split between film and droplet based on comparisons to experimental data. This model was implemented in COBRAG to refine the liquid distribution between the liquid film and the droplets. 2) The improvements to the numerical method to mitigate water packing in numerical cells at locations of sharp interfacial property changes and low pressure applications. The quenching model and the methods package developed during the research work provide a detailed spatial representation of the subchannel thermal-hydraulic properties for a single BWR bundle for ATWS-I and LOCA conditions. Given the importance of the physical phenomena at play in a BWR channel for safety analyses of various reactor accident scenarios and the interest to generate more precise and updated experimental data, this modeling methodology provides a starting ground in improving the predictive capabilities of three-field transient analysis codes until a more stable and viable approach is ascertained for both ATWS-I and LOCA applications. Benchmark results in the current study demonstrate that the development and inclusion of this newly proposed model for rewetting modeling along with the necessary COBRAG modifications is able to match experimental data for high pressure and high temperature quenching. The development of the 2D-Conduction Controlled quenching model and renodalization scheme along with the COBRAG modification results in an enhanced capability to model the rewetting phenomenon in a Lagrangian-Eulerian framework for two-phase flow and three fields numerical model. The result of this thesis developed an advanced COBRAG code version with transient capabilities. It was demonstrated that this advanced COBRAG code reasonably models experimental results representative of BWR LOCA and ATWS-I analyses

Rewetting processes during PWR reflood

Rewetting of heated surfaces is important in many physical processes and has important technological applications. Understanding of this phenomenon is required in many engineering and scientific fields. It is one of the most crucial phenomena to be considered for the safety analysis of the design basis Loss‐of‐Coolant Accident (LOCA) in light water reactors (Pressurized Water and Boiling Water Reactors). To mitigate the consequences of LOCA, water is fed into the reactor core via an emergency core cooling system; in the PWR, this water is fed to the core via the lower plenum (“bottom reflooding”) and in the BWR, this water is sprayed onto the top of the core (“top reflooding”). In both the cases, a quench front is formed which moves rather slowly. Ahead of quench front, complex and chaotic processes are occurring over a very small axial region where high temperature gradient exists. The heat transfer mechanism is not well known in this region. In this work, the detailed physics of the rewetting processes has been investigated both theoretically and experimentally. The thermal hydraulic behaviour of hot vertical channels during emergency core cooling conditions would be expected to be flow direction‐dependent, it was important to consider the two cases (top reflooding and bottom reflooding) separately. It was possible for the first time, to the author’s knowledge, to apply the fast response infra‐red thermal imaging system to study the rewetting process during top and bottom reflooding of heated vertical surfaces. The important contribution of this work was the use of this new technology to sense the variation of temperature with time at multiple nearby locations at the quench front. In the top reflood experiments, a heated stainless steel pate was quenched by a falling film flow. Through an infrared‐transparent substrate embedded in the plate and coated with platinum, temperature measurements at a location near the rewetting front were achieved using infrared thermal imaging system. The temperature/time traces showed fluctuations in temperature indicating occurrence of intermittent contacts at the quench front. A high speed video camera was also employed to capture rewetting processes by the visual observation of the rewetting front from the top surface. In the visual observations, the liquid film has been seen making intermittent contacts with the hot surface. In these experiments, the effect of the flow rate and the degree of sub‐cooling of the feed liquid has been studied. The rewetting temperature and the characteristic length of the intermittent contact region have been deduced from the experimental results. Experiments were also done to measure temperature changes at the rewetting front for the case of bottom reflooding of a heated tube using a similar technique to that employed for the studies of top reflooding. The results suggested that the rewetting behaviour was different depending on whether the reflood rate was high or low. For high reflood rate, the observations are consistent with the regime above the rewetting front being of the inverted annular type and, for lower reflooding rates, the results are consistent with the rewetting front corresponding to a film dryout in annular flow. An important finding from these experiments is the identification of transient temperature fluctuations in the transition region for the high flooding rate case. These are similar to those observed in the top reflood case and it seems very likely that these fluctuations are associated with intermittent wetting of the surface in this region. An attempt has been made to model rewetting phenomena in which the mechanism of heat transfer at the quench has been proposed. The postulated mechanism is transient near‐surface cooling resulting from intermittent solid‐liquid contacts, followed by recovery of the surface temperature of the metal substrate, with explosive vaporization occurring when the homogeneous nucleation temperature is restored at the metal‐water interface. A one‐dimensional rewetting model was constructed to explain the cyclical process; this model predicted the cyclical behaviour, with the expected qualitative dependence on system parameters. Its predictions are quantitatively consistent with experimental observation, in that the unsteady model analysis brackets the experimentally observed periodicity of the quasi‐steady actual process. The one‐dimensional model of the process has been complemented by twodimensional simulations using a commercial finite element code (ANSYS). In these simulations, an intermittent contact region has been modelled by imposing a heat transfer coefficient over a certain length between dry and wet regions. A parametric study was performed to see the effect of the rewetting velocity, the wet side heat transfer coefficient, intermittent contact heat transfer coefficient, and the length of intermittent contact region

Flow and heat transfer in pressurised water reactor reflood

This thesis describes work relating to the reflood phase of a Large Break Loss-of-Coolant Accident (LB-LOCA) in Pressurised Water Reactor (PWR). Three related types of experiment have been carried in this context, namely studies of particle motion in an annulus geometry simulating drop motion in a ballooned fuel element, studies of single phase flow in a 3×3 tube bundle simulating a ballooned fuel element and studies of reflooding of a hot tube in which it was possible to photograph the region above the rewetting front using axial view photography. In the particle tracking studies, Particle Tracking Velocimetry (PTV) was used to determine typical particle tracks in an annulus test section in which the inner surface was ballooned to simulate the clad ballooning likely to occur during the reflood phase of an LB-LOCA. Excellent agreement was obtained between the measured particle tracks and ones calculated using the STAR-CD CFD code. The second set of experiments focussed on investigating the effect of pin ballooning on the vapour flow. An idealised, simulated PWR bundle containing a 3×3 rod arrangement with a central ballooned pin was designed and constructed and, using a novel isokinetic probe sampling technique, the axial deviation in mass flow of an outer sub-channel was measured. Again, good agreement was obtained between the flows measured and those calculated from the STAR-CD code. To further elucidate the rewetting process itself and the behaviour of the associated two-phase flow, an axial-viewing reflood (AVR) rig has been designed and constructed. Within this facility, experiments have been carried out to examine the thermal-hydraulic effects occurring during bottom-up reflooding of a single hot tube. A high-speed high-temperature axial viewing technique has been developed and applied to observe the quench front, and any precursory droplet production, deposition and entrainment ahead of the propagating quench front

ANALISA PENGARUH SUHU AWAL PELAT PANAS PADA PROSES QUENCHING CELAH SEMPIT REKTANGULAR

Pemahaman terhadap manajemen termal apabila terjadi suatu kecelakaan parah reaktor nuklir seperti melelehnya bahan bakar dan teras reaktor, menjadi prioritas utama untuk menjaga integritas bejana tekan reaktor. Dengan demikian hasil lelehan bahan bakar dan teras reaktor (debris) tidak keluar dari bejana tekan reaktor dan mengakibatkan dampak lain yang lebih besar ke lingkungan. Salah satu cara yang dilakukan untuk menjaga integritas bejana tekan reaktor adalah dengan melakukan pendinginan terhadap panas berlebih yang dihasilkan akibat dari kecelakaan tersebut. Untuk mempelajari dan mendapatkan pemahaman mengenai hal tersebut, maka dilakukan penelitian mengenai pengaruh suhu awal pelat panas dalam proses quenching (pendinginan secara tiba-tiba) celah sempit rektangular. Penelitian difokuskan pada penentuan suhu rewetting dari pendinginan pelat panas dengan suhu awal pelat 220 0C, 400 0C, dan 600 0C dengan laju aliran air pendingin 0,2 liter/detik. Eksperimen dilakukan dengan menginjeksikan air pada laju aliran 0,2 liter/detik pada suhu air pendingin 85 0C ke dalam celah sempit rektangular. Data hasil pengukuran digunakan untuk mengetahui suhu rewetting yang terjadi pada pendinginan pelat panas tersebut. Tujuannya adalah untuk memahami pengaruh suhu awal pelat panas terhadap rewetting pada proses quenching di celah sempit rektangular. Hasil yang diperoleh menunjukkan bahwa titik rewetting pada pendinginan pelat panas 220 0C, 400 0C, dan 600 0C terjadi pada suhu rewetting yang berbeda-beda. Pada suhu awal pelat panas 220 0C, suhu rewetting terjadi pada 220 0C yaitu langsung ketika air dilewatkan melalui celah sempit rektangular. Pada suhu awal pelat panas 400 0C, suhu rewetting terjadi pada 379,51 0C. Dan pada suhu awal pelat panas 600 0C, suhu rewetting terjadi pada 426,63 0C. Perbedaan suhu awal pelat panas yang sangat signifikan menyebabkan terjadinya perubahan sifat fisik benda uji, berbedanya rejim pendidihan yang dialami oleh fluida yang melewati celah sempit, perubahan nilai kalor spesifik bahan, perubahan nilai konduktifitas termal, dan perbedaan suhu wall superheated-nya. Perubahan-perubahan yang terjadi tersebut menyebabkan peningkatan suhu rewetting seiring dengan kenaikan suhu awal pelat panas.Kata kunci: rewetting, quenching, celah sempit, keselamatan nuklir The understanding about thermal management in the event of a severe accident such as the melting nuclear reactor fuel and reactor core, became a priority to maintain the integrity of reactor pressure vessel. Thus the debris will not out from the reactor pressure vessel and resulting impact of more substantial to the environment. One way to maintain the integrity of the reactor pressure vessel was cooling of the excess heat generated due to the accident. Toget understanding of this aspect, the research focused on the effect of the initial temperature of the hot plate in the rectangular narrow gap quenching process. The initial temperature effect on quenching process is related to cooling process (thermal management) when the occurrence of a nuclear accident due to loss of coolant accident or severe accident. In order to address the problem, it is crucial to conduct research to get a better understanding of thermal management regarding to nuclear cooling accident. The research focused on determining the rewetting temperature of hot plate cooling on 220 0C, 400 0C, and 600 0C with 0.2 liters/sec cooling water flowrate. Experiments were carried out by injecting 85 0C cooling water temperature into the narrow gap at flowrates of 0.2 liters/sec. Data of transient temperature measurements were recorded using a data acquisition system in order to know the rewetting temperature during the quenching process. This study aims to understand the effect of hot plate intial temperature on rewetting during rectangular narrow gap quenching process. The results obtained show that the rewetting point on cooling the hot plate 220 0C, 400 0C and 600 0C occurs at varying rewetting temperatures. At 220 0C hot plate initial temperature, the rewetting temperature occurs on 220 0C. At 400 0C hot plate initial temperature, the rewetting temperature occurs on 379.51 0C. At 600 0C hot plate initial temperature, the rewetting temperature occurs on 426.63 0C. Significant differences of hot plate initial temperature leads to changes in physical properties of material, different boiling regimes occurs when fluid passing through a narrow gap, changes on specific heat of material, changes on thermal conductivity of material, and the differences of wall superheated temperature. Rewetting temperature will increase due to increasing on hot plate initial temperature. Keywords: rewetting, quenching, narrow gap, nuclear safet

ショウトツ フンリュウ ニヨル コウオンメン ノ キュウソク レイキャクチュウ ノ ヒテイジョウ デンネツ トクセイ

Transient heat transfer has been investigated experimentally with a subcooled water jet during quenching of hot cylindrical blocks made of copper, brass and steel for initial surface temperatures from 250 to 600 oC. The jet velocity was from 3 to 15 m/s and jet subcooling was from 5 to 80 K with a jet diameter of 2 mm. When the jet first struck the hot surface, the visible leading edge of the moving liquid (wetting front) became stagnant for a certain period of time in the small impinged region and splashed out from that region before wetting the entire surface. This wetting delay may be described as resident time which is a strong function of block material and jet subcooling and also a function of block initial temperature and jet velocity. New correlations for the resident time and the surface temperature at resident time at wetting front position have been proposed in this study which agree well with the experimental data. During the movement of the wetting front, the surface temperature at the wetting front drops to 120-200 oC and the surface heat flux reaches its maximum value due to forced convection nucleation boiling. The maximum heat flux is a strong function of the position on the hot surface, jet velocity, block material properties and jet subcooling. A new correlation for maximum heat flux is also proposed. When the resident time is short, the rate of movement of the maximum heat flux position increases with the increase of jet velocity and subcooling and decreases with the increase of block initial temperature. These trends are opposite for long resident time. During the movement of the wetting front over the hot surface, a darker moving vigorous boiling region is observed at the leading area of moving liquid. The width of this vigorous boiling region is described as the ‘boiling width’. Boiling width affects the heat flux estimation and distribution in jet impingement quenching. Boiling width increases with radial position. Higher conductivity of the test section material results in the higher v