Akikazu Kurihara

Thermal Influence on Steam Generator Heat Transfer Tube During Sodium-Water Reaction Accident of Sodium-Cooled Fast Reactor

Abstract Sodium-water reaction is a design-basis accident of a sodium fast reactor. A breach of the heat transfer tube in a steam generator (SG) results in contact of liquid sodium with water. The typical phenomenon is that the pressurized water blows off and is mixed with the liquid sodium surrounding SG tubes. The design and safety concern is a possibility of the secondary failure of nearby heat transfer tubes that could cause undesirable development of the accident. One needs to evaluate the temperature transients of the heat transfer tubes in the reaction region for safety evaluation. In the present study, a computational method is developed for this purpose. It solves the sodium thermal hydraulics and the heat conduction in the adjacent heat transfer tubes. An experiment performed at the Japan Atomic Energy Agency is analyzed with the method developed in this study. It is found that analyzed temperatures are in good agreement with the experimental data. Based on the experimental and computational results, multiphase multicomponent flow characteristics are depicted. Furthermore, the heat transfer coefficient is evaluated using the instantaneous heat flux and temperature obtained from the numerical simulation.

Visualization on the behavior of inert gas jets impinging on a single glass tube submerged in liquid sodium

In order to accurately model sodium–water reaction jets in steam generators of fast breeder reactors, knowledge of size distributions or mean diameters of liquid sodium droplets entrained into the reaction jets is prerequisite. In the present study, argon-gas jet behaviors, without chemical reaction, injected into liquid sodium were successfully visualized using an endoscope and a glass tube, and the size distributions and mean diameters of liquid sodium droplets entrained into the gas jet were also obtained in the bubbling regime. Most of the liquid sodium droplets were observed to be intermittently produced in the vicinity of a gas nozzle in the present study. The droplet size distributions of entrained sodium droplets were found to agree well with the Nukiyama–Tanasawa distribution function when the arithmetic mean diameter was used. The Sauter mean diameters obtained in the present study were also found to be well correlated with an empirical equation proposed by Epstein et al. The present study shows that the existing knowledge, which is based on the results of water experiments, is suitable in terms of accuracy in practice.

Void fraction distributions of inert gas jets across a single cylinder with non-wetting surface in liquid sodium

Little work on the void fraction behaviors along structural materials with poor-wettability for liquid metals has been performed. In the present study, void fraction behaviors around a single cylinder with non-wetting surface condition were quantitatively discussed by using a gas jet–cylinder system where the impinging jet flow, the boundary layer flow, the separation flow, and the wake flow appear. One cylinder with a non-wetting surface and two cylinders with a wetting surface were used to vary the wettability for liquid sodium, and void fraction distributions were measured around the cylinders. In the case of wetting condition, void fraction distributions around the cylinder decrease clearly in the backward region of the cylinder, and liquid-rich region is formed due to bubble separation from the cylinder surface. On the other hand, under non-wetting condition, because of two-phase flow without bubble separation on the cylinder surface, void fraction distributions show almost steady values around the cylinder compared to those with wetting surface. The void behaviors on a non-wetting surface were also confirmed by a visualization experiment conducted in water. The observed differences can be basically attributed to the work of adhesion required for liquid–solid interfacial separation.

Estimation of Heat Transfer Coefficient and Flow Characteristics on Heat Transfer Tube in Sodium-Water Reaction

In the steam generator of a sodium-cooled fast reactor, high-pressure water flows inside heat transfer tubes while liquid sodium flows on the shell side. Heat is exchanged through the tube wall. When the tube fails, water vapor leaks into the sodium stream, and a sodium-water reaction is initiated. This reaction occurs rapidly and generates a high-temperature jet. It then becomes possible for neighboring tubes to experience a secondary failure due to overheating. With regard to the secondary failure, an estimate of heat transfer from fluid to the tube is important for safety evaluation. In the present study, a numerical analysis has been carried out to determine the heat transfer coefficient from temperature data obtained in a sodium-water reaction experiment. By updating the heat transfer coefficient, an inverse problem of heat transfer has been solved in the analysis based on the result of the SWAT-1R experiment. It is found that the heat transfer coefficient fluctuates largely during the reaction. The heat transfer coefficient is affected by the flow characteristics. Hence, we characterize the flow pattern near the heat transfer tube at typical periods in the phenomenon progression.

Development of a multiphysics analysis system for sodium-water reaction phenomena in steam generators of sodium-cooled fast reactors

A multiphysics analysis system for sodium-water reaction phenomena in a steam generator of sodium-cooled fast reactors was newly developed. The analysis system consists of the mechanistic numerical analysis codes, SERAPHIM, TACT, and RELAP5. The SERAPHIM code calculates the multicomponent multiphase flow and sodium-water chemical reaction caused by discharging of pressurized water vapor. Applicability of the SERAPHIM code was confirmed through the analyses of the experiment on water vapor discharging in liquid sodium. The TACT code was developed to calculate heat transfer from the reacting jet to the adjacent tube and to predict the tube failure occurrence. The numerical models integrated into the TACT code were verified through some related experiments. The RELAP5 code evaluates thermal hydraulic behavior of water inside the tube. The original heat transfer correlations were corrected for the tube rapidly heated by the reacting jet. The developed system enables evaluation of the wastage environment and the possibility of the failure propagation.

Multiphysics Analysis System for Tube Failure Accident in Steam Generator of Sodium-Cooled Fast Reactor

Multiphysics analysis system was newly developed to evaluate possibility of failure propagation occurrence under heat transfer tube failure accident in a steam generator of sodium-cooled fast reactors. The analysis system consists of the computer codes, SERAPHIM, TACT, RELAP5, which are based on the mechanistic numerical models. The SERAPHIM code calculates the multicomponent multiphase flow involving sodium-water chemical reaction. In this study, numerical models for the chemical reaction about production of a sodium monoxide and its transport process were constructed to enable evaluation of a wastage environment. The TACT code was developed to calculate heat transfer from the reacting jet to the adjacent tube and to predict the tube failure occurrence. The TACT code was integrated by the numerical models of the fluid-structure thermal coupling, the temperature and stress evaluation, the wastage evaluation and the failure judgment. The RELAP5 code evaluates thermal hydraulic behavior of water inside the tube. The original heat transfer correlations were corrected for the rapidly heated tube in the present work.Copyright

Structure and Erosion Characteristics of Underexpanded Inert-Gas Jets Impinging on a Single Cylinder Submerged in Liquid Sodium

When a heat transfer tube wall in a steam generator of a sodium-cooled fast reactor fails, high-pressure steam leaks into low-pressure liquid sodium side. Then the chemical reaction between steam and liquid sodium causes the erosive and corrosive reaction jet, and the secondary failure of a neighboring heat transfer tube is expected. The objectives of the present experimental study are to understand the structure of an underexpanded impinging gas jet which is injected into liquid sodium and to grasp a relationship between the gas jet velocity and the erosion characteristics of liquid sodium droplets which are entrained into the gas jet.In the present study, the structure of an underexpanded inert gas jet impinging on a single cylinder within liquid sodium was clarified using the visualization method, which has been constructed by the authors. Then the erosion tests were carried out at the most erosive geometry, which was confirmed by the visualization experiments.From the visualization, the underexpanded impinging gas jet in liquid sodium could be classified into four regions: core region, droplet flow region, liquid film flow region, and separation region. The erosion phenomenon on the cylinder surface was observed only when the droplet flow region was impinged. In this region, the liquid droplet impingement type erosion was confirmed. The erosion rate, the ratio of the volume loss of specimens to the duration time, was correlated as the function of the gas jet velocity at the end of the external expansion region.Copyright

Visualization on Inert Gas Jets Impinging to a Glass Tube Submerged in Liquid Sodium

For the sodium-water reaction accident, it is important to grasp the structure of gas jets submerged in liquid sodium and associated droplet sizes. In this study, we successfully obtained visualized images of inert gas jets injected into liquid sodium. Formation processes of liquid sodium droplets entrained into the gas jets and drop-size distributions are discussed.Visualization was conducted on upward argon gas jets impinging to a glass tube in stationary liquid sodium pool. The gas jet behavior was observed by using a bore scope inserted inside of the glass tube. The gas jet velocities at the nozzle exit were 13 50 m/s, liquid droplets began to be formed. The formation processes of liquid droplets were classified into (1) spray type; a liquid film formed on the nozzle is blown and atomized by the gas jets and (2) entrainment type; attributed to the instability of a gas cavity-liquid phase interface. But the difference in the drop-size distribution of the both formation processes was not significant. Mean droplet diameter of liquid sodium was confirmed to be agreed well with an empirical equation suggested from a model experiment using water.Copyright

Kinetic Study of Sodium-Water Reaction Phenomena by Differential Thermal Analysis

In a sodium-cooled fast reactor (SFR), if a heat transfer tube in the steam generator (SG) is failed, high pressurized water vapor blows into the liquid sodium and sodium-water reaction (SWR) takes place. SWR may cause damage to the surface of the neighboring heat transfer tubes by thermal and chemical effects. Therefore, it is important to clearly understand the SWR for safety assessment of the SG.From recent study, sodium (Na)–sodium hydroxide (NaOH) reaction as secondary surface reaction of the SWR phenomena in a SFR was identified by ab initio method [1]. However, kinetics of this reaction is a still open question. It is important to obtain quantitative rate constant of sodium monoxide (Na2O) generation by Na-NaOH reaction because Na2O may accelerate the corrosive and erosive effects.Differential thermal analysis (DTA) provides us with the valuable information on the kinetic parameters, including activation energy, pre-exponential factor (frequency factor) and reaction rate constant. Thus, kinetic study of Na–NaOH reaction has been carried out by using DTA technique. The parameters, including melting points of Na and NaOH, phase transition temperature of NaOH, Na-NaOH reaction temperature and the decomposition temperature of sodium hydride (NaH) were identified from DTA curves. Na, NaOH, and Na2O as major chemical species were observed from the X-ray diffraction (XRD) analysis of the residues after the DTA experiment. It was inferred that Na2O could be generated as a reaction product. Based on the measured reaction temperature, the first-order rate constant of Na2O generation was obtained by the application of the laws of chemical kinetics. From the estimated rate constant, it was found that Na2O generation should be considered during SWR.The results can be the basis for developing a chemical reaction model used in a multi-dimensional sodium-water reaction code, SERAPHIM, being developed by the Japan Atomic Energy Agency (JAEA) toward the safety assessment of the SG in a SFR.Copyright

Investigation of correlation diagram between heat transfer coefficient and void fraction under sodium-water reaction

Sodium-water reaction (SWR) in a steam generator of sodium-cooled fast reactor (SFR) is a significant phenomenon for safety assessment of the system. One of the top concerns in the SWR is an overheating rupture phenomenon in which a neighbor heat transfer tube fails instantaneously because of a deterioration of structural integrity under a high temperature condition. Hence, the heat transfer coefficient on the tube surface is of importance. Since hydrogen gas is generated in the SWR and liquid water will evaporate quickly due to depressurization, the reaction region is covered with a multi-phase flow structure, and thus the value of the heat transfer coefficient will vary widely. In the present paper, a correlation diagram has been developed between the heat transfer coefficient and the void fraction based on one dimensional homogeneous flow simulation. Furthermore, the transient of void fraction in SWAT-1R experiment is investigated using the diagram.