Akihiro Uchibori

Computational Methodology of Sodium-Water Reaction Phenomenon in Steam Generator of Sodium-Cooled Fast Reactor

A new computational methodology of sodium-water reaction (SWR), which occurs in a steam generator of a liquid-sodium-cooled fast reactor when a heat transfer tube in the steam generator fails, has been developed considering multidimensional and multiphysics thermal hydraulics. Two kinds of reaction models are proposed in accordance with a phase of sodium as a reactant. One is the surface reaction model in which water vapor reacts directly with liquid sodium at the interface between the liquid sodium and the water vapor. The reaction heat will lead to a vigorous evaporation of liquid sodium, resulting in a reaction of gas-phase sodium. This is designated as the gas-phase reaction model. These two models are coupled with a multidimensional, multicomponent gas, and multiphase thermal hydraulics simulation method with compressibility (named the ‘SERAPHIM’ code). Using the present methodology, a numerical investigation of the SWR under a pin-bundle configuration (a benchmark analysis of the SWAT-1R experiment) has been carried out. As a result, the maximum gas temperature of approximately 1,300_C is predicted stably, which lies within the range of previous experimental observations. It is also demonstrated that the maximum temperature of the mass weighted average in the analysis agrees reasonably well with the experimental result measured by thermocouples. The present methodology will be promising to establish a theoretical and mechanical modeling of secondary failure propagation of heat transfer tubes due to such as an overheating rupture and a wastage.

Improvement of Gas Entrainment Prediction Method—Introduction of Surface Tension Effect—

A gas entrainment (GE) prediction method has been developed to establish design criteria for the largescale sodium-cooled fast reactor (JSFR) systems. The prototype of the GE prediction method was already confirmed to give reasonable gas core lengths by simple calculation procedures. However, for simplification, the surface tension effects were neglected. In this paper, the evaluation accuracy of gas core lengths is improved by introducing the surface tension effects into the prototype GE prediction method. First, the mechanical balance between gravitational, centrifugal, and surface tension forces is considered. Then, the shape of a gas core tip is approximated by a quadratic function. Finally, using the approximated gas core shape, the authors determine the gas core length satisfying the mechanical balance. This improved GE prediction method is validated by analyzing the gas core lengths observed in simple experiments. Results show that the analytical gas core lengths calculated by the improved GE prediction method become shorter in comparison to the prototype GE prediction method, and are in good agreement with the experimental data. In addition, the experimental data under different temperature and surfactant concentration conditions are reproduced by the improved GE prediction method.

Development of unstructured mesh-based numerical method for sodium–water reaction phenomenon in steam generators of sodium-cooled fast reactors

ABSTRACT When pressurized water or vapor leaks from a failed heat transfer tube in a steam generator of sodium-cooled fast reactors, a high-velocity and high-temperature jet with sodium–water chemical reaction may cause wastage on the adjacent tubes. For safety assessment of the steam generator, a computational fluid dynamics code called SERAPHIM calculating compressible multicomponent multiphase flow with sodium–water chemical reaction has been developed. The original SERAPHIM code is based on the finite difference method. In this study, unstructured mesh-based numerical method for the SERAPHIM code was developed to advance a numerical accuracy for the complex-shaped domain including multiple heat transfer tubes. Numerical analysis of an underexpanded jet experiment was performed as part of validation of the unstructured mesh-based SERAPHIM code. The calculated pressure profile showed good agreement with the experimental data. To investigate the effect of the introduction of the unstructured mesh and to confirm applicability of the numerical method for the actual situation, water vapor discharging into liquid sodium was analyzed. The calculated behavior of the reacting jet agreed with the previous experimental knowledge. It was demonstrated that the proposed numerical method could be applicable to evaluation of the sodium–water reaction phenomenon.

Numerical Analysis of Melting/Solidification Phenomena Using a Moving Boundary Analysis Method X-FEM

Abstract A numerical analysis method for melting/solidification phenomena has been developed to evaluate feasibility of the several candidate techniques in the nuclear fuel cycle. Our method is based on the extended finite element method, which has been used for moving boundary problems. The basic idea of the extended finite element method is to incorporate the signed distance function into the standard finite element interpolation to represent a discontinuous gradient of the temperature at a moving solid-liquid interface. This technique makes it possible to simulate movement of the solid-liquid interface without the use of a moving mesh. Construction of the finite element equation from the energy equation in the case of melting/solidification problems has been discussed and is reported here. The technique of quadrature and the method to solve the governing equations for the problem involving liquid flows have also been constructed in the present work. The numerical solutions of the basic problems - a one-dimensional Stefan problem, solidification in a two-dimensional square corner, and melting of pure gallium - were compared to the exact solutions or to the experimental data. Through these verifications, validity of the newly developed numerical analysis method has been demonstrated.

Development of Unstructured Mesh-Based Numerical Method for Sodium-Water Reaction Phenomenon

Abstract When pressurized water or vapor leaks from a failed heat transfer tube in a steam generator (SG) of sodium-cooled fast reactors, a high-velocity, high-temperature jet with sodium-water chemical reaction may cause wastage on the adjacent tubes. For safety assessment of the SG, a computational fluid dynamics code SERAPHIM, in which a compressible multicomponent multiphase flow with sodium-water chemical reaction is computed, has been developed. The original SERAPHIM code is based on the finite difference method. In this study, an unstructured mesh-based numerical method was developed and introduced into the SERAPHIM code to advance a numerical accuracy for a complex-shaped domain including multiple heat transfer tubes. The multiphase flow under the tube failure accident is calculated by the multifluid model considering compressibility. The governing equations are solved by the Highly Simplified Marker And Cell (HSMAC) method. The original HSMAC method was modified for compressible multiphase flows in the unstructured mesh. Validity of the unstructured mesh-based SERAPHIM code was investigated through the analysis of an underexpanded jet experiment, which is a key phenomenon in the tube failure accident. The calculated pressure profile showed good agreement with the experimental data. Numerical analysis of water vapor discharging into liquid sodium was also performed. The calculated behavior of the reacting jet agreed with the previous experimental knowledge. It was demonstrated that the proposed numerical method could be applicable to evaluation of the sodium-water reaction phenomenon.

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.

Velocity of Entrainments Formed by High Velocity Air Jet Flow in Stagnant Water

This study was intended to examine sodium entrainment behavior in the case that a hole was formed on a tube wall in the steam generator of a fast breeder reactor and high pressure and high temperature water jetted out into sodium. Flow visualization experiments of an air jet in liquid were performed. The test vessel was 270 mm wide, 5 mm depth and 300 mm high. The air jet was blown vertically upward into stagnant liquid in the test vessel from a rectangular cross-section nozzle of 1 mm wide, 5 mm depth and 20 mm long which was located at the bottom of the test vessel. A flow state of the jet in the liquid was recorded with a high speed video camera at the fastest 150,000 frame/s. The test liquid was water and kerosene. Filament-like ears and wisps pulled out from the wavy interface were noticed on the interface between liquid and the air jet. The ears and the wisps were broken off and entrained into the air jet. The droplets broke up to small entrainments. This process seemed quite similar to the entrainment process in the annular dispersed flow in a pipe. Entrainment was initiated at a little bit downstream from the nozzle outlet. The entrainment inception point moved downstream as the air jet velocity increased. Axial directional entrainment velocity increased as the air jet velocity increased and the entrainment proceeded downstream. Transversal directional entrainment velocity was much slower than the axial directional entrainment velocity. The variation of the entrainment velocity in the transversal direction was not so prominent. The entrainments produced at the interface of the air jet moved to gather at the center portion of the air jet as those were accelerated.Copyright

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

Entrainment Into High Speed Air Jet Blowing Out From a Hole to Stagnant Water

Flow visualization experiments of an air jet in liquid were performed. The test vessel was 270 mm wide, 5 mm depth and 300 mm high. The air jet was blown vertically upward into stagnant liquid in the test vessel from a nozzle of 1 mm wide, 5 mm depth and 20 mm long which was located at the bottom of the test vessel. A flow state of the jet in the liquid was recorded with a high speed video camera at fastest 5×105 f/s. The test liquid was water and kerosene. Experiments were performed at atmospheric pressure and ambient temperature. Filament-like ears and wisps pulled out from the wavy interface were noticed on the interface between liquid and the air jet. The ears and wisps were broken off and entrained into the air jet. The droplets broke up to small entrainments. This process seemed quite similar to the entrainment process in the annular dispersed flow in a pipe. As the air jet velocity increased, the number of entrainments created by the air jet increased lineally and the smaller entrainments increased. The correlation for the entrainment diameter distribution which was developed for the annular dispersed two-phase flow in a pipe predicted well the present results. The correlations for the entrainment diameter developed for entrainments in the annular dispersed two-phase flow in a pipe and for droplets that were blown out into open space above a water pool by a nitrogen gas jet that blew into water vertically upwards considerably underpredicted the experimental results. Measured entrainment rates were considerably lower than the prediction of the correlation for the annular dispersed two-phase flow in a pipe.Copyright

Visualization of Entrainment and Surface Behavior of High Speed Air Jet Blowing Out From a Hole to Stagnant Water

A two dimensional air jet was blown out from a nozzle into water in a thin vessel. The behavior of the interface between water and the air jet and also the air jet were recorded with a high speed video camera. Filament-like ears and wisps pulled-out from the wavy water surface were noticed in the recorded photos. Droplets are formed from these. Droplet diameters were obtained from the recorded photos. As the air velocity increased, the number of droplets created by the air jet increased lineally and the smaller droplets increased. The correlation for the droplet diameter distribution developed for the annular dispersed two-phase flow in a pipe predicted well the present results. The correlations for the droplet diameter developed for the annular dispersed two-phase flow in a pipe and for the jet blowing out from the stagnant water pool considerably underpredict the experimental results.Copyright