Development of Numerical Simulation for Jet Break Up Behavior in Complicated Structure of BWR Lower Plenum (7) Measurement of Fragment Diameter by Image Processing Technique

In order to decommission nuclear reactors and to improve the safety of BWR, it is important to estimate the falling behavior of molten core jet in the reactor vessel of BWR when an accident occurred as can be seen from Fukushima Daiichi nuclear power plant accident. Since the BWR lower plenum is consisted with various complicated structures, it is suggested that the jet falling behavior is affected by these structures. Thus we are developing the numerical simulation method to estimate the molten core falling behavior in BWR. To verify the code for the case of the BWR core melt accident, it is necessary to obtain the experimental data and validate the code by comparing the numerical results with the experimental results. The purpose of this study is to investigate the influence of these structures on behavior of jet breakup and fragmentation, and to construct the benchmarks of the numerical simulation experimentally. We used molten core simulant material and simulate the molten core falling behavior, focusing on the hydrodynamic behavior. The 1/10 planar type test section simulated the arrangement of complicated structures in the BWR lower plenum is used. Jet injection experiments were conducted under some conditions that experimental parameters were flow rate and nozzle diameter. To clarify the influence of complicated structures on the jet behavior, experiments were performed in the conditions with and without structures. Jet falling behaviors were recorded by a high speed video camera. The fragment diameters were measured from image by means of image processing techniques. Visual measurement is usually used to measure fragment diameter, but it will contain the arbitrariness and the amount of fragments are small. Since the outline of fragment is easy to recognize by the difference of refractive index between gas and liquid, image processing for measuring the diameters is used in gas liquid flow. On the other hand, it is difficult to recognize the interface in liquid-liquid flow. We developed the new image processing filter for detecting the outline of fragments precisely and established the image processing method including this filter. We measured about ten thousand fragments precisely and automatically. The measurement of fragment diameter was implemented by the image processing method mentioned above. The histogram of fragment diameter distribution shows that it can be fitted by the lognormal distribution in condition with and without structures. We calculated the volume median diameters in all conditions. The diameters were smaller that depended on the increasing injection velocity. Comparing between condition with and without structures, the fragment diameters became small in condition with structures than without structures. Since the velocity of tip of the jet was larger in condition with structures (Saito et al., J. Nucl. Sci. Tech, 2015), the velocity gradient between the jet and ambient fluid also would be larger. The shear force strongly acting on the interface made the diameter small.

Critical Heat Flux Enhancement in Water-Based Nanofluid With Honeycomb Porous Plate on Large Heated Surface

Heat removal through pool boiling is limited by the phenomena of critical heat flux (CHF). CHF enhancement is vitally important in order to satisfy demand for the cooling technology with high heat flux in In Vessel Retention (IVR). Various surface modifications of the boiling surface, e.g., integrated surface structures and coating of a micro-porous have been proven to effectively enhance the CHF in saturated pool boiling. We have been proposed a novel method of attaching a honeycomb structured porous plate on a considerably large heater surface. Previous results, by the authors in [1] reported that CHF has been enhanced experimentally up to more than approximately twice that of a plain surface (approximately 2.0 to 2.5 MW/m2) with a diameter of 30 mm heated surface. However, it is necessary to demonstrate the method together with infinite heater surface within laboratory scale. It is important that cooling techniques for IVR could be applicable to a large heated surface as well as remove high heat flux. Objective of this study is to investigate the CHF of honeycomb porous plate and metal solid structure in nanofluid boiling or water boiling on a large heated surface. Water-based nanofluid offers good wettability and capillarity. While metal solid structure is installed on honeycomb porous plate to increase the number of vapor jet. Experimental results of honeycomb porous plate and combination of honeycomb porous plate and metal solid structure in water-based nanofluid boiling shows that CHF is increased up to twice [2] and thrice, respectively compared to plain surface in water boiling. To the best of the author’s knowledge, the highest value (3.1 MW/m2) was obtained for a large heated surface having a diameter of 50 mm which is regarded as infinite heated surface. This demonstrates potential for general applicability to have more safety margin in IVR method.

Utilization of Thorium in LWR Fuels Aiming at Thermal Conductivity Improvements

One of the main drawbacks of uranium dioxide, which is used in almost all nuclear power reactors, is its low thermal conductivity. As a consequence, temperature at the center of fuel pellet is relatively high, because heat is poorly conducted away. To reach a higher level of safety, maximal temperature in any fuel pellet is one of the main limiting parameters, which restrict the fuel thermal output. This paper deals with the use of thorium in LWR fuels with the objective of fuel pellet maximal temperature reduction. Research investigating homogenous distribution of thorium dioxide (thoria) in uranium dioxide fuel has already been done and did not lead to considerable thermal conductivity improvements. The aim of this study is to investigate heterogeneous distribution of thorium in commonly used uranium dioxide fuel in the form of uranium and thorium pellets placed together.

Experimental Study on Splashing During Liquid Jet Impingement Onto a Liquid Film

A liquid jet is of considerable importance in many industrial fields including jet cleaning, jet engine and combustion. As an important example, the Monju nuclear power plant in Japan experienced a sodium leak in 1995. This led to a fire accident because the sodium reacted with oxygen in the air. To predict the significance of the fire accident, accurate evaluation of the amount of splashed droplets caused by the sodium jet impingement is of great importance. In this work, the relationship between the condition of a liquid jet and the amount of splashed droplets is explored experimentally. In the experiments, a liquid jet was emanated vertically downward from a circular nozzle onto a liquid film formed on a horizontal plate. Visualization using a high speed camera was performed to observe the condition of the liquid jet. From the nozzle, the mode of the liquid jet changed jet, lump and drop. Here, the jet mode means the continuous jet with smooth surface, the lump mode the continuous jet with disturbed surface and the drop mode the broken jet. Dependences of the transition length to each mode on the important parameters such as the jet velocity and the nozzle diameter were investigated. Measurement was also conducted for the splash ratio that is defined as the ratio of the amount of splashed droplets to the jet flow rate. It was found that the splash ratio is high when the liquid jet is in the drop mode at the impact point. It was shown that the splash ratio can be correlated well as a function of the impact Weber number and the Strouhal number of the droplets impinging the liquid film.

Study on Stability of Liquid Jet for Liquid Lithium Target of Boron Neutron Capture Therapy (BNCT)

For the development of the liquid lithium (Li) jet target system for the boron neutron capture therapy (BNCT), the stability of the Li jet is important. The purpose of the present study is to investigate the characteristics of the disturbances and the droplet formation of a water jet flow at high velocity. The experimental studies for a water jet flow were performed to simulate the liquid Li jet flow. The nozzles in the experiment had a rectangular flow channel with the gap of 0.5 mm and the length of 10 mm and 70 mm. The water flow velocities in these nozzles were 5 m/s, 10 m/s or 15 m/s. The stability of the water jet flow was investigated by the observation of the surface disturbance using a high speed video camera. The formation of water droplet from the water jet flow was detected, and the characteristics of the droplet formation were analyzed using Phase Doppler Anemometry (PDA). Then, the surface disturbance of the jet flow was characterized by the characteristics of the droplet generation. In the experiment with 10mm length of nozzle, a lot of droplets are generated from the jet surface and the surface is considered to be unstable. On the other hand the smooth jet surface and the stable jet are made near nozzle outlet. In the experiment with 70mm length of nozzle, few droplets are generated from the surface of the jet and the surface of the jet is very smooth especially for the place near nozzle outlet. It was concluded that the droplet generation from the surface of the water jet was promoted by the distortion of the jet surface. Also the surfaces of the jet flow made by the nozzle having the length of 70 mm was smoother than those in the tests with the nozzle having the length of 10 mm. The large eddy in the flow must be dumped before the nozzle outlet because the turbulence was fully developed in the nozzle.

Evaluation of Thermodynamic Properties of Cerium in Liquid Cadmium Cathode Within LiCl-KCl Salt System

Thermodynamic properties of Ce-Cd intermetallic compound were investigated in LiCl-KCl-CeCl3-CdCl2 molten salt system at various temperatures. Six Ce-Cd intermetallic compounds, CeCd, CeCd2, CeCd3, Ce13Cd58, CeCd6, and CeCd11 could be observed via cyclic voltammetry (CV) and the relative Gibbs free energies for the intermetallic formations were estimated by the analysis of the anodic peaks in the CV experiments. Furthermore, an open circuit chronopotentiometry which is known as a suitable method for measuring thermodynamic values of intermetallic compounds were performed. The linear trends of the relative Gibbs free energies were found by using potential difference from CV and CP methods. It can be noted that CV method can be easy and fast tool to estimate relative Gibbs energy for intermetallic compounds of Cd-Ms (metals). For the determination of standard Gibbs free energy, enthalpy, and entropy for the intermetallic formation, CP results were used. The linear trend of the Gibbs energy were obtained against temperature changing from 698 K to 823 K. From the linear relationship, the enthalpy and entropy of the formation were calculated.

Implementation of Fault Tolerant Control Scheme for Point Kinetic Reactor Model With Temperature Feedback in Matlab

As a highly safety-critical system, it is insufficient for the Nuclear Power Plant (NPP) Instrumentation and Control (I&C) system to simply rely on a conventional control schemes or controllers which only satisfy stability and performance specification to the perturbation of the nominal plant. Since the current operating or newly-built I&C systems are based on transferring or adapting modern high performance electronic devices, it provides the hardware foundation and possibility to incorporate more advanced control systems into nuclear systems to achieve higher safety and stable performance, even in unexpected faulty situations. Active Fault Tolerant Control (FTC) is one of the choices for such advanced control. Active FTC encompasses the following components: 1. nominal controller design, 2. sensors and actuators fault detection and isolation, and 3. fault estimation and fault accommodation. In this research, approaches for each component are integrated into an active FTC scheme. Following this, the active FTC scheme is applied to a point kinetic rector model with fuel temperature and coolant temperature effect to reactivity. Simulation results show that the active FTC scheme designed in this research can effectively track the global power set point, even under situations with single fault from actuator or sensors.

Investigation of the Effects of Hydrogen Atoms Concentration on the Tungsten Sigma 5 (310) Symmetric Tilt Grain Boundary

The objective of this work is to investigate the effects of the concentration of hydrogen atoms at the tungsten Σ5 (310) grain boundary (GB) on the GB energy and the GB pulling force. Tungsten (W) is the most preferred plasma facing material (PFM) for the future nuclear fusion reactors such as in the proposed DEMO (demonstration power plant) and ITER (international thermonuclear experimental reactor — which is at the moment the largest Tokamak nuclear fusion reactor under construction in the world. Tungsten is considered as a PFM because of its excellent thermal properties, low-sputtering yield and high melting point. However, hydrogen (H) atoms have an affinity for tungsten grain boundary, and are trapped there permanently. In addition, it makes it prone to failure. W will be exposed to extremely high fluences of H isotopes. The low-energy H isotopes will be retained in the tungsten material leading to the formation of blisters in W and causes degradation of the mechanical and thermal properties of W. Therefore for safety reason, the effect of hydrogen atoms at the tungsten Σ5 (310) symmetric tilt-GB needs to be investigated. This will greatly aid in better designing of fusion wall materials. In addition, the full understanding of the tungsten grain boundary energy and the force required to pull the grain boundary apart will also help in better material selection. Classic molecular dynamics method was used for the investigation. LAMMPS-a sophisticated, classical atomic and molecular dynamics modelling and simulation software, is a vital tool adopted for the investigation of the effects of hydrogen atoms in tungsten Σ5 (310) symmetric tilt-GB. Tersoff potential for W-H interactions was used for the modelling. The size of the simulation box is 10 × 100 × 10 lattices and it consists of 21262 W atoms. Periodic boundaries were used for all sides of the system. Conjugate gradient method was used for the minimization. The trajectories of the atoms were visualized using visual molecular dynamics (VMD) software. The GB energies are calculated to be −924.060 J/m2, −923.898 J/m2 and −743.414 J/m2 for pure W, W with one H atom and W with 30 H atoms are at the GB respectively. In addition, the forces required to separate the GB apart are 0.0279551eV/Ang for pure W, 0.024789eV/Ang and 0.0185eV/Ang for W with one H and 30 H atoms at the GB respectively. The result shows that hydrogen acts as a grain boundary embrittler and weakens the GB strength. The GB energy reduces as the concentration of hydrogen at the tungsten GB increases. In addition, the more the hydrogen atoms at the tungsten GB the lesser the value of the force required to pull the GB apart. However, the GB energies and pulling forces starts increasing slowly when H atoms exceed a certain number depending on the H atoms distribution around the W GB.

The Fast Three-Dimensional Space-Time Neutron Kinetic Model for Cartesian Geometry and Cylindrical Geometry

In order to raise computation speed on the premise of enough numerical accuracy, the Predictor-Corrector Improved Quasi-Static (PC-IQS) method and Nodal Green’s Function Method (NGFM) were combined to solve the three-dimensional space-time neutron diffusion kinetics problems for Cartesian geometry. In addition, the improved quasi-static method and the Krylov algorithm were applied to solve the three-dimensional space-time neutron diffusion kinetics problems for cylindrical geometry. Based on the proposed model, the program of three-dimensional neutron space-time kinetic code has been tested by the two-dimensional and three-dimensional transient numerical benchmarks. The numerical results obtained by this work were in good agreement with the reference solutions.

Experimental Investigation of Subcooled Choking Flow in a Steam Generator Tube Crack

Steam generator tubes have a history of small cracks and even ruptures, which lead to a loss of coolant from the primary side to the secondary side. These tubes have an important role in reactor safety since they serve as one of the barriers between radioactive and non-radioactive materials of a nuclear power plant. A rupture then signifies the loss of the integrity of the tube itself. Therefore, choking flow plays an integral part not only in the engineered safeguards of a nuclear power plant, but also to everyday operation. There is limited data on actual steam generators tube wall cracks. Here experiments were conducted on choked flow of subcooled water through two samples of axial cracks of steam generator tubes taken from US PWR steam generators. The purpose of the experimental program was to develop database on critical flow through actual steam generator tube cracks with subcooled liquid flow at the entrance. The knowledge of this maximum flow rate through a crack in the steam generator tubes of a pressurized water nuclear reactor will allow designers to calculate leak rates and design inventory levels accordingly while limiting losses during loss of coolant accidents. The test facility design is modular so that various steam generator tube cracks can be studied. Two sets of PWR steam generators tubes were studied whose wall thickness is 1.285 mm. Tests were carried out at stagnation pressure up to 6.89 MPa and range of subcoolings 16.2–59°C. Based on these new choking flow data, the applicability of analytical models to highlight the importance of non-equilibrium effects was examined.