Kazuya Shibata

Effect of Nuclear Fuel Particle Movement on Nuclear Criticality in a Rotating Cylindrical Vessel

In conventional criticality safety evaluations of nuclear facilities, nuclear fuel particle behavior could not be considered because any appropriate computational tools did not be developed. A new criticality evaluation tool, which can consider the particle behavior, is developed by coupling a Discrete Element Method with a continuous energy Monte Carlo type code. Criticality evaluations considering the nuclear particle movement in a rotating cylindrical tank are performed by applying this tool. Effects of the powder-filling ratio and the rotating speed on nuclear criticality are investigated in the present study. The nuclear powder free surface is fluctuated more remarkably as the filling ratio and the rotating speed are higher. The fluctuation makes the effective multiplication factor lower because of neutron leakage from the system by the increase of the superficial area and reduction of the atomic number densities due to the flowability.

Development of a criticality evaluation method involving the granular flow of the nuclear fuel in a rotating drum

Abstract Particle dynamics of nuclear fuel material has not been considered in conventional nuclear criticality evaluations. However, the particle motion influences nuclear criticality significantly. In the present study, the criticality calculation is combined with the discrete element method (DEM) to investigate the effects of the particle macroscopic behavior on nuclear criticality. Particle motion is analyzed in a rotating drum by the DEM, and then, the nuclear calculation is carried out. This paper focuses on particle size distribution, size segregation, and change of surface area of the particle bed. The particle size distribution has an important influence on the nuclear criticality evaluation because it affects not only the particle movement but also the atomic number densities in the bed. The surface area of the particle bed shows a close correlation with the multiplication factor. On the other hand, the size segregation does not have a significant effect on nuclear criticality.

A Three-Dimensional Numerical Analysis Code for Shipping Water on Deck Using a Particle Method

A three-dimensional numerical analysis code for shipping water on deck was developed using MPS (Moving Particle Semi-implicit) method [1–2]. MPS is a particle method for incompressible flows. A ship motion model was developed using the particle method. Shipping water was analyzed using this code. The fluid behavior, the waveform of pressure, and the time integration of pressure on an imitated deck were in agreement with experiment. A visualization system for calculation result was also developed.Copyright

Improvement of boundary conditions for non-planar boundaries represented by polygons with an initial particle arrangement technique

ABSTRACT The boundary conditions represented by polygons in moving particle semi-implicit (MPS) method (Koshizuka and Oka, Nuclear Science and Engineering, 1996) have been widely used in the industry simulations since it can simply simulate complex geometry with high efficiency. However, the inaccurate particle number density near non-planar wall boundaries dramatically affects the accuracy of simulations. In this paper, we propose an initial boundary particle arrangement technique coupled with the wall weight function method (Zhang et al. Transaction of JSCES, 2015) to improve the particle number density near slopes and curved surfaces with boundary conditions represented by polygons in three dimensions. Two uniform grids are utilized in the proposed technique. The grid points in the first uniform grid are used to construct boundary particles, and the second uniform grid stores the same information as in the work by Zhang et al. The wall weight functions of the grid points in the second uniform grid are calculated by newly constructed boundary particles. The wall weight functions of the fluid particles are interpolated from the values stored on the grid points in the second uniform grid. Because boundary particles are located on the polygons, complex geometries can be accurately represented. The proposed method can dramatically improve the particle number density and maintain the high efficiency. The performance of the previously proposed wall weight function (Zhang et al.) with the boundary particle arrangement technique is verified in comparison with the wall weight function without boundary particle arrangement by investigating two example geometries. The simulations of a water tank with a wedge and a complex geometry show the general applicability of the boundary particle arrangement technique to complex geometries and demonstrate its improvement of the wall weight function near the slopes and curved surfaces.

Numerical analysis of fluid-structure and fluid-rigid body interactions using a particle method

An algorithm is presented for fluid-structure and fluid-rigid body interactions using a particle method. The algorithm is based on weak coupling, where fluid and solid analyses are explicitly connected in each time step. Fluid dynamics is solved by the Moving Particle Semi-implicit (MPS) method proposed by Koshizuka and Oka in 1996. Elastic solid dynamics is solved by the MPS method proposed by Song et al. in 2000. Rigid body motion is calculated by the particle method proposed by Tanaka et al. in 2007. The external force from the fluid to the rigid bodies is calculated by either volume integral or surface integral. When the volume integral is employed, both fluid and rigid bodies are calculated as fluid at first in each time step and then the rigid body dynamics is solved and the shapes are reconstructed. For fluid-elastic solid coupling, surface integral is necessary. Calculation examples using the present algorithms are shown.Copyright

Verification and validation of explicit moving particle simulation method for application to internal flooding analysis in nuclear reactor building

ABSTRACT The Fukushima nuclear accident raised the importance of flooding study in nuclear reactor buildings. It is known that the external flooding can be attributed to natural causes, while the internal flooding is caused by the piping ruptures, tank failure or the actuation of fire suppression systems. The building flooding process can damage the safety-related components and systems, which needs to study carefully. In order to simulate this phenomenon which is accompanied by complex flow with free surface, a particle method based on Lagrangian approach named explicit moving particle simulation (EMPS) method, is employed in this analysis. Verification and validation analyses are carried out. The verification problems are a hydrostatic analysis and a water spreading to investigate the differences of the particle wall and polygon wall boundary models, while the validation studies of two experiments of dam-break induced flooding show good agreements. Afterwards, the internal flooding process in AP1000 is simulated by assuming a break of the in-containment refueling water storage tank as an example. The results achieved so far indicate that the EMPS method is capable to simulate the internal flooding process in the nuclear reactor buildings.

Numerical Study on Coarse Grain Modeling of the DEM for Industrial Scale Gas-Solid Flow Systems

The Discrete Element Method (DEM) is widely used in various numerical simulations related to granular media. The DEM is a Lagrangian approach where individual particle is calculated based on the Newton’s second law of motion. Therefore, the DEM enables us to investigate the granular flow characteristics at the particle level. On the other side, the DEM has a difficulty to be applied in large-scale powder systems because the calculation cost becomes too expensive when the number of particles is huge. To solve this issue, we have developed a coarse grain modeling as a large scale model of the DEM. The coarse grain particle represents a group of original particles. The coarse grain model was used in typical gas-solid and solid-liquid two phase flows so far, where the particle size was relatively large, namely, cohesive force did not act between the solid particles. In the present study, the coarse grain model is evolved to simulate fine particles by considering the interparticle van der Waals force. The adequacy of the coarse grain model is proved by comparing the simulation results of original particle system. Through this study, the coarse grain model is shown to simulate the cohesive particle behavior precisely.Copyright

Numerical Simulation of Jet Breakup Using Particle Method

Numerical analysis of jet breakup is carried out using Moving Particle Semi-implicit (MPS) method in x-y two dimensions. In the MPS method, particle interaction models are prepared for differential operators and the governing equations are discretized without grids. Surface tension is also modeled as particle interactions. Effects of the Weber number and the Froude number on the jet breakup length agree well with experimental data.Copyright

Numerical Analysis of Droplet Size Distribution Using Particle Method

A continuous jet changes to droplets where jet breakup occurs. In this study, two-dimensional numerical analysis of jet breakup is performed using MPS method (Moving Particle Semi-implicit Method) which is a particle method for incompressible flows. The continuous fluid surrounding the jet is neglected. The size distribution of droplets is in agreement with the Nukiyama-Tanasawa distribution which has been widely used as an experimental correlation. Effects of the Weber number and the Froude number on the size distribution are also obtained from the calculation.Copyright