Yuichi Yamamoto

Analytical study on effects of BWR fuel spacer on droplet deposition

In a boiling water nuclear reactor (BWR), liquid film dryout may occur on a fuel rod surface when the fuel assembly power exceeds the critical power. The spacers supporting fuel rods affect on the thermal-hydraulic performance of the fuel assembly. The spacer is designed to enhance critical power significantly. If spacer effects for two-phase flow could be estimated analytically, the cost and time for the development of the advanced BWR fuel would be certainly decreased. The final goal of this study is to be able to analytically predict the critical power of a new BWR fuel assembly without any thermal-hydraulic tests. Initially, we developed the finite element code to estimate spacer effects on the droplet deposition. Then, using the developed code, the spacer effects were estimated for various spacer geometries in a plane channel and one subchannel of BWR fuel bundle. The estimated results of the spacer effects showed a possibility to analytically predict the critical power of a BWR fuel assembly.

Analysis of Flows around a BWR Spacer by the Two-Fluid Particle Interaction Method

The Moving Particle Semi-implicit (MPS) method, a particle interaction method developed in recent years, is formulated by representing the differential operators in Navier-Stokes equations as the interaction between particles characterized with a kernel function and adopts a mesh-free algorithm. The MPS method is particularly suitable for treating liquid breakup. We extended the MPS method to a two-fluid system, introduced a potential-type surface tension, and developed a kernel function for the interface between liquid and gas to simulate two-phase flows. This extended MPS method, which we call Two-Fluid MPS (TF-MPS) method, has been verified through a number of analyses of two-phase flow experiments. The objectives of this study are to verify the applicability of TF-MPS method to a flow around a BWR spacer and to make up constitutive correlations for macroscopic methods. In this paper, we describe the formulation and the calculation algorithm of TF-MPS method, and present the results of the verification studies.

Analysis of jet flows with the two-fluid particle interaction method

The particle interaction method called MPS (Moving Particle Semi-implicit) method has been developed in recent years, which is formulated by representing the differential operators in Navier-Stokes equation as the interaction between particles characterized with a kernel function and adopts a mesh-free algorithm. This method is suitable especially for treating liquid breakup. We extended the MPS method to two-fluid system, introduced a potential-type surface tension, and modified the calculation algorithm to simulate jet flows. The objective of this study is to evaluate the interfacial area (or, so called binary contact area) of immiscible two-fluid systems with a chemical reaction, where one is injected as a jet into a pool of the other fluid. As a first step, we investigated if the proposed method is capable of reproducing the hydrodynamics of jet flow by analyzing Tanasawas experiment. In this paper, we describe the formulation and the calculation algorithm of the method, and results of the verification studies.

Next Generation Safety Analysis Methods for SFRs—(8) Analyses of Eutectics Between Fuel and Steel in Metal Fuel With FPMD Code VASP

There are two main objectives in this study. One is to estimate atomic diffusion coefficients in eutectic reaction between metal fuel and cladding materials in order to establish the atomic diffusion model for the COMPASS code. The other is to estimate their material properties such as Young’s modulus in high temperature up to near melting points in core disruptive accidents (CDAs) in Sodium-cooled Fast Reactors (SFRs). We used the first principle molecular dynamics (FPMD) code VASP to realize the two objectives. We tried to understand the initiation mechanism of eutectics based on change of electronic state energy accompanied by change of Kohn-Sham energy, including phonon effect. In this project [1], three methods, phase diagram calculation (CALPHAD), classical molecular dynamics (CMD), and FPMD, are employed to understand the mechanism of eutectics and to introduce dynamic characteristics in eutectic phenomena into the COMPASS code.Copyright

Next Generation Safety Analysis Methods for SFRs—(4) Development of a Computational Framework on Fluid-Solid Mixture Flow Simulations for the COMPASS Code

The COMPASS code is designed based on the moving particle semi-implicit (MPS) method to simulate various complex mesoscale phenomena relevant to core disruptive accidents (CDAs) of sodium-cooled fast reactors (SFRs). The MPS method, which is a fully Lagrangian method, can be extended for fluid-solid mixture flow simulations in a straightforward approach. In this study, a computational framework for fluid-solid mixture flow simulations was developed for the COMPASS code. In the present framework, the passively moving solid (PMS) model, which is originally proposed to describe the motion of a rigid body in a fluid, used to simulate hydrodynamic interactions between fluid and solids. In addition, mechanical interactions between solids were modeled by the distinct element method (DEM). Since the typical time step size in DEM calculation, which uses an explicit time integration scheme, is much smaller than that in MPS calculation, a multi-time-step algorithm was introduced to couple these two calculations. In order to verify the proposed computational framework for fluid-solid mixture flow simulations, a series of experiments of water-dam break with multiple solid rods was simulated using the COMPASS code. It was found that simulations considering only fluid-solid interactions using the PMS model can not reasonably represent typical behaviors of solid rods observed in the experiments. However, results of simulations taking account of solid-solid interactions using DEM as well as fluid-solid ones were in good agreement with experimental observations. It was demonstrated that the present computational framework enhances the capability of the COMPASS code for mesoscale simulations of fluid-solid mixture flow phenomena relevant to CDAs of SFRs. To improve the computational efficiency for fluid-solid mixture flow simulations, it will be necessary to optimize the time step size used in DEM calculations by adjusting DEM parameters based on additional experiments and numerical tests.Copyright

Next Generation Safety Analysis Methods for SFRs—(5) Structural Mechanics Models of COMPASS Code and Verification Analyses

A five-year research project started in FY2005 (Japanese Fiscal Year, hereafter) to develop a code based on the Moving Particle Semi-implicit (MPS) method for detailed analysis of core disruptive accidents (CDAs) in sodium-cooled fast reactors (SFRs). The code is named COMPASS (Computer Code with Moving Particle Semi-implicit for Reactor Safety Analysis). CDAs have been almost exclusively analyzed with SIMMER-III [2], which is a two-dimensional multi-component multi-phase Eulerian fluid-dynamics code, coupled with fuel pin model and neutronics model. The COMPASS has been developed to play a role complementary to SIMMER-III in temporal and spatial scale viewpoint; COMPASS for mesoscopic using a small window cut off from SIMMER-III for macroscopic. We presented the project’s outline and the verification analyses of elastic structural mechanics module of the COMPASS in ICONE16 [1]. The COMPASS solves physical phenomena in CDAs coupling fluid dynamics and structural dynamics with phase changes, that is vaporization/condensation and melting/ freezing. The phase changes are based on nonequilibrium heat transfer-limited model and all “phase change paths” considered in SIMMER-III are implemented [20]. In FY2007, the elastoplastic model including thermal expansion and fracture are formulated in terms of MPS method and implemented in the COMPASS, where the model adopts the von Mises type yield condition and the maximum principal stress as fracture condition. To cope with large computing time, “stiffness reduction approximation” was developed and successfully implemented in the COMPASS besides parallelization effort. Verification problems are set to be suitable for analyses of SCARABEE tests, EAGLE tests and hypothetical CDAs in real plants so that they are suggesting issues to be solved by improving the models and calculation algorithms. The main objective of SCARABEE-N in-pile tests was to study the consequences of a hypothetical total instantaneous blockage (TIB) at the entrance of a liquid-metal reactor subassembly at full power [21]. The main objectives of the EAGLE program consisting of in-pile tests using IGR (Impulse Graphite Reactor) and out-of-pile tests at NNC/RK are; 1) to demonstrate effectiveness of special design concepts to eliminate the re-criticality issue, and 2) to acquire basic information on early-phase relocation of molten-core materials toward cold regions surrounding the core, which would be applicable to various core design concepts [22, 23]. In this paper, the formulations and the results of functional verification of elastoplastic models in CDA conditions will be presented.Copyright

Next generation safety analysis methods for SFRS (3) thermal hydraulics models of Compass code and experimental analyses

A computer code, named COMPASS, is being developed employing the Moving Particle Semi-implicit (MPS) method for various complex phenomena of core disruptive accidents (CDAs) in the sodium-cooled fast reactors (SFRs). The COMPASS is designed to analyze multi-physics problems involving thermal hydraulics, structure and phase change, in a unified framework of MPS method. In FYs2006 and 2007 (Japanese Fiscal Year, hereafter), the development of basic functions of COMPASS was completed and fundamental verification calculations were carried out. In FY2007, the integrated verification program using available experimental data for key phenomena in CDAs was also started. In this paper, we show the basic verification calculations for the phase change model of COMPASS and the results of experimental analyses, together with the outline of the formulation of MPS method and the conceptual design of the COMPASS code.© 2009 ASME

R&D of the Next Generation Safety Analysis Methods for Fast Reactors With New Computational Science and Technology: 6 — Study of Eutectic Reaction Between Metals: FPMD Approach

A five-year research project started in FY2005 to develop a code based on the MPS (Moving Particle Semi-implicit) method for detailed analysis of core disruptive accidents (CDAs) in sodium-cooled fast reactors (SFRs). The code is named COMPASS (Computer Code with MPS for Reactor Safety Analysis) [1]. In this project, both mixed-oxide (MOX) and metal fuels are considered as a fuel material component. One of the main features of the project is to investigate eutectic reactions between the metal fuel and the cladding/structure materials with phase diagram calculation, classical and first principle molecular dynamics (CMD and FPMD) methods. This paper presents outcomes from study with FPMD.Copyright

Detailed analyses of specific phenomena in core disruptive accidents of sodium-cooled fast reactors by the compass code

A five-year research project has been initiated in 2005 to develop a code based on the MPS (Moving Particle Semi-implicit) method for detailed analysis of specific phenomena in core disruptive accidents (CDAs) of sodium-cooled fast reactors (SFRs). The code is named COMPASS (Computer Code with Moving Particle Semi-implicit for Reactor Safety Analysis). The specific phenomena include 1) fuel pin failure and disruption, 2) molten pool boiling, 3) melt freezing and blockage formation, 4) duct wall failure, 5) low-energy disruptive core motion, 6) debris-bed coolability, and 7) metal-fuel pin failure. Validation study of COMPASS is progressing for these key phenomena. In this paper, recent COMPASS results of detailed analyses for the several specific phenomena are summarized. Simulations of GEYSER and THEFIS experiments were performed for dispersion and freezing behaviors of molten materials in narrow flow channels. In particular, the latter experiment using melt-solid mixture is also related to fundamental behavior of low energy disruptive core. CABRI-TPA2 experiment was simulated for boiling behavior of molten core pool. Expected mechanism of heat transfer between molten fuel and steel mixture was reproduced by the simulation. Analyses of structural dynamics using elastoplastic mechanics and failure criteria were performed for SCARABEE BE+3 and CABRI E7 experiments. These two analyses are especially focused on thermal and mechanical failure of steel duct wall and fuel pin, respectively. The present results demonstrate COMPASS will be useful to understand and clarify the specific phenomena of CDAs in SFRs in details.Copyright

Next Generation Safety Analysis Methods for SFRs—(6) SCARABEE BE+3 Analysis With SIMMER-III and COMPASS Codes Featuring Duct-Wall Failure

Governing key phenomena in core disruptive accidents (CDAs) in sodium-cooled fast reactors (SFRs) are supposed to be (1) fuel pin failure and disruption, (2) molten pool boiling, (3) melt freezing and blockage formation, (4) duct wall failure, (5) low-energy disruptive core motion, (6) debris-bed coolability, and (7) metal-fuel pin failure with eutectics between fuel and steel [1]. Although the systematic assessment program for SIMMER-III [4–7] has provided a technological basis that SIMMER-III is practically applicable to integral reactor safety analyses, further model development and validation efforts should be made to make future reactor calculations more reliable and rational. For mechanistic model development, a mesoscopic approach with the COMPASS code [1, 2, 3] is expected to advance the understanding of these key phenomena during event progression in CDAs. The COMPASS code has been developed since FY2005 (Japanese Fiscal Year, hereafter) to play a complementary role to SIMMER-III. In this paper, the overall analysis of SCARABEE-BE+3 test with the SIMMER-III and those with COMPASS, focusing the duct wall failure in a small temporal and spatial window cut from the SIMMER-III analysis results of the test, are described.Copyright