Noriyuki Shirakawa

Development of severe accident analysis code SAMPSON in IMPACT project

IMPACT is the name of a program and of specific simulation software, which will perform full-scope and detailed calculations of various phenomena in a nuclear power plant for a wide range of event scenarios. The four years of the IMPACT project Phase 1 have been completed, and each analysis module of the prototype version of the severe accident analysis code SAMPSON has been developed and verified by comparison with separate-effect test data. Verification of the integrated code with combinations of up to 11 analysis modules has been conducted, with the Analysis Control Module, to demonstrate the code capability and integrity. A 10-inch cold leg failure Loss of Coolant Accident in the Surry Plant was the assumed initiating event. The system analysis was divided into two cases; one was an in-vessel retention analysis when gap cooling was effective, the other was an analysis of phenomena when the event was extended to ex-vessel due to the reactor pressure vessel failure when gap cooling was not sufficient. U...

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.

Development of Molten Core Relocation Analysis module MCRA in the severe accident analysis code SAMPSON

IMPACT is a complex software system under development at the Nuclear Power Engineering Corporation, that includes the severe accident analysis code (SAMPSON). SAMPSON is an integration of twelve modules and will be capable of simulating hypothesized severe accidents in a nuclear power plant in the final phase of the IMPACT project. As one of these modules, the Molten Core Relocation Analysis (MCRA) module simulates the relocation behavior of a molten core during a severe accident. MCRA adopts a multi-phase, multi- component, multi-velocity field model to simulate severe accident phenomena mechanistically. Herein, we describe flow regimes, interfacial area modeling and physics models such as the momentum exchange and phase change models. Two separate effect analyses are presented as verification of MCRAs models following the model descriptions. First, the fluid dynamics of the multi-velocity field model was verified in the calculation of nitrogen gas bubbling through water in Leungs experiment. Second, data of the JRC-Ispra KROTOS-37 experiment were adopted for verification of the multi-velocity field model and vaporization/condensation model. Calculation results of both analyses compared well with the experimental data. Finally, MCRAs integrated function involving the melting/freezing model was demonstrated by calculating two-phase flow with a fuel rod and molten fuel.

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.

The effect of bubble size on the radial distribution of void fraction in two-phase flow in a circular tube

SIMMER-IV, a three-dimensional version of SIMMER-III, has been developed at JNC to study the safety of fast reactors mechanistically. SIMMER-IV was applied to the experimental analysis of bubbly flow to verify the models. The results of an experiment performed by Valukina et al. showed that the radial distribution of void fraction changes suddenly as the bubble size changes. In the present work, the analysis focuses on reproducing the change of the radial void distribution. The diffusion term, “lateral lift force,” and “wall force” were introduced in sequence into its momentum conservation equation. Naturally, the diffusion term improved the radial distribution of vertical velocity but did not explain the change of the radial void distribution in the experiment. Subsequent introduction of the lateral lift force increased the void fraction along the wall, but it did not change the radial void distribution as the bubble size changed. Finally, the wall force was introduced and, in accordance with the experimental results, the dependence of radial void distribution on the bubble size was proved. The behavior of a molten pool that is bubbly with steel steam plays an important role in the evaluation of LMFR severe accidents. This study is expected to improve the evaluation.

Hydrogen Bond and Crystal Deformation of Cellulose in Sub/Super-critical Water

The characteristics of a hydrogen bond and the stability of Iβ-phase crystalline cellulose in sub/super-critical water (up to 750 K) were investigated by molecular dynamics (MD) simulations with the GROMOS87 force field and the flexible SPC model. The population of hydrogen bonds between cellulose chains decreases as the temperature or the water density increases. The increase of the temperature also decreases the lifetime of the hydrogen bond between cellulose and water and between celluloses, where the lifetimes are shorter than that between water molecules. The fluctuation of the crystalline structure was observed and it degraded with chains leaving due to the breaking of hydrogen bonds. A model estimating the break-period of a chain link by hydrogen bonds is proposed. The break-period estimated by this model based on probability and lifetime of hydrogen bonds between chains agrees to with that obtained by molecular dynamical calculation.

Analysis of International Standard Problem No. 45, QUENCH06 Test at FZK by Detailed Severe Accidents Analysis Code, IMPACT/SAMPSON

The QUENCH06 test at Forschungszentrum Karlsruhe (FZK) selected to be the international standard problem (ISP) No. 45 by OECD/NEA has been analyzed by the IMPACT/SAMPSON code, a detailed analysis code for severe accidents in an LWR. The code has been used to perform blind and open phase analyses of the quench phenomenon and Zr/water reaction on the actual PWR cladding. Conclusions obtained from the analyses are as follows: (1) Overall, the blind phase analysis predicted relatively well the results of QUENCH06 test with respect to thermal hydraulics in the test bundle and its degradation due to Zr/water reaction. (2) The difference in the final accumulated hydrogen generation between the blind phase analysis and the test was 19% under parabolic rate constants by Cathcart-Pawel and Urbanic-Heidrick with a transition temperature of 1,853 K. (3) Embrittlement failure criteria for Zircaloy cladding against quenching adopted from the SCDAP/RELAP5 (MATPRO) predicted almost correctly the failure time of the QUENCH06 test. (4) The open phase analysis has traced the temperature change of the inner fuel cladding very well and has yielded almost the measured hydrogen release, considering inner surface oxidation after cladding failure. (5) The IMPACT/SAMPSON code has been validated against the quench phenomenon and the Zr/water reaction observed in the QUENCH06 test at FZK.

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