Hidemasa Yamano

The development of simmer-III, an advanced computer program for lmfr safety analysis, and its application to sodium experiments

Abstract SIMMER-III is a general two-dimensional, three-velocity-field, multiphase, multicomponent, Eulerian, fluid dynamics code coupled with a space-time and energy-dependent neutron transport kinetics model. The philosophy behind the SIMMER-III development was to generate a versatile and flexible tool, applicable for the safety analysis of various reactor types with different neutron spectra and coolants including the new accelerator-driven systems for waste transmutation. Currently, a three-dimensional version is also available, coined SIMMER-IV. The main backbone for analyses, however, is still SIMMER-III. SIMMER-III has proven especially well suited for fast spectrum systems such as the liquid-metal-cooled fast reactor where it is one of the key codes for safety analysis, including its application within licensing procedures. To serve especially the last purpose, the code must be made sufficiently robust and reliable and be tested and validated extensively. A comprehensive and systematic assessment program of the code has been conducted. This paper gives the major achievements of this assessment program. The SIMMER-III code handles by default liquid-metal-cooled fast reactor core materials - fuel, steel, coolant, control rod, and fission gas, in solid, liquid, and vapor states. The total of 27 density and 16 energy components are modeled in three velocity fields and one structure field in order that important fluid motions in a degraded core are simulated adequately. The spatial differencing method is based on Eulerian staggered mesh with a higher-order differencing scheme to mitigate numerical diffusion. An improved analytic equation-of-state model provides good accuracy especially at high temperature and pressure. Multiple flow-regime treatment is available over the entire void fraction range. An interfacial area convection model improves the flexibility of the code by tracing transport and history of interfaces and thereby better represents physical phenomena. A generalized and flexible code framework, along with improved numerical stability and accuracy, allows us to apply it to a variety of simple and complex multiphase flow problems. The code assessment program is an ongoing effort. Two major milestones have been achieved in the past by completing two assessment campaigns, Phase 1 and Phase 2: Phase 1 for fundamental code assessment of individual models and Phase 2 for integral code assessment for key phenomena relevant to liquid-metal-cooled fast reactor safety. Through this systematic code assessment program, comprehensive validation of the physical models has been conducted step-by-step. The assessment program has demonstrated that SIMMER-III is a state-of-the-art code with advanced models sufficiently flexible for simulating transient multiphase phenomena occurring during core disruptive accidents. This paper concentrates on the specifics of the code, mainly reflected in its application to sodium experiments related to the safety of liquid-metal-cooled fast reactors.

Self-Leveling Onset Criteria in Debris Beds

In a core-disruptive accident of a sodium-cooled fast breeder reactor, core debris may settle on the coresupport structure and/or in the lower inlet plenum of the reactor vessel because of rapid quenching and fragmentation of molten core materials in the subcooled sodium plenum. Coolant boiling is the mechanism driving the self-leveling of a debris bed that causes significant changes in the heat-removal capability of the beds. In the present study, we develop criteria establishing the onset of this self-leveling behavior that we base on a force balance model assuming a debris bed with a single-sized spherical particle. The model considers drag, buoyancy, and gravity acting on each particle. A series of experiments with simulant materials verified the applicability of this description of self-leveling. Particle size (between 0.5–6 mm), shape (spherical and nonspherical), density (namely of alumina, zirconia, lead, and stainless steel), along with boiling intensity, bed volume, and even experimental methods were taken into consideration to obtain general characteristics of the self-leveling process. We decided to use depressurization boiling to simulate an axially increasing void distribution in the debris bed, although bottom heating was also used to validate the use of the depressurization method. On the self-leveling onset issues, we obtained good agreement between model predictions and experimental results. Extrapolation of our model to actual reactor conditions is discussed.

A scenario of core disruptive accident for Japan sodium-cooled fast reactor to achieve in-vessel retention

As the most promising concept of sodium-cooled fast reactors, the Japan Atomic Energy Agency (JAEA) has selected the advanced loop-type fast reactor, so-called JSFR. The safety design requirements of JSFR for Design Extension Condition (DEC) are the prevention of severe accidents and the mitigation of severe-accident consequences. For the mitigation of severe-accident consequences, in particular, the In-Vessel Retention (IVR) against postulated Core Disruptive Accidents (CDAs) is required. In order to investigate the sufficiency of these safety requirements, a CDA scenario should be constructed, in which the elimination of power excursion and the in-vessel cooling of degraded core materials are evaluated so as to achieve IVR. In the present study, the factors leading to IVR failure were identified by creating phenomenological diagrams, and the effectiveness of design measures against them were evaluated based on experimental data and computer simulations. This is an unprecedented approach to the construction of a CDA scenario, and is an effective method to objectively investigate the factors leading to IVR failure and the design measures against them. It was concluded that mechanical/thermal failures of the reactor vessel due to power-excursion/thermal-load could be avoided by adequate design measures, and a clear vision for achieving IVR was obtained.

Experimental Studies and Empirical Models for the Transient Self-Leveling Behavior in Debris Bed

Studies on the self-leveling behavior in debris beds are crucial in the assessment of core-disruptive accidents (CDAs) that could occur in sodium-cooled fast reactors (SFRs). To clarify this behavior, a series of experiments have been performed in which nitrogen gas has been percolated uniformly through a particle bed. In these experiments, solid particles and water contained in a rectangular tank simulate respectively fuel debris and coolant. Based on the data obtained, an empirical model was developed to describe the transient variation in the bed inclination angle during the self-leveling process. Good agreement has been obtained between calculated and experimental values. Verification of the model has been confirmed through detailed analysis of the effects of experimental parameters such as particle size, particle density, and gas flow rate. Its applicability to extended conditions was further discussed by performing modeling simulations and comparing results against experimental data obtained from a larger-scale experimental system that employed a conventional boiling method. With further improvements, the model will be tested under more realistic reactor conditions and is expected to benefit future analyses and simulations of CDAs in SFRs.

Development of multicomponent vaporization/condensation model for a reactor safety analysis code SIMMER-III: Theoretical modeling and basic verification

Abstract It is believed that the numerical simulation of thermal-hydraulic phenomena of multiphase, multicomponent flows in a reactor core is essential to investigate core disruptive accidents (CDAs) of liquid-metal fast reactors. A new multicomponent vaporization/condensation (V/C) model was developed to provide a generalized model for a fast reactor safety analysis code SIMMER-III, which analyzes relatively short-time-scale phenomena relevant to accident sequences of CDAs. The model characterizes the V/C process associated with phase transition through heat-transfer and mass-diffusion limited models to follow the time evolution of the reactor core under CDA conditions. The heat-transfer limited model describes the nonequilibrium phase-transition processes occurring at interfaces, while the mass-diffusion limited model is employed to represent effects of noncondensable gases and multicomponent mixture on V/C processes. Verification of the model and method employed in the multicomponent V/C model of SIMMER-III was performed successfully by analyzing a series of multicomponent phase-transition experiments.

CHARACTERISTICS OF SELF-LEVELING BEHAVIOR OF DEBRIS BEDS IN A SERIES OF EXPERIMENTS

During a hypothetical core-disruptive accident (CDA) in a sodium-cooled fast reactor (SFR), degraded core materials can form roughly conically-shaped debris beds over the core-support structure and/or in the lower inlet plenum of the reactor vessel from rapid quenching and fragmentation of the core material pool. However, coolant boiling may ultimately lead to leveling of the debris bed, which is crucial to the relocation of the molten core and heat-removal capability of the debris bed. To clarify the mechanisms underlying this self-leveling behavior, a large number of experiments were performed within a variety of conditions in recent years, under the constructive collaboration between the Japan Atomic Energy Agency (JAEA) and Kyushu University (Japan). The present contribution synthesizes and gives detailed comparative analyses of those experiments. Effects of various experimental parameters that may have potential influence on the leveling process, such as boiling mode, particle size, particle density, particle shape, bubbling rate, water depth and column geometry, were investigated, thus giving a large palette of favorable data for the better understanding of CDAs, and improved verifications of computer models developed in advanced fast reactor safety analysis codes.

Thermophysical Properties of Lead-Bismuth Eutectic Alloy in Reactor Safety Analyses

A consistent set of thermophysical properties of a lead-bismuth eutectic (LBE) alloy was developed for use in safety analyses of lead-alloy-cooled fast reactor systems. The vapor and liquid thermodynamic states of LBE were modeled up to and above the critical point based on a van-der-Waals type of equation. We assumed that LBE vapor is composed of monatomic lead and bismuth and diatomic, bismuth components, and that liquid LBE is a non-ideal mixture of lead and bismuth. Recommended equations were also presented for the transport properties and surface tension of liquid LBE.

Technological Feasibility of Two-Loop Cooling System in JSFR

An advanced large-scale sodium-cooled fast reactor named JSFR adopts an innovative two-loop cooling system. This cooling system design raises major technological issues: hydraulic and structural integrity due to the increase in one-loop coolant flow rate, safety design against the break or failure in one-loop piping, and ensuring the reliability of the decay heat removal system (DHRS). The present paper describes the investigation of the piping structural integrity due to flow-induced vibration using a 1/3-scale hot-leg piping test. The structural integrity of the hot-leg piping in the JSFR design has been confirmed by a flow-induced vibration analytical methodology, verified with the experimental data. Additional experimental results have revealed that hydraulic issues including gas entrainment and vortex cavitation could be prevented by some design measures. By applying appropriate safety design, the two-loop system has been confirmed to be valid against the break or failure in one-loop piping by a safety evaluation in this study. The DHRS with natural circulation is designed in conformity with the two-loop system by introducing adequate safety designs. In this paper, the validity of this DHRS is given by a probabilistic safety assessment and safety evaluation.

Unsteady Elbow Pipe Flow to Develop a Flow-Induced Vibration Evaluation Methodology for Japan Sodium-Cooled Fast Reactor

This paper describes the current status of flow-induced vibration evaluation methodology development for primary cooling pipes in the Japan sodium-cooled fast reactor (JSFR), with particular emphasis on recent research and development activities that investigate unsteady elbow pipe flow. Experimental efforts have been made using 1/3-scale and 1/10-scale single-elbow test sections for the hot-leg pipe. The 1/10- scale experiment simulating the hot-leg pipe indicated no effect of pipe scale in comparison with the 1/3- scale experiment under inlet-rectified-flow conditions. The next experiment using the 1/3-scale test section was performed to investigate the effect of swirl flow at the inlet. Although the flow separation region was deflected at the downstream from the elbow, the experiment clarified a less significant effect of swirl flow on pressure fluctuation onto the pipe wall. An additional experiment was intended to study the effect of elbow curvature. The experiments with water revealed no clear flow separation in a larger curvature elbow case than that of the JSFR. Since the interference of multiple elbows should be investigated to understand turbulent flow in the cold-leg pipe geometry, 1/15-scale experiments with double elbows were carried out to clarify that flow in the first elbow influenced a flow separation behavior in the second elbow. Simulation activities include Unsteady Reynolds Averaged Navier Stokes equation (URANS) approach with a Reynolds stress model using a commercial computational fluid dynamics (CFD) code and Large Eddy Simulation (LES) approach using an in-house code. A hybrid approach that combined with RANS and LES was also applied using a CFD code. Several numerical results appear in this paper, focusing on its applicability to the hot-leg pipe experiments. These simulations reasonably agreed with the experimental data using the 1/3-scale test section.

Transient Heat Transfer Characteristics Between Molten Fuel and Steel with Steel Boiling in the CABRI-TPA2 Test

Abstract In the TPA2 test of the CABRI-RAFT program, which is part of a fast reactor safety study, fuel-to-steel heat transfer characteristics within a molten fuel/steel mixture system have been investigated. This test was performed in the French CABRI reactor and used a test capsule that contained fresh 12.3%-enriched UO2 pellets with embedded stainless steel balls. Following a preheating phase, the capsule was subjected to a transient overpower that resulted in fuel melting and steel vaporization. The observed steel vapor pressure buildup was quite low, which suggested the presence of a mechanism that significantly reduced the fuel-to-steel heat transfer. A detailed experimental data evaluation by SIMMER-III led to one possible interpretation that the steel vaporization at the surface of the steel ball blanketed the steel from the molten fuel.