Hirotaka Tagami

An investigation on debris bed self-leveling behavior with non-spherical particles

Studies on debris bed self-leveling behavior with non-spherical particles are crucial in the assessment of actual leveling behavior that could occur in core disruptive accident of sodium-cooled fast reactors. Although in our previous publications, a simple empirical model (based model), with its wide applicability confirmed over various experimental conditions, has been successfully advanced to predict the transient leveling behavior, up until now this model is restricted to calculations of debris bed of spherical particles. Focusing on this aspect, in this study a series of experiments using non-spherical particles was performed within a recently developed comparatively larger scale experimental facility. Based on the knowledge and data obtained, an extension scheme was suggested with the intention to extend the base model to cover the particle-shape influence. The proposed scheme principally consists of two parts – with one part for correcting the terminal velocity of a single non-spherical particle, which is the key parameter in our base model, and the other for representing the additional particle–particle interactions caused by the shape-related parameters. Through detailed analyses, it is found that by coupling this scheme, good agreement between experimental and predicted results can be achieved for both spherical and non-spherical particles given current range of experimental conditions.

Experimental evaluation of debris bed characteristics in particulate debris sedimentation behaviour

After a core-disruptive accident (CDA) in sodium-cooled fast reactor, degraded core material can form debris beds on core-support structure or in the lower inlet plenum of the reactor vessel. This paper reports an experimental evaluation on debris bed formation characteristic in CDA. Investigation of debris bed characteristic during debris sedimentation on core catcher plate is important from recriticality concern and also from cooling considerations to ensure the safety of the reactor main vessel in CDA. In the present study, to evaluate this characteristic, a series of experiments was performed by gravity driven discharge of solid particles as simulant debris from a nozzle into a quiescent water pool in isothermal condition at room temperature. The discharged solid particles with a maximum amount of 10 L finally accumulate on the debris tray, forming a bed with a convex or concave mound depending on the experimental parameters. The nozzle diameter, nozzle height, debris density, debris diameter and debris volume are taken as the experimental parameters. Currently, three types of spherical particles, namely Al2O3, ZrO2 and stainless steel (SS) with diameter of 2, 4, or 6 mm are employed to study the effect of key experimental parameter on debris bed mound shape. In addition, 2 mm non-spherical particles of SS were also utilized to investigate the effect of debris shape on altering mound profile. In experimental investigation with different debris volume, both developing and fully developed mound shapes were observed based on the effect of debris size, density and nozzle diameter. In this study, the investigated particle velocity of main stream settling particles was found increasing with nozzle diameter, which caused a decrement of mound height with an increment of mound dimple area. In nozzle height effect, shrinking of concavity on mound shape was observed with decreasing manner of impact velocity while height is reducing. From the visualization results of the experimental investigations, transformation of bed shape from convex to concave was observed with increasing repose angle incase of 4 mm Al2O3 particle. In general, transformation of bed shape was observed by increasing either nozzle diameter or particle density for all particle type. The present results could be useful to validate numerical models and simulation codes of particulate debris sedimentation.Copyright

Safety Evaluation of Prototype Fast-Breeder Reactor: Analysis of ULOF Accident to Demonstrate In-Vessel Retention

In the original licensing application for the prototype fast-breeder reactor, MONJU, the event progression during an unprotected loss-of-flow (ULOF), which is one of the technically inconceivable events postulated beyond design basis, was evaluated. Through this evaluation, it was confirmed that radiological consequences could be suitably limited even if mechanical energy was released. Following the Fukushima-Daiichi accident, a new nuclear safety regulation has become effective in Japan. The conformity of MONJU to this new regulation, hence, should be investigated.The objectives of the present study are to conduct a preliminary evaluation of ULOF for MONJU reflecting the knowledge newly obtained after the original licensing application, and to gain the prospect of In-Vessel Retention (IVR) for the conformity of MONJU to the new regulation.In the evaluation of event progressions during ULOF, the whole sequence was categorized into 1) initiating, 2) transition, and 3) post-accident-material-relocation/post-accident-heat-removal (PAMR/PAHR) phases. In the initiating phase, fuel pin disruption caused by coolant boiling would result in axial fuel dispersion in subassembly (SA). In the transition phase, molten-core pool would be formed due to the failure of SA walls, and the molten fuel would be discharged through the control-rod guide tubes (CRGTs). In the PAMR/PAHR phase, molten fuels discharged through CRGTs would be relocated and be stably cooled in the lower plenum by decay-heat removal. The methodology of the present study and its results can be summarized as below:1) The initiating phase was evaluated by SAS4A code reflecting the models and parameters for fuel-pin disruption and fuel dispersions based on the CABRI experiments. Contrary to the original licensing evaluation showing 380 MJ in mechanical energy release under conservative conditions, the present evaluation showed that no significant energy release would take place.2) The transition phase was evaluated by 3-dimensional SIMMER-IV code reflecting the models and parameters for CRGT failure and molten-fuel discharge based on the EAGLE experiments. Contrary to the past 2-dimensional evaluation showing 150 MJ in mechanical energy release under conservative conditions, the present evaluation showed that the released mechanical energy would be remarkably reduced because the non-physical axisymmetric/coherent fuel compaction peculiar to 2-dimensional evaluation was appropriately mitigated in 3-dimensional evaluation.3) The PAMR/PAHR phase was evaluated by S-COPD, FLUENT codes and heat-balance calculations reflecting the present evaluation of the precedent phases. Contrary to the past evaluation involving the uncertainties in molten-fuel fragmentation and debris-bed formation, the present evaluation showed that stable cooling of discharged core materials could be achieved even if fragmentation was incomplete.The preliminary evaluation in the present study showed that no significant mechanical energy release would take place, and that thermal failure of the reactor vessel could be avoided by the stable cooling of disrupted-core materials. This result suggests that the prospect of IVR against ULOF, which lies within the bounds of the original licensing evaluation and conforms to the new nuclear safety regulation, will be gained.Copyright

Numerical Simulation of the Self-Leveling Phenomenon by Modified SIMMER-III

During a hypothetical core-disruptive accident in a sodium-cooled FBR, degraded core material can form debris beds on the core-support structure and/or in the lower inlet plenum of the reactor vessel, due to the rapid quenching and fragmentation of the core material melt. Heat convection and vaporization of the sodium will lead ultimately to leveling the debris bed that is of crucial importance to the relocation of the molten core, the recriticality evaluation and the heat removal capability of the debris bed. There is, therefore, a great need for more studies focusing on this topic, especially the much needed numerical simulation. The widely-used fast reactor safety analysis code, SIMMER-III, has difficulties in this simulation because of the lack of modeling for mechanistic interactions among particles in the current version. However, the extensive experimental analysis and the previously-proposed analytical model provide SIMMER-III the possibility of taking consideration of the extra influence of solid particles in this phenomenon. Thus, the debris fluidization model and the boiling regulation model are proposed and introduced into SIMMER-III. Calculations, by the modified SIMMER-III, against several representative experiments with typical self-leveling behavior have been performed and compared with the evaluated items recorded in experiments. The good agreements on these items suggest the modified SIMMER-III can simulate the self-leveling behavior with reasonable precision, especially on the onset of self-leveling, although further model improvement is necessary to represent the transient behavior of bed leveling more reasonably.Copyright

Development of the evaluation methodology for the material relocation behavior in the core disruptive accident of sodium-cooled fast reactors

The in-vessel retention (IVR) of core disruptive accident (CDA) is of prime importance in enhancing safety characteristics of sodium-cooled fast reactors (SFRs). In the CDA of SFRs, molten core material relocates to the lower plenum of reactor vessel and may impose significant thermal load on the structures, resulting in the melt-through of the reactor vessel. In order to enable the assessment of this relocation process and prove that IVR of core material is the most probable consequence of the CDA in SFRs, a research program to develop the evaluation methodology for the material relocation behavior in the CDA of SFRs has been conducted. This program consists of three developmental studies, namely the development of the analysis method of molten material discharge from the core region, the development of evaluation methodology of molten material penetration into sodium pool, and the development of the simulation tool of debris bed behavior. The analysis method of molten material discharge was developed based on the computer code SIMMER-III since this code is designed to simulate the multi-phase, multi-component fluid dynamics with phase changes involved in the discharge process. Several experiments simulating the molten material discharge through duct using simulant materials were utilized as the basis of validation study of the physical models in this code. It was shown that SIMMER-III with improved physical models could simulate the molten material discharge behavior, including the momentum exchange with duct wall and thermal interaction with coolant. In order to develop an evaluation methodology of molten material penetration into sodium pool, a series of experiments simulating jet penetration behavior into sodium pool in SFR thermal condition were performed. These experiments revealed that the molten jet was fragmented in significantly shorter penetration length than the prediction by existing correlation for light water reactor conditions, due to the direct contact and thermal interaction of molten materials with coolant. The fragmented core materials form a sediment debris bed in the lower plenum. It is necessary to remove decay heat safely from this debris bed to achieve IVR. A simulation code to analyze the behavior of debris bed with decay heat was developed based on SIMMER-III code by implementing physical models, which simulate the interaction among solid particles in the bed. The code was validated by several experiments on the fluidization of particle bed by two-phase flow. These evaluation methodologies will serve as a basis for advanced safety assessment technology of SFRs in the future.

Numerical study on sedimentation behavior of solid particles used as simulant fuel debris

This paper presents numerical simulations using the discrete element method (DEM) to model sedimentation behavior of solid debris particles, which is significant for estimates of the coolability of debris beds. A series of experiments of gravity driven discharge of solid particles into a quiescent water pool was used to validate the DEM simulation method. We evaluated the effects of three crucial factors: particle density, particle diameter, and nozzle diameter on three key quantitative parameters: particle dispersion angle, particle fall time in the pool, and the height of the deposited particle bed to express the particle sedimentation behavior. The three crucial factors play a significant role in the particle sedimentation behavior. We compared the experimental and simulated results of the particle dispersion angle and particle fall time in the pool, and the height and shape of the deposited particle bed. The general trend of the simulation results indicates a reasonable agreement with the experimental observations. The simulations exhibit the potential applicability of the DEM-based simulation technique for the prediction of particle sedimentation behavior.

Numerical Simulation of Self-Leveling Behavior in Debris Bed by a Hybrid Method

The postulated core disruptive accidents (CDAs) are regarded as particular difficulties in the safety analysis of liquid-metal fast reactors (LMFRs). In the CDAs, the self-leveling behavior of debris bed is a crucial issue to the relocation of molten core and heat-removal capability of the debris bed. The fast reactor safety analysis code, SIMMER-III, which is a 2D, multi-velocity-field, multiphase, multicomponent, Eulerian, fluid dynamics code coupled with a fuel-pin model and a space- and energy-dependent neutron kinetics model, was successfully applied to a series of CDA assessments. However, strong interactions among rich solid particles as well as particle characteristics in multiphase flows were not taken into consideration for fluid-dynamics models of SIMMER-III. In this article, a developed hybrid method, by coupling the discrete element method (DEM) with the multi-fluid model of SIMMER-III, is applied in the numerical simulation of self-leveling behavior in debris bed. In the coupling algorithm the motions of gas and liquid phases are solved by a time-factorization (time-splitting) method. For particles, contact forces among particles and interactions between particles and fluid phases are considered through DEM. The applicability of the method in such complicate three phase flow is validated by taking the simulation of a simplified self-leveling experiment in literature. Reasonable agreement between simulation results and corresponding experimental data shows that the present method could provide a promising means for the analysis of self-leveling behavior of debris bed in CDAs.Copyright

Validation of a 3D Hybrid CFD-DEM Method Based on a Self-Leveling Experiment

The postulated core disruptive accidents (CDAs) are regarded as particular difficulties in the safety analysis of liquid-metal fast reactors (LMFRs). In the CDAs, core debris may settle on the core-support structure and form conic bed mounds. Heat convection and vaporization of coolant sodium will level the debris bed, which is named “self-leveling behavior” of debris bed. To reasonably simulate such transient behavior, as well as thermal-hydraulic phenomena occurring during a CDA, a comprehensive computational tool is needed. The SIMMER code is a successful computer code developed as an advanced tool for CDA analysis of LMFRs. It is a multi-velocity-field, multiphase, multicomponent, Eulerian, fluid dynamics code coupled with a fuel-pin model and a space- and energy-dependent neutron kinetics model. Until now, the code has been successfully applied to simulations of key thermal-hydraulic phenomena involved in CDAs as well as reactor safety assessment. However, strong interactions among rich solid particles as well as particle characteristics in multiphase flows were not taken into consideration for its fluid-dynamics models. Therefore, a hybrid computational method was developed by combining the discrete element method (DEM) with the multi-fluid models to reasonably simulate the particle behaviors, as well as the thermal-hydraulic phenomena of multiphase fluid flows. In this study, 3D numerical simulation of a simplified self-leveling experiment is performed using the hybrid method. Reasonable agreement between simulation results and corresponding experimental data demonstrated the validity of the present method in simulating the self-leveling behavior of debris bed.© 2014 ASME

Development of assessment method for a self-leveling behavior of debris bed and analyses of experiments

When core melt occurs in severe accident in Sodium Cooled Fast Reactor (SFR), molten core material moves to the lower plenum in reactor vessel and fragmented by fuel coolant interaction. These fragmented particles, so called debris, accumulate on the structure surface to form debris bed. If the thickness of the debris bed exceeds the coolable thickness of the decay heat, boiling of sodium occurs inside the debris bed. It is found from past in-pile experiments that the sodium flow and boiling inside the debris bed caused by a decay heat planarize the debris bed to lower the debris bed thickness. This mechanism is called self-leveling of debris bed. In the accident sequence of SFR, when fuel debris locally accumulates beyond the coolable thickness, fuel debris remelts with decay heat and they cannot be retained in-vessel. However, it is expected that the debris bed thickness lowers the coolable thickness with self-leveling phenomenon and they can be safely retained in-vessel. This is why an appropriate assessment for self-leveling behavior is important for safety analysis of SFR with the object of safety cooling of fuel debris. Therefore, the object of this study is to develop new analytical methods to simulate unique phenomena in self-leveling behavior and implement it to SFR safety analysis code. The characteristic of self-leveling is that when the larger external forces caused by environmental fluids are larger than a threshold value, the debris bed is fluidized. The new methods are developed with assuming that the debris bed behaves as Bingham fluid from this feature. They are categorized into two main parts. The first part is particle interaction models to model the effect of particle-particle contacts and collisions. Particle pressure and particle viscosity related to particle-particle collisions and contacts, respectively, are applied to pressure and viscosity term in the particle momentum equation. The second part is a large deformation method, which simulates Bingham fluid characteristic of debris bed. This method numerically judges a onset of debris bed fluidization which depends on a shear stress strength. An experimental study of self-leveling behavior, in which the particle bed behavior driven by bubbles inflow from the bottom of bed in gas-solid-liquid three-phase flow was observed, is analyzed to validate the new methods. Simulation results well reproduced the transient changes of particle bed, whose elevation angle and form deformation becomes gradually small and obscure, respectively. Their dependencies on particle size and density are also well simulated with new methods. The assessment results show that these methods provide a basis to develop analytical methods of self-leveling behavior of debris bed in the safety assessment of SFRs.Copyright