Debris Bed Coolability Research Articles

A study on modeling of boiling heat transfer in core debris bed of SFR

In case of a hypothetical severe accident in a Sodium-cooled Fast Reactor (SFR), coolability of the debris bed in the post-accident phase plays a vital role in mitigating the accident and ensuring the structural integrity of the reactor vessel. Few numerical studies are reported in literature, in which the boiling heat transfer in debris bed is expressed as equivalent heat conduction using similarity law between heat conduction and two-phase heat transfer. However, these studies assumed steady state mass conservation for the boiling zone and neglected the gravity force. Hence, a detailed study has been carried out for various particle sizes and porosities of SFR debris to investigate the influence of above considerations. The effect of gravity on debris bed coolability is studied using steady state model of Lipinski, which showed that gravity has a non-negligible effect, for particle size of 0.3 mm and porosity of 0.5. However, the gravitation force was found to have a negligible effect in dryout heat flux estimation for the bottom cooled configuration. A transient numerical model is developed for simulating the boiling phenomena in debris beds and validated with the published experimental results. The assumption of steady state mass conservation is verified by carrying out transient analysis, which indicated early prediction of the dryout inception. For time dependent heat generation case, the unsteady mass conservation predicted higher DHF compared to constant heat generation.

An experimental investigation on debris bed formation from fuel coolant interactions of metallic and oxidic melts

During postulated severe accidents in a light water reactor (LWR), the core melt (corium) may relocate to the lower head and fail the reactor pressure vessel (RPV). The corium is expected to undergo fuel coolant interactions (FCI) if the reactor cavity is flooded with water. Both FCI energetics and resulting debris bed coolability are of paramount importance to reactor safety, since the ex-vessel corium poses a threat to the containment integrity if steam explosion occurs or the debris bed is uncoolable, leading to release of radioactive fission products to the environment. The present study is intended to quantify the characteristics of a debris bed resulting from FCI, which are crucial to debris bed coolability. Different from the previous studies with only oxidic materials, various materials, including metallic ones of Sn, Sn-Bi and Zn as well as oxidic one of Bi2O3-WO3, were employed as the simulants of corium (mixture of UO2/ZrO2/Zr/Fe) in the present study to investigate the effects of melt materials, melt superheat and coolant subcooling on debris bed formation in a water pool. High-speed photography was applied to visualize melt jet breakup, droplets fragmentation, as well as fragments sedimentation on the pool floor. Other obtained data are debris bed shape (profile) and porosity, as well as morphology and size distribution of debris particles. The comparative results of various tests provided insights toward filling the knowledge gap on debris bed characteristics under different melt materials and compositions.

Characteristics and Mechanisms of Debris Bed Formation Behavior in Severe Accidents of Sodium-Cooled Fast Reactors: Experimental and Modeling Studies

A Sodium-cooled Fast Reactor (SFR) is one of the optimized candidates in Generation IV nuclear reactor systems, but safety is an essential issue for SFR development and application. Most knowledge was accumulated through SFR safety investigations, especially for Core Disruptive Accidents (CDAs). During the CDA of SFRs, the molten materials in the core region are likely to discharge into subcooled sodium and form a debris bed on the lower region of the reactor vessel. Noticing that elaboration on the characteristics and mechanisms of Debris Bed Formation (DBF) behavior should be essential for the subsequent analysis of debris bed coolability and accident progression through various experimental and modeling studies, much knowledge was obtained during the past decades. Motivated to promote future investigations on CDAs of SFRs, the previous experiments and modeling studies on DBF behavior are systematically reviewed and discussed in this paper. The experimental results showed that the flow-regime and accumulated-bed characteristics during DBF were influenced by varying parameters and realistic conditions. Through the modeling studies, several empirical models were proposed for predicting the flow regime and accumulated-bed characteristics in DBF. In addition, to promote further development of research, the future prospects concerning DBF behavior are also described.

Experimental investigation on debris bed formation from metallic melt coolant interactions

Motivated by risk quantification of severe core meltdown accidents which may occur light water reactors, an extensive DEFOR (DEbris FORmation) test series with various molten oxidic materials was conducted previously at Royal Institute of Technology to investigate the characteristics of debris beds formed from oxidic melt-water interactions, which are important to the coolability of debris beds and thus the retention of core melt. Yet, little attention has been paid to metallic melt which exist in the core melt. The present study is intended to fill in the knowledge gap, i.e., to characterize metallic melt-water interactions when a melt jet falls into a deep water pool. Nine DEFOR-M tests are carried out to clarify the effects of melt jet diameter, free fall height and water pool depth on metallic debris formation characteristics. Molten tin of 20 kg is employed as the simulant of metallic melt. A melt sensor and a mass sensor are installed to detect the discharging period of coherent jet and the mass accumulation of debris bed, respectively. A high-speed camera is applied to record the process of jet breakup, debris fragmentation and solidification, and debris bed formation. The experimental results show that the water pool depth has a significant influence on steam explosion and debris bed formation characteristics. Moreover, the jet breakup length is sensitive to jet diameter and free fall height. Finally, the features of metallic debris beds are compared with those of oxidic ones in respects of debris bed's configuration and porosity, particles' morphology and size distribution.

A simple correlation to estimate agglomerated debris formation based on experiments of melt jet-breakup using a metallic melt

The molten core (corium or melt) relocation in severe accidents in a light water reactor can cause agglomeration by the merging of melt particles with others, and form massive debris (agglomerated debris) when the particles that detach from a melt jet accumulate on the floor of the containment vessel. This paper presents a modification of a simple correlation to estimate the mass fraction of the agglomerated debris proposed in the previous study. The modification is based on experiments of melt jet-breakup using a metallic melt. The mass fraction of the agglomerated debris needs to be estimated because the agglomerated debris can severely affect the debris bed coolability. The present study focused on the effects of changes in the experimental conditions for melt jet injection (nozzle diameter and inlet velocity) on the mass fraction of the agglomerated debris to expand the experimental database of the previous study that focused only on the effects of changes in the melt and coolant temperatures, and coolant depth on the mass fraction of the agglomerated debris. The visualized observations using a high-speed camera and morphological investigation of recovered debris revealed the effects of changes in the nozzle diameter and inlet velocity on the mass fraction of the agglomerated debris. The extrapolation of the modified simple correlation revealed the mass fraction of the agglomerated debris under the anticipated reactor conditions.

Evaluation of the dryout region of heat-generating conical porous beds

This study evaluates how multi-phase fluids flow and how a large volume becomes a dryout region in a volumetrically heat-generating conical porous bed formed in the pre-flooded reactor cavity of a hypothetical 1,000 MWe-class NPP whose reactor cavity is circular and 9 m in diameter. At repose angles of 25°, 35°, and 45°, effective particle diameters of 0.65, 0.9 and 1.65 mm with porosity of 0.37, the system pressures from 1 bar to 5 bar and the power densities of 400, 600, and 800 kW/m3, the coolability of debris beds is assessed. It is observed that dryout occurs in the debris beds for all the analyzed cases and that the volumetric ratio of dryout region is from 1.6% to 90.4%. The evaluation finds that the dryout region decreases as the effective particle diameter increases, the system pressure increases, the repose angle increases and the power density decreases.

Empirical modeling of ex-vessel debris bed formation in a pre-flooded reactor cavity

To secure the containment integrity of a nuclear power plant (NPP) in severe accident situation of ex-vessel corium release into the reactor cavity, a pre-flooding strategy of a reactor cavity has been adopted in various NPPs, including Korean PWRs. Molten corium released into the pre-flooded water pool is fragmentized and forms a particulate debris bed at the bottom of the cavity. To prevent the containment failure, it is important to properly remove the heat from the debris bed, which can be accomplished by water ingression into the bed. The water ingression characteristics are highly dependent on the debris bed shape. In this study, several main parameters affecting the debris bed shape were determined with respect to phenomena of free-fall & sedimentation, particle dispersion by steam spike and convection flow, and self-leveling. Then empirical model was developed and validated using the results of DEFCON (Debris bed Formation and COolability experimeNt) experiments. The model’s error was within ±10% when only free-fall & sedimentation were present. When steam-spike or convection was added to free-fall & sedimentation condition, the error increased to within ±20%. When all three were simultaneously present (free-fall & sedimentation, steam-spike, and convection), the model predicted most of the experimental results within ±20% error, except for several cases where convection had a significant impact. The effect of self-leveling itself was within ±10%. Considering all phenomena related to particulate debris bed formation, most cases had an error of ±20%. In addition, the debris bed shape formed by poly-disperse particles was predicted appropriately by the mode using the mass mean diameter of the particle size distribution, with an error range of ±20%. Consequently, it is expected that the proposed model of the debris bed shape can be utilized in an integrated severe accident analysis code for the evaluation on the ex-vessel debris bed coolability in the pre-flooded reactor cavity.

Experimental study on coolability of mixed particle size debris bed under top flooding

During severe accident in reactor, a debris bed is formed after the core melt interacts with the coolant, this is the key stage to realize the melt retention in vessel and terminate the accident process. Studying the drying characteristics of such stacked particle beds can expand the safety margin of the In-Vessel Retention (IVR) strategy. In this paper, based on the debris bed's characteristics of prototype fuel coolant interaction (FCI) experiment, a one-dimensional cylindrical debris bed with irregular and multi-sized sand particles was constructed. At the uniformly stacked case, the typical characteristics were observed. Two vapor phase concentration areas appeared in the debris bed before and after drying, bubble blocking area was formed in the middle of the debris bed before drying, then a dry zone appeared at the bottom of the debris bed due to lack of liquid, the dry area extended first radially and then axially. Meanwhile, the influence of system pressure is significant, dryout heat flux (DHF) under 1.0 MPa is almost twice than normal pressure. At the sand particles layered case, it was found that DHF increased by 40%∼50% compared with uniformly stacked condition, the bubble blocked area and dry area coincided, and occurred in the middle of the debris bed. Finally, a DHF model based on Reed pressure drop model is established for one-dimensional uniform sand debris bed, and the calculated values are in good agreement with the experimental values.

Mapping of quench front propagation in a heated cylindrical particle bed – MONET tests

In case of a severe accident in nuclear reactor, a debris bed may be formed from the fragmentation of the molten core at different stages of the accident. Debris bed coolability is an important issue, as it will determine the accident progression, its termination, and mitigation. One of the major concerns of debris bed coolability investigations is to understand and evaluate the effect of water re-flooding. Many experiments have been conducted to study the two-phase flow and heat transfer in porous medium configuration. However, the understanding of debris bed quenching with multi-dimensional characteristics are limited. Here, a test facility MCCI-mitigatiON through passive cooling Effect Test (MONET) apparatus is established to conduct the porous debris bed re-flooding investigation. The objective is to understand the mechanism of heat transfer and quench front movement inside the cylindrical heated debris bed during quenching by cooling water supplied from the bottom of the bed. Specifically, the effect of Homogeneous bed (with different external boundary conditions) and radially stratified bed (with different internal porosity configurations) conditions of the cylindrical debris bed on the quench front movement has been investigated. The particulate debris bed is formed with two different types of particles (SS 304, alumina) and sizes (2–3.5 mm). The effect on the quench front movement inside the particle bed has been reported by using temperature measurements together with direct visualizations. Present findings suggest non-uniform quench front propagation pattern even for the homogeneous bed conditions. While the tests with the stratified bed indicate a clear influence of low porosity region on the quench front and vapor movement.

Morphological analysis of debris obtained from melt coolability experiment using CaO-B<SUB align=right>2O<SUB align=right>3 corium simulant

The present study is focused on debris analysis obtained from melt coolability experiments. Melt gets fragmented upon interaction with water and get quenched while forming a porous debris bed. This debris bed is coolable by natural circulation. Quenching of the melt pool has been discussed in the central region in the test section. The agglomeration of melt debris and the formation of solid cake regions adversely affected the coolability of the porous debris bed. The characterisation study of debris is carried out. The debris size range was found from 1 to 3 cm. The porosity of the debris bed was measured about 53% using the volumetric method. The data represents a systematic study of the debris agglomeration phenomenon and debris size distribution which is necessary for the development of new ways to mitigate agglomerations of debris to enable the prediction of the debris coolability in various melt coolability scenarios.

Experimental investigation on the coolability of nuclear reactor debris beds using seawater

During the Fukushima nuclear reactor accident, seawater was injected into the reactor core to cool the decay heat from the heat generating porous debris bed. However, the impact of dissolved salts in water on the coolability of a debris bed is not well understood. This paper is one of the first works to investigate the cooling mechanisms for debris bed using seawater. An experimental system was built where the packed bed test section was volumetrically heated via a high frequency induction heater. The temperature along the axis of the test section was measured using a high resolution temperature sensor based on Optical Frequency Domain Reflectometry. Experimental results show that the dryout heat flux increases with an increase in concentration, with pure water having a dryout volumetric heat flux of 1.8 MW/m3 and salt water of concentration 7% having a dryout volumetric heat flux of 2.8 MW/m3. However, this enhanced coolability using salt water is short term, as the crystallization fouling mechanism becomes significant over time and leads to gradual temperature excursion in the bed. The fouling mechanism eventually leads to a cross section of the debris bed to plug with the deposited salts, and this leads to a pressure build up in the lower regions of the debris bed.

Experiments of melt jet-breakup for agglomerated debris formation using a metallic melt

In severe accidents in a light water reactor, the relocated molten core (so-called corium or melt) can form a debris bed. The debris bed coolability is a critical issue for prevention and mitigation of the molten core–concrete interactions. Agglomeration has a serious impact on assessment of debris bed coolability if agglomeration forms massive debris (so-called agglomerated debris) by merging of melt particles with others when the melt particles accumulate on a floor. This paper presents the results of melt jet-breakup experiments for agglomerated debris formation using a simulant metallic melt. The experiments injected a melt jet of a low-melting point metal through a circular nozzle into a test section filled with coolant water. The particles were generated due to the melt jet-breakup accumulated on to a catcher, which is a flat plate made of stainless steel, installed in the test section. A high-speed video camera imaged particle formation and accumulation on the catcher plate. Agglomerated debris was confirmed by morphological investigation of the recovered debris. The experimental results revealed the effects of the melt jet injection conditions (melt temperature, coolant temperature, and coolant depth) on the mass fraction of agglomerated debris. On the basis of the experimental results, we proposed a simple correlation to estimate the mass fraction. The simple correlation successfully reproduced the mass fraction of agglomerated debris obtained in the DEFOR-A test [Kudinov et al., Nucl. Eng. Des., 301 (2013), 284–295]. The experimental data base presented in this paper makes further contributions to the modeling and validation of mechanistic models or simulation tools for agglomerated debris formation.

Coupled MELCOR/COCOMO analysis on quench of ex-vessel debris beds

The cornerstone of severe accident strategy of Nordic BWRs is to flood the reactor cavity for the long-term coolability of an ex-vessel debris bed. As a prerequisite of the long-term coolability, the hot debris bed formed from fuel coolant interactions (FCI) should be quenched. In the present study, coupling of the MELCOR and COCOMO codes is realized with the aim to analyze the quench process of an ex-vessel debris bed under prototypical condition of a Nordic BWR. In this coupled simulation, MELCOR performs an integral analysis of accident progression, and COCOMO performs the thermal–hydraulic analysis of the debris bed in the flooded cavity. The effective diameter of the particles is investigated. The discussion on the bed’s shape shows a significant effect on the propagation of the quench front, due to different flow patterns. Compared with MELCOR standalone simulation, the coupled simulation predicts earlier cavity pool saturation and containment venting.

Development and application of a surrogate model for quick estimation of ex-vessel debris bed coolability

During a hypothetical severe accident of a Nordic boiling water reactor (BWR), an ex-vessel particulate debris bed is expected to form in the flooded lower drywell due to melt-coolant interactions after vessel failure. The key parameter to evaluate debris bed coolability is the dryout heat flux (DHF) or dryout power density, representing the limit of heat removal capacity by the coolant. Several numerical codes such as COCOMO have been developed to simulate thermal hydraulics in multi-dimensional debris beds and predict the cooling limit, but they are computationally expensive and not suitable for probabilistic risk analysis. This paper aims to develop a surrogate model which can serve as a quick-estimate tool for the dryout power density of a heap-like debris bed in a saturated water pool. The dryout power density predicted from the COCOMO code is treated as the full model. A characteristic factor is introduced as the dryout power density ratio between the multi-dimensional debris bed (predicted by COCOMO code) and the corresponding one-dimensional debris bed (predicted by Lipinski 0-D model). The characteristic factor is correlated by the Kriging method with six parameters: bed porosity, particle diameter, debris mass, bed slope, cavity radius and containment pressure. After the surrogate model is trained and validated, it is employed to analyze the coolability of prototypical debris beds of a reference Nordic BWR, given the bed mass and containment pressure from MELCOR simulation. Coolability maps are produced as quick look-up diagrams for identification of coolable domain with the variation of porosity, particle diameter and slope angle. A preliminary uncertainty analysis is performed to demonstrate the effect of uncertain input parameters on non-coolable domain.

Minimum diameter limit of particle size distribution and its effect on coolability of debris bed

The particle size distribution (PSD) of the debris bed is a major parameter influencing its coolability with respect to the two-phase pressure drop of the porous media. Most studies have focused on representative particle diameter as opposed to size distribution. Based on a severe accident situation (strong two-phase convection), we introduced a rearrangement of small particles and developed a corresponding size distribution formula by modifying the Rosin-Rammler distribution.A minimum particle diameter model was proposed as a function of dryout heat flux (DHF) based on a minimum fluidization velocity concept. In addition, a truncated Rosin-Rammler distribution was developed and a Sauter Mean Diameter (SMD) was formulated in the form of the gamma function with a new parameter: the minimum particle diameter (Dmin). Sensitivity analysis of DHF in a 1-D top flooded debris bed showed two notable results for the effect of Dmin on coolability. First, Dmin significantly reduced DHF uncertainty due to the distribution constant. Moreover, the calculated DHF increased by introducing the concept of a minimum particle diameter. In conclusion, treatment of small particles in the debris bed is essential for determining debris bed coolability. Additionally, the truncated Rosin-Rammler distribution could be utilized to provide the precise SMD in a porous media.

Study on high-speed calculation of debris bed coolability for sodium-cooled fast reactors

Study on high-speed calculation of debris bed coolability for sodium-cooled fast reactors

Study on application of artificial neural network to debris bed coolability calculations for sodium-cooled fast reactors

Study on application of artificial neural network to debris bed coolability calculations for sodium-cooled fast reactors

Design of core catchers for sodium cooled FBRs – Challenges

The whole core meltdown scenario in sodium cooled fast reactors is considered under design extension criteria and has a very low probability of occurrence. To mitigate such a hypothetical severe accident in fast breeder reactors, a core catcher has been provided to accommodate the core debris within the primary containment boundary. In sodium cooled fast breeder reactor (FBR), an in-vessel core catcher, is provided to receive and disperse the fuel debris arising out of core meltdown during Hypothetical Core Disruptive Accident (HCDA). The core catcher prevents hot debris from reaching and damaging the main vessel. It also enables adequate heat transfer by natural convection to keep debris in stable conditions. The present study focuses on design of core catcher with sacrificial barriers and techniques to improve natural cooling of debris on the core catcher for FBRs. Studies are carried out towards the selection of suitable sacrificial material for core catcher, numerical analysis on multilayer and thermal-hydraulic analysis on multi jets core catcher concepts for selection of best suitable technique for debris bed coolability. The core catcher concept with sacrificial barrier and cooling pipes is found to possess enhanced coolability of debris bed. Experimental studies on compatibility of sacrificial ceramic material with sodium indicate strong dependence on microstructure. It has also been found that the multilayer core catcher with MgO as delay bed of 50 mm thickness is adequate to reduce the bottom temperature of core catcher below the design safety limit. Judicious mix of all these findings is expected to have unique design to accommodate whole core meltdown in future fast reactors.

Two-phase flow pressure loss through packed particle bed for wide void fraction range

The understanding of the two-phase flow phenomena in porous media is a critical issue for debris bed coolability assessment in severe nuclear power plant accidents. Hence, two-phase-flow friction force modeling has been extensively attempted, based on pressure-loss measurement data. However, most previous two-phase flow pressure loss experiments were conducted at a restricted void-fraction range up to approximately 0.6, which is considerably lower than the dryout condition. Therefore, in this research, two-phase flow tests in packed particle beds are conducted to gather two-phase pressure loss data in the high-void-fraction region. The test beds are constructed by packing mm-scale particles, 3–7 mm in diameter. The experimental data are analyzed and compared with the predicted values of the previous two-phase friction models in term of not only the pressure gradient but also the interfacial friction force between the liquid and gas phases. As there was no valid data for the high-void-fraction region during the previous model development processes, an appropriate model for the interfacial friction force at void fractions greater than approximately 0.6 is unavailable. However, the interfacial friction force plays a significant role in the coolability assessment of the ex-vessel debris bed because it determines the water ingression rate under a cocurrent flow condition. The obtained experimental results in this research indicate the necessity to further improve the model in order to decrease the uncertainty in severe accident risk quantification.

Experimental study on evaluation of effective diameter for non-spherical particles in predicting pressure gradients of water/air two-phase flow in particle beds

To evaluate the adequacy of the effective diameter (dsd and deq) of non-spherical particles in predicting pressure gradients of upward air/water two-phase flow through well-packed beds, a set of experiments were performed using four kinds of cylindrical particles under isothermal conditions varying the superficial air velocity (0–0.8 m/s) with no water inflow at the bottom. The measured pressure gradients were compared with the existing models and the proposed model from our previous study adopting dsd or deq. Consequently, the proposed model adopting deq predicts the pressure gradients of two-phase flow through non-spherical particle beds much better than the existing models, which is reducing the uncertainty in evaluating long-term coolability of ex-vessel debris bed composed of irregular shaped particles.