Molten fuel-coolant interaction phenomena with application to carbide fuel safety

This paper reviews the major phases occurring during an energetic molten fuel/coolant interaction (MFCI), the categories of interaction and modes of contact between molten fuel and liquid coolant, the film boiling destabilization and collapse mechanisms, and the important fragmentation mechanisms of the melt. Two major models that describe the processes involved in an MFCI event are discussed: the spontaneous nucleation model and the pressure detonation model. Finally, the MFCI experiments involving carbide fuel and liquid sodium are reviewed and the potential for an energetic interaction between molten carbide fuel and liquid sodium is discussed. Recommendations are given for future work on MFCI phenomena relative to the carbide fuel/sodium system.     

Study on thermal-hydraulic behavior during molten material and coolant interaction

In a core disruptive accident (CDA) of a Fast Breeder Reactor, the post accident heat removal (PAHR) is crucial for the accident mitigation. The molten core material should be solidified in the sodium coolant in the reactor vessel. The material, being fragmented while solidification and forming debris bed, will be cooled in the coolant.

In the experiment, molten material jet is injected into water to experimentally obtain the visualized information of the fragmentation and boiling phenomena during PAHR in CDA. The experiment shows that the break up of the molten material into fine fragments is observed at the front, side and middle part of the jet during very short time interval. The distributed particle behavior of the molten material jet is observed with high-speed video camera. And the visual data is analyzed with Particle Imaging Velocimetry (PIV).

The experimental results are compared with the existing theories. Consequently, the marginal wavelength on the surface of a water jet is close to the value estimated based on the Rayleigh–Taylor instability. Moreover, the fragmented droplet diameter obtained from the interaction of molten material and water is close to the value estimated based on the Kelvin–Helmholtz instability.     

Reactor structural response to molten fuel-coolant interactions

In order to realistically determine the structural response of a liquid metal fast breeder reactor to a molten fuel-coolant interaction (MFCI), an MFCI region was incorporated into the two-dimensional, hydrodynamic containment code, REXCO-H. In this way, it is possible to account for the two-dimensional hydrodynamic response, as well as for the effect of vessels and plates, upon the expansion process in the MFCI region.The MFCI model has been extended in order to increase the usefulness of the code under a variety of conditions. The sodium equation of state has been improved using basic thermodynamic relations and recent data to incorporate temperature dependent properties. Heat transfer models available to describe the MFCI include not only a quasi-steady-state model, but also a parametric model, including the fuel heat of fusion. Nonhomogenous MFCI regions can be treated by assigning different parameters to each zone within a region, including volume fractions of fuel, sodium, steel, and void, as well as initial fuel and coolant temperatures and fraction of molten fuel.Several cases have been studied in order to delineate the effect of various parameters on the peak pressures generated in the MFCI zones. These include effect of initial fuel and coolant temperatures, void fraction, amount of molten fuel and/or vessel wall compliance. The response of a typical reactor configuration is evaluated for a given set of initial conditions.     

Fragmentation of continuous molten copper droplets penetrating into sodium pool

The progression of hypothetical core disruptive accidents (CDAs) in metal fuel cores is strongly affected by exclusion of molten metal fuel from the core region due to molten fuel–coolant interaction (FCI). As a basic study of FCI, the present paper focuses on the fragmentation characteristics of continuous molten copper droplets with a total mass from 20 to 50 g penetrating into a sodium pool. The results show that the fragmentation of the continuous molten copper droplets is sensitive to the change of the hydrodynamic and thermal conditions when the instantaneous contact interface temperature (T_i) is lower than the turning point (T_tp) and insensitive at T_i > T_tp. Compared with the fragmentation of a single droplet, the fragmentation of continuous droplets is accelerated and enhanced due to the collision between the droplets and the upward microjets. The present mass median diameter (D_m) or dimensionless mass median diameter (D_m/D₀) of continuous copper droplets shows a distribution with smaller values than those of single copper droplet, and larger values than those of copper jets under similar thermal and hydrodynamic conditions. These results are promising to assure the termination of accidents in CDAs and useful to the core design with enhanced safety in FBRs.     

Molten fuel-coolant interaction during a reactivity initiated accident experiment

The results of a reactivity-initiated accident experiment, designated RIA-ST-4, are discussed and analyzed with regard to molten fuel-coolant interaction (MFCI). In this experiment, extensive amounts of molten UO₂ fuel and zircaloy cladding were produced and fragmented upon mixing with the coolant. Coolant pressurization up to 35 MPa and coolant overheating in excess of 940 K occurred after fuel rod failure. The initial coolant conditions were similar to those in boiling water reactors during a hot startup (that is, coolant pressure of 6.45 MPa, coolant temperature of 538 K, and coolant flow rate of 85 cm³/s). It is concluded that the high coolant pressure recorded in the RIA-ST-4 experiment was caused by an MFCI and was not due to gas release from the test rod at failure, Zr/water reaction, of UO₂ fuel vapor pressure. The high coolant temperature indicated the presence of superheated steam, which may have formed during the expansion of the working fluid back to the initial coolant pressure; yet, the thermal-to-mechanical energy conversion ratio is estimated to be only about 0.3%.     

Fragmentation behavior during molten material and coolant interactions

A safe design for a fast breeder reactor (FBR) requires post-accident heat removal (PAHR) for any potential core disruptive accident (CDA). It is important to ensure that the molten core material solidifies in the sodium coolant in the reactor vessel even if all of the core material has melted. In the present experiment, molten material was injected into water to experimentally obtain the information on the molten material jet entering the coolant and its fragmentation. Visual information was obtained with a high-speed video camera, showing that fragmentation behavior on the side of the jet was different from that on the jet front, and that the injection nozzle diameter significantly influenced the jet breakup length, while the molten jet temperature and the coolant temperature did not influence the jet breakup length. Comparison of the diameters of fragments of the solidified molten material thus obtained with fragmentation theory shows that the median fragment diameter is between the critical Weber number theory and the most-unstable wavelength of the instability theory of surface waves at a gas liquid interface.The quench behavior of the molten jet in coolant was calculated for FBR conditions by using the model that reflects actual fragmentation behavior. It was clarified that the mass of molten material in the coolant pool is related to the fragment diameter under FBR conditions.     

Dissolution behavior of core structure materials by molten corium in boiling water reactor plants during severe accidents

For the decommissioning of the Fukushima Daiichi Nuclear Power Plant, it is necessary to consider the access route to the fuel debris for its removal, which can be determined by knowing the corruption situation of the core support structure. To predict the damage condition of reactor vessel, dissolution behavior of the core structure material should be understood. In this study, the dissolution behavior of core structure materials (stainless steel) by molten metallic corium (stainless steel + B₄C) originated from control rod and its cladding was investigated. As a result of immersion experiment, it was found that there were two types of dissolution mode in this system: (1) chemical dissolution by eutectic reaction between Fe and B and (2) physical dissolution caused by the grains falling off from solid steel due to infiltration of molten metal. Moreover, on the basis of kinetic analysis, it was considered that the chemical dissolution in this system was slow. Therefore, the dissolution is considered to mainly occur through the mechanism that physical dissolution precedes chemical dissolution.     

Analytical tool development for coarse break-up of a molten jet in a deep water pool

A computer code JASMINE-pre was developed for the prediction of premixing conditions of fuel–coolant interactions and debris bed formation behavior relevant to severe accidents of light water reactors. In JASMINE-pre code, a melt model which consists of three components of sub-models for melt jet, melt particles and melt pool, is coupled with a two-phase flow model derived from ACE-3D code developed at JAERI. The melt jet and melt pool models are one-dimensional representations of a molten core stream falling into a water pool and a continuous melt body agglomerated on the bottom, respectively. The melt particles generated by the melt jet break-up are modeled based on a Lagrangian grouped particle concept. Additionally, a simplified model pmjet was developed which considers only steady state break-up of the melt jet, cooling and settlement of particles in a stationary water pool. The FARO corium quenching experiments with a saturation temperature water pool and a subcooled water pool were simulated with JASMINE-pre and pmjet. JASMINE-pre reproduced the pressurization and fragmentation behavior observed in the experiments with a reasonable accuracy. Also, the influences of model parameters on the pressurization and fragmentation were examined. The calculation results showed a quasi-steady state phase of melt jet break-up during which the amount of molten mass contained in the premixture was kept almost constant, and the steady state molten premixed masses evaluated by JASMINE-pre and pmjet agreed well.     

Transformation and fragmentation behavior of molten metal drop in sodium pool

In order to clarify the fragmentation mechanism of a metallic alloy (U–Pu–Zr) fuel on liquid phase formed by metallurgical reactions (liquefaction temperature = 650 °C), which is important in evaluating the sequence of core disruptive accidents for metallic fuel fast reactors, a series of experiments was carried out using molten aluminum (melting point = 660 °C) and sodium mainly under the condition that the boiling of sodium does not occur. When the instantaneous contact interface temperature (T_i) between molten aluminum drop and sodium is lower than the boiling point of sodium (T_c,bp), the molten aluminum drop can be fragmented and the mass median diameter (D_m) of aluminum fragments becomes small with increasing T_i. When T_i is roughly equivalent to or higher than T_c,bp, the fragmentation of aluminum drop is promoted by thermal interaction caused by the boiling of sodium on the surface of the drop. Furthermore, even under the condition that the boiling of sodium does not occur and the solid crust is formed on the surface of the drop, it is confirmed from an analytical evaluation that the thermal fragmentation of molten aluminum drop with solid crust has a potential to be caused by the transient pressurization within the melt confined by the crust. These results indicate the possibility that the metallic alloy fuel on liquid phase formed by the metallurgical reactions can be fragmented without occurring the boiling of sodium on the surface of the melt.     

Development of the models for advection-diffusion of eroded concrete into debris and concrete volume reduction in molten core-concrete interactions

Models for the three-dimensional (3D) advection, diffusion, and volume reduction of eroded concrete into molten core are being developed. As part of the assessment of the reactor interior at TEPCO's Fukushima Daiichi Nuclear Power Plant, analytical models of molten core–concrete interaction (MCCI) to predict locations and condition of molten core (debris) have been improved in the debris spreading analysis (DSA) module of the severe accident analysis code SAMPSON. In addition to the primary model for 3D natural convection with simultaneous spreading, melting, and solidification in an open space, the analysis model to treat phenomena in a closed space, such as debris eroding laterally under concrete floors at the bottom of the sump pits, had been improved. This modeling with practical applicability is referred to as the full-3D MCCI model. This paper presents modeling of the advection and diffusion of eroded concrete into debris melt and calculation processes that were installed for simulating volume reduction when concrete decomposed. They were developed and incorporated into the full-3D MCCI model. The advanced DSA module with the models noted above was validated using MCCI test data. The calculated erosion rates agreed with the test data within a margin of about 16%.     

Simulation of molten metal freezing behavior on to a structure

In the severe accident analysis of liquid metal reactors (LMRs), understanding the freezing behavior of molten metal onto the core structure during the core disruptive accidents (CDAs) is of importance for the design of next-generation reactor. CDA can occur only under hypothetical conditions where a serious power-to-cooling mismatch is postulated. Material distribution and relocation of molten metal are the key study areas during CDA. In order to model the freezing behavior of molten metal of the postulated disrupted core in a CDA of an LMR and provide data for the verification of the safety analysis code, SIMMER-III, a series of fundamental experiments was performed to simulate the freezing behavior of molten metal during penetrating onto a metal structure. The numerical simulation was performed by SIMMER-III with a mixed freezing model, which represents both bulk freezing and crust formation. The comparison between SIMMER-III simulation and its corresponding experiment indicates that SIMMER-III can reproduce the freezing behavior observed on different structure materials and under various cooling conditions. SIMMER-III also shows encouraging agreement with experimental results of melt penetration on structures and particle formation.     

Fuel Fragmentation and Mechanical Energy Conversion Ratio at Rapid Deposition of High Energy in LWR Fuels

The fuel fragmentation is one of the important subjects in the field of molten fuel-coolant interaction (MFCI) since it is one of basic processes of the MFCI, and it has not yet been made clear enough. Accordingly, U02 fuel fragmentation was studied in a postulated reactivity initiated accident (RIA) condition by the Nuclear Safety Research Reactor (NSRR). The distribution of the size of fuel fragments was obtained through the experiments and the mechanism of fuel fragmentation was studied. Also, the relation between the conversion ratio of the mechanical energy to the thermal and the degree of fuel fragmentation was obtained experimentally.

It was revealed that the distribution of fuel fragments was well described in the form of logarithmic Rosin-Rammler's distribution law. The fuel fragmentation was found to be explained by the Weber-type hydraulic instability model and the internal pressurization model. It was also shown that the mechanical energy conversion ratio was inversely proportional to the volume-surface mean diameter defined as the ratio of the total volume of fragments to the total surface, and furthermore that it was influenced by the coolant subcooling and the volumetric ratio of fuel to water.     

Distance for fragmentation of a simulated molten-core material discharged into a sodium pool

A series of experiments has been carried out to obtain experimental knowledge on the distance for fragmentation of a molten core material discharged into the sodium plenum during postulated core disruptive accidents of sodium-cooled fast reactors. In the current experiments, 0.9 kg of molten aluminum (initial temperature: around 1473 K) was discharged into a sodium pool (diameter: 0.11 m, depth: 1 m, initial temperature: 673 K) through a nozzle (inner diameter: 20 mm). Visual observation of the fragmentation behavior was performed using an X-ray imaging system. The following experimental results were obtained. (1) Liquid column of molten aluminum was intensively fragmented almost simultaneously with a rapid expansion of sodium vapor in the vicinity of the column. (2) Due to the intensive fragmentation, penetration of the liquid column was limited to approximately 100 mm or so from the sodium level. (3) The molten aluminum was rapidly cooled after the intensive fragmentation. Based on these results, the distance for fragmentation of the liquid column was estimated to be 100 mm in the experiments. Through the current experiment, useful knowledge was obtained for the future development of an evaluation method of the distance for fragmentation of the molten core material.     

An evolution of molten core cooling strategies

Fundamental mechanisms behind the molten core cooling strategies are revisited to provide an insight for a proper implementation of severe accident management guideline (SAMG) and a development of an engineered safety feature. From the results of a qualitative evaluation and a quantitative plant analysis, weak points of the current severe accident management guideline for an operating plant are identified and a revision of the molten core cooling strategies is proposed. In addition, technical issues for various kinds of core catcher concepts are discussed.     

Phase segregation model and molten pool thermal-hydraulics during molten core-concrete interaction

A new model for melt thermal-hydraulics during molten core concrete interaction (MCCI) is presented. This model assumes that phase segregations occur in the melt, leading to a crust formation composed of refractory materials (UO₂ZrO₂). The interface temperature between this crust and the liquid melt is linked to the solid fraction and is calculated on the basis of a thermal equilibrium assumption. The solid fraction is also controlled by conduction heat transfer through the solid crust. It is shown that the temperatures measured in the ACE experiments are recalculated within a maximum deviation of 10%, (when referenced to the solidus temperature) without any adjustment. Other important consequences for this new approach are outlined: for physical properties, physico-chemical interactions, fission products behavior, mixing with sacrificial materials, and crust stabilities.     

反应堆压力容器外水冷条件下贯穿件完整性分析

严重事敝下堆芯熔融物坍塌到反应堆压力容器(RPV)下封头时,可能造成贯穿件因高温熔融物热侵袭而失效,使压力容器丧失完整性,熔融物进入到反应堆堆腔中,导致熔融物堆内滞留(IVR)失效.在分析贯穿件脱落和熔融物流入贯穿件两种失效模式基础上,分别运用VTA程序和修正的整体凝固模型(MBF)计算贯穿件焊缝的熔化程度、热膨胀产生的摩擦力,估算贯穿件内熔融物流动的距离.结果表明,在成功实施反应堆压力容器外水冷(EVVC)措施条件下,300 MW压水堆核电厂压力容器的下封头不会因贯穿件失效而丧失完整性,堆芯熔融物小能通过贯穿件失效向堆腔迁移.     

Transient cooling of shallow debris beds in the first two FARO-LWR experiments

The investigation of the early behaviour of reactor core debris beds in high pressure water and the heating of the bottom plate on which the debris rests are important aspects of the FARO-LWR out-of-pile tests. In the first two tests 18 and 44 kg of molten U0₂-ZrO₂ fell through 1 m of water and led to the fragmentation of of the melt in both tests and to the formation of shallow debris beds of 3 and 7 cm in height. The unfragmented melt ended up as frozen cakes on the bottom plate. The latter got heated rather little in comparison with the heating of the overlying water by the debris bed. From global energy balances of these tests, heat fluxes from the transient debris beds to the water could be evaluated. They turned out to be between 7.8 and 10.5 MW m⁻² during the first 10 s and about 3.9 MW m-⁻² from 10 to 24 s. In particular the first values are considerably higher than dryout heat fluxes measured in experiments with steady state debris beds. This implies that the shallow and initially very hot FARO debris beds must have cooled down rapidly and that they did not dry out significantly. Probably some fluidization occurred in the early life of these beds.     

熔融液滴与水作用细粒化实验研究

针对核反应堆发生严重堆芯熔化事故时可能发生的燃料与冷却剂的相互作用以及蒸汽爆炸的复杂过程,对高温熔融金属液滴与水作用的细粒化过程进行了实验研究.对不同工况下实验产物的形状进行了比较分析,并对熔融液滴初始温度、水温、下落高度及材料物性对细粒化过程的影响进行了研究.本文还采用高速摄像仪对熔融液滴的细粒化过程进行了观测.结果表明:熔融液滴初始温度、水温和材料物性对细粒化程度的影响较大;本实验参数范围内下落高度对细粒化程度的影响不大.     

Fragmentation of a single molten metal droplet penetrating into sodium pool. IV. Thermal and hydrodynamic effects on fragmentation in copper

To characterize the relationship between thermal and hydrodynamic effects on fragmentation of molten metallic fuels, with the interaction of the sodium coolant under a wide range of thermal and hydrodynamic conditions, in this paper, we focus on the fragmentation characteristics of a single molten copper droplet (1 and 5 g) with an ambient Weber number (We _a) from 102 to 614 and superheating conditions from 15 to 574°C, which penetrates into a sodium pool at an initial temperature from 298 to 355°C. In our experiments, fine fragmentations of the single molten copper droplets with a high We _a were clearly observed even under a supercooled condition that is well below the copper melting point of 1083°C. The dimensionless mass median diameters (D _m /D ₀) of molten droplets with a high We _a are less than the molten droplets with a low We _a under the same thermal condition. When We _a was approximately >200, the hydrodynamic effect on fragmentation became dominant over the thermal effect under a relatively low superheating condition. For a higher We _a range, the comparisons indicated that the fragment sizes of the molten copper droplets had similar distributions to those of copper and metallic fuel jets and stainless steel droplets even with different thermophysical properties and a 1000-fold mass difference, which implied the possibility that the fragment size characteristics of the molten metal jets could be evaluated by the interaction of a single droplet with the sodium coolant without consideration of dropping modes and mass.     

A thermal stress mechanism for the fragmentation of molten UO2 upon contact with sodium coolant

An important aspect of fuel-coolant interaction problems relative to various hypothetical LMFBR accidents is the fragmentation of molten oxide fuel on contact with sodium coolant. In order to properly analyze the kinetics of such an event, an understanding of the breakup process and an estimate of the size and dispersion of such fragmented fuel must be known. A thermal stress initiated mechanism for fragmentation is presented using elastic stress theory for the cases of both temperature-dependent and independent mechanical properties. Included is a study of the effect of the choice of surface heat transfer boundary condition and the compressibility of the unsolidified inner core. Results of parametric calculations indicate that the thermal stresses induced in the thin outer shell and the pressurization of the inner molten core are potentially responsible for the fragmentation. For UO₂ in Na the calculated stresses are extremely high, while for aluminum in water they are much smaller and a strong function of the surface heat transfer boundary condition. Qualitatively, these results compare favorably with small scale dropping experiments, that is, molten UO₂ quenched in Na undergoes fragmentation while aluminum in water usually results in little breakup. The experimentally observed increase in breakup with decreasing coolant temperature is also in qualitative agreement with the thermal stress-induced mode of fragmentation.