Motivated from the fuel–coolant interaction phenomena in boiling water reactors, in the present work, effect of natural convection flows, set during the melt dripping, on the nature of formation of debris bed, of solidified particles, at the bottom floor is studied. Standard shape solid particles are used to simulate the dripping melt and their paths are tracked using a particle tracking technique to acquire additional data such as particles velocity, travel time and path. The experimental studies performed on PDS-P facility are designed to study the separate effects and generate the data for codes validation. A novel particle tracking technique allowed quantification of kinetic properties for every particle. The results helped in refinement of the particles distribution on the floor, quantifying the debris bed shape. Higher pool depths and natural convection flows rates are seen effective in enhancing the distribution of debris particles, creating shallow and well spread debris bed.

The jet fragmentation is a crucial process during fuel coolant interactions (FCI) which result in droplets dispersed in coolant and may cause a steam explosion. The present work deals with experimental studies of metallic melt jet fragmentation followed by numerical simulations that support the experimental findings. Two test series were carried out at both large and small scales, to achieve significant variation in Weber number (We). It was found that the jet fragmentation patterns followed the R-T and K-H instabilities at lower and higher We, respectively. Various methods were applied to estimate jet breakup length at different scales, due to measurement challenges at larger We (large scale). At smaller scale, an averaged jet breakup length was proposed to consider the dynamic behavior of coherent jet. Early solidification of low superheat melt interacting with water having moderate subcooling was observed in the forms of frozen jet and large-size fragments. The numerical simulations considering only hydrodynamic effects (no consideration of vapor film) reproduced the jet fragmentation patterns but under-estimated jet breakup lengths from the experiments, probably due to the absence of vapor film around the melt jet in simulations, which differs from the reality where vapor film around the melt delay the instabilities development and hence the jet breakup.

During a hypothetical severe accident in a light water reactor, the core and internal structures melt down and the molten materials (corium) may fall into a water pool, forming a debris bed from so-called fuel coolant interactions (FCI), whose characteristics are important to corium coolability. The present study is concerned with the characterization of an ex-vessel debris bed forming form the FCI of a metal rich corium jet falling into a water pool in the reactor cavity. Five scoping tests were carried out on the DEFOR-M test facility using metallic Tin melt at different superheats. The molten tin was employed as the simulant of metal (Zr/Fe) rich corium melt. The processes of melt jet fragmentation and debris formation was recorded by high-speed cameras. The contour and volume of the resulting debris bed were measured by a three-dimensional laser scanning system, and the debris particles were sieved for their size distribution. The experimental results revealed the effects of melt superheat and water subcooling on the characteristics of a debris bed, including the bed's configuration and porosity, the particles' morphology and size distribution. A preliminary comparison of debris bed characteristics between metallic melt and oxidic melt was also provided.

This paper investigates the dynamics of melt infiltration through a preheated porous debris bed which is of importance to severe accident modeling in nuclear power plants. Proper understanding of the flow physics and affecting parameters is needed to define flow regime(s) according to combination of the driving forces, i.e. capillary and gravity. A model development and validation therefore should consider various effects and competing mechanisms. After a careful study of the governing equations and scaling rules, a known analytical model is validated against existing experimental data from REMCOD experiment. The predictions of this model are in good agreement with the experimental data. Furthermore, a global sensitivity analysis identifies the most influential parameters and reveals the need for further experiments with different range of affecting parameters. The results underline the importance of permeability as the most influential parameter.

The re-melting of multi-component debris is important for both in-vessel and ex-vessel phases of severe accident progression in nuclear power plants. However, current knowledge is limited with respect to understanding the associated complex phenomena and their interactions. In this paper, the phenomenon of melt infiltration through a porous debris bed with and without solidification is examined by synthesizing the data obtained from ongoing experimental research (REMCOD facility). In this regard, results obtained from 12 experiments are analyzed. Eight tests were conducted for melt infiltration through debris at temperatures above solidification. At this condition, two flow regimes are identified for the melt flow inside the hot porous debris, which is initially dominated by capillary forces and hydrostatic head and then later by the gravity forces. In addition, 4 tests were performed for melt penetration into cold debris where melt infiltration is limited by solidification. It was found that the depth of penetration is correlated with the difference between "sensible heat of melt" and "the amount of heat required to heat the bed up to the melting point of specific melt composition."

Spray cooling is a versatile technology for various cooling applications involving high surface heat fluxes. Experimental facility was built to study heat transfer performance of an upward multi-nozzle array of water sprays impacting a surface of heated plate made of reactor vessel grade steel. The effect of inclination angles of the steel surface on the cooling performance was investigated to assess heat transfer in complex semispherical/ semielliptical geometry of large reactor lower head and to address possible application of spray cooling in severe accident management (SAM) of light water reactors (LWRs) based on In-vessel melt retention with external reactor vessel cooling (IVR-ERVC). Joule heating of SA302B steel foil of 0.15 mm thickness and surface area of 96 cm² enabled prototypic heat fluxes to be evacuated during reactor accident. A 2×3 array of full jet narrow-coned pressure-swirl spray nozzles was used to reproduce multi-nozzle cooling. The tests were conducted as a series of consequent steady states realized at stepwise increasing power and surface heat fluxes up to the maximum values of 29 kW and 2.97 MW/m² limited in the specific facility design. Seven surface inclinations, between 0^o and 90^o were tested and no significant variations in spray cooling performance with the inclination of the heated surface was found. The results indicated a promising prospect of using a multi-nozzle array system for cooling of large surface area of reactor lower head. Much higher heat fluxes can be safely extracted by spray cooling in comparison with the critical heat fluxes that appeared at RPV water pool cooling at natural convection. The maximum value of heat flux at direct spray impact zones and its drop-off slightly from the center to the periphery of the spray cone was detected in the tests. The water flow rate and liquid subcooling significantly influenced maximum steel surface temperature but had no noticeable effects on surface temperature uniformity. The spray-to-spray interaction had no observable effects on local surface temperatures, however, the colliding zones where four spray cones have visible effects on local surface temperatures due to poor liquid momentum. The results also showed that more uniform liquid film distribution could be obtained for some inclinations because of improved liquid drainage, which in turn leads to maintaining low surface temperatures. 

Cooling by water spray is a well-known technology that can reach significantly higher Critical Heat Flux (CHF) compared to other cooling methods. For the light water reactor safety, the in-vessel retention (IVR) by external reactor vessel cooling (ERVC) is a comprehensive severe accident management strategy to arrest and confine the corium in the lower head of the reactor pressure vessel. Heat fluxes up to 1.5 MW/m² have already been assumed attainable in low-power nuclear reactors while cooling required in high-power reactors is expected to reach 2.5 MW/m². Instead of reactor lower head flooding and relying on cooling due to natural convection, a viable and more efficient alternative is to spray the external surface of the vessel. Given all the advantages of spray cooling reported in the literature, a lab-scale experimental facility was built to validate the efficiency of multi-nozzle spray cooling of a downward-facing heated surface inclined at different angles up to 90^o. The facility employed a 2×3 matrix of spray nozzles to cool the FeCrAl alloy foil with an effectively heated surface area of 96 cm² using water as the coolant. Heat loads and surface inclinations were varied parameters in the test matrix. The results show that no significant variations in spray cooling performance concerning the inclination of the heated surface. A surface heat flux of 2.5 MW/m² was achieved at every inclination of the downward-facing surface. The results also indicate that more uniform liquid film distribution could be obtained for some inclinations, which in turn leads to maintaining low surface temperature. The obtained surface heat flux margin by spray cooling indicates that it is feasible to adopt IVR-ERVC strategy for a large power reactor.

Key phenomena in the cooling states of underwater debris beds were classified based on the premise that a target debris bed has a complicated geometry, nonhomogeneous porosity, and volumetric heat. These configurations may change due to the molten jet breakup, droplet agglomeration, anisotropic melt spreading, two-phase flow in a debris bed, particle self-leveling and penetration of molten metals into a particle bed. Based on these classifications, the modular code system THERMOS was designed for evaluating the cooling states of underwater debris beds. Three tests, DEFOR-A, PULiMS, and REMCOD were carried in six phases to extend the existing database for validating implemented models. Up to Phase-5, the main part of these tests has been completed and the test plan has been modified from the original one due to occurrences of unforeseeable phenomena and changes in test procedures. This paper summarizes the entire test plan and representative data trends prior to starting individual data analyses and validations of specific models that are planned to be performed in the later phases. Also, it tries to timely report research questions to be answered in future works, such as various scales of melt-coolant interactions observed in the shallow pool PULiMS tests.

Topic of present study emerges from the hypothetical nuclear severe accident in light water reactor (LWR) where molten fuel core may interact with a reactor pool causing steam explosion. Fundamental research is important to substantiate the knowledge of steam explosion energetics. In order to get the basic understanding of single droplet behavior during steam explosion phenomena, the present study is carried out to deal with hydrodynamic deformation of a droplet during preconditioning, the initial phase of fuel coolant interactions. The three-dimensional simulations are carried out with the help of volume of fluid (VOF) method using the CFD code ANSYS FLUENT. The deformation modes and deformation rate of a droplet initialized with different Weber numbers (We) are analyzed, followed by the separate-effect studies of droplet density and viscosity. As a next part, a droplet is exposed to high-intensity pressure pulses in order to study the deformation/breakup of the droplet at sudden accelerations. Finally, the deformation rates of the droplets starting solidification are analyzed, where the droplets are modeled with a modified surface tension.

During a hypothetical severe accident of a light water reactor (LWR), molten corium could fall in the form of jet into a water pool. The jet fragmentation is crucial process during fuel coolant interactions (FCI) which fragment into droplets and disperse in the coolant, and it may cause a steam explosion. This paper deals with a study of computational fluid dynamics on the melt jet falling into a water pool in order to get qualitative and quantitative understanding of initial premixing phase of FCI. The preliminary objectives to pursue are modeling of jet fragmentation and estimation of the jet breakup length. A commercial CFD code ANSYS FLUENT 14.0 is used for the 2D numerical analysis employing Volume of Fluid (VOF) method. The problem and simulation conditions are similar to the jet breakup tests carried out at KTH (Manickam et al., 2014). Initially, a fragmentation/breakup pattern of the jet is discussed. Further, the effect of jet diameter and the jet injection velocity on the jet breakup length is studied, with a wide range of ambient Weber number (We_a) from 1.25 to 1280. The numerical results compared with the experimental data are in a reasonable agreement. The impacts of physical properties of melt (density, viscosity and surface tension) on the jet breakup lengths are also investigated and presented. Finally the droplet size distributions are obtained based on the simulation results. These preliminary data may be helpful to substantiate the understanding of the phenomena during melt jet interactions with coolant.