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Abstract: T liquidus was proposed as the interface temperature (), for various severe accident situations for thermalhydraulic steady state. This proposal was made on the basis of the analysis of solidification front stability in thermalhydraulic steady state for volumetrically heated corium pools and was extended to reactor transients with slow solidification rates that are controlled by the long-term decrease in residual power. The conclusions were corroborated by prototypic corium and variable solidification rates obtained by experimental approaches () for corium containing small amounts of silica or none at all. When the concentration in silica increases (approximately above 10wt%), it was concluded from the experiments that a plane-front situation could not be obtained. The present work offers a theoretical approach to the maximum time delay that is necessary for mass transfer and full phase-segregation in volumetrically heated liquid pools bounded by a crust. It is concluded that full segregation is obtained for in-vessel situations within time delays that are shorter or of the same order of magnitude as the characteristic time for the corium pool to form and evolve to a quasi-steady-state situation. The characteristic time delay for mass transfer associated with simulant material experiments is also determined. Phase segregation can also be obtained for corium–concrete interaction, provided that the silica content is less than approximately 10wt%. However in the latter case, more complex phenomena occur at the interface due to the interaction with sparging gas (such as porous medium formation) which requires a different model approach. [Copyright &y& Elsevier]

This paper addresses the phenomenon of random packing of sedimenting grains in the context of metallic alloy solidification from a dynamics perspective. More precisely, we investigate the evolution of the grains from the initial steady-state sedimentation until the final mechanical equilibrium-packing-is achieved. The packing dynamics of two grain geometries (spherical and nonconvex) is investigated for a hydrodynamic condition of a low ratio of grain inertia to viscous dissipation, given by a low Stokes number, St, of o(10^{-3}), which is typical in solidification. An experimental setup that consists of a vertical column filled with glycerol is employed to reproduce the packing of the sedimenting grains by means of two different protocols (sequential and collective), enabling to track the trajectory of the grains. The grain packing is also simulated by means of a discrete element method for both grain geometries in case of collective packing protocol, complementing the experimental work-trajectory tracking-with the time evolution of the local solid fraction, of the local number of contacting neighbors and of the grain orientation during the packing dynamics.

The primary phase grain size is a key parameter to understand the formation of the macrosegregation pattern in large steel ingots. Most of the characterization techniques use two-dimensional measurements. In this paper, a characterization method has been developed for equiaxed dendritic grains in industrial steel castings. A total of 383 contours were drawn two-dimensionally on twelve 6.6 cm²slices. A three-dimensional reconstruction method is performed to obtain 171 three-dimensional grains. Data regarding the size, shape and orientation of equiaxed grains is presented and thereby shows that equiaxed grains are centimeter-scale complex objects. They appear to be a poly-dispersed collection of non-isotropic objects possessing preferential orientations. In addition, the volumetric grain number density is 2.2×107 grains/m3, which compares to the 0.5×107 grains/m3 that can be obtained with estimation from 2D measurements. The 2.2×107 grains/m3 value is ten-times smaller than that previously used in the literature to simulate the macrosegregation profile in the same 6.2 ton ingot.

The packing of free-floating crystal grains during solidification has a strong impact on the phase-change process as well as on the structure and the defects in the solidified material. The packing fraction is affected by the particular dendritic morphology of the grains and by their low inertia resulting from the small density difference between solid and liquid. Understanding the grain packing phenomenon during metal alloy solidification is not experimentally possible since packing is coupled to many other phenomena. We therefore investigate the packing of equiaxed dendrites on a model system, consisting of fixed-shape nonconvex model particles sedimenting in conditions hydrodynamically similar to those encountered in solidifying metals. We perform numerical simulations by a discrete-element model and experiments with transparent liquids in a sedimentation column. The combination of experiments and simulations enables us to determine the packing fraction as a function of (i) the grain morphology, expressed by a shape parameter, and (ii) the hydrodynamic conditions, expressed by the particle Stokes number.