Cold Crucible Research Articles

Thermoconvective flow of molten glass heated by direct induction in a cold crucible

Thermoconvective flow of molten glass heated by direct induction in a cold crucible

Solution Growth of 3C-SiC Single Crystals by Cold Crucible Technique

We have successfully grown 3C-SiC(111) single crystals 10mm x 10mm in dimension on 6H-SiC(0001) substrate by the solution growth method using cold crucible technique. The growth rate of 60μm/hr was achieved. The use of Si-Ti-C ternary solution as well as the electromagnetic stirring are responsible for the relatively high growth rate in solution method. The threading dislocation density is low and the etch pit density amounts to 105-106 /cm2 at the lowest region. Polytype of the grown layer has changed from 3C to 6H with an increase in the dip depth of substrate. A mathematical model was applied to get better understanding of what happened in the crucible.

Steam Explosion Experiments Using Nuclear Reactor Materials in the TROI Facilities

A series of TROI steam explosion experiments was performed using various prototypic melts. The melt was pure zirconia, eutectic corium (70: 30 weight percent of UO2: ZrO2) or iron-added eutectic corium. In this series, an experiment with pure zirconia, two experiments with eutectic corium, and three experiments with iron-added corium were carried out. A steam explosion was found to be somewhat related to the melt composition and an external triggering. As with most of the previous tests, zirconia melt led to a steam explosion again. It is quite certain that a zirconia melt will more than likely result in an energetic steam explosion. Meanwhile, eutectic corium led to an energetic steam explosion by applying an external trigger, but it had a weak steam spike without an external trigger. The explosivity of eutectic corium cannot be ignored, as an external trigger led to an energetic steam explosion. Iron-added corium did not lead to an energetic steam explosion. The reason for this is likely to be a relatively low melt temperature (superheat) when compared to zirconia melt or oxidic corium melt, resulting from the melting method used in the TROI experiments, an induction heating applied to a cold crucible. The iron-added corium at a low temperature was solidified easily so a steam explosion did not occur.

Characterization of Ti‐Ta Alloys Synthesized by Cold Crucible Levitation Melting

Characterization of Ti‐Ta Alloys Synthesized by Cold Crucible Levitation Melting

To eliminate the composition transient zone in directional solidification of TiAl alloys

The technique of directional solidification (DS) with cold-crucible and seeding is used to control the lamellar orientations in the solidified structures of TiAl alloys. However, the concentration transient zone (CTZ), a characteristic of alloy DS, may result in failure of the structure control because the CTZ separates the seed and the directionally solidified peritectic structure of TiAl alloys due to the β-phase formation. A composition adjusting method is proposed to eliminate the CTZ in this paper. Measurements were carried out to determine the macro-composition distributions in the directionally solidified Ti–Al%–Si% samples with and without the composition adjustment. The CTZ length measured in the directionally solidified sample without composition adjustment shows to be comparable to that predicted by a zone-melting-equivalent modeling performed in the present work. The experimental results for the CTZ control in zone-melting-like DS process with a cold crucible are exhibited. These results show that the CTZ control is feasible.

Vitrification of a simulator of Savannah River site (USA) wastes with high iron and aluminum content on bench and commercial facilities with a cold crucible

Bench and commercial-facility experiments have been performed on cold-crucible vitrification of a simulator of high-level wastes from the Savannah River site (USA). The wastes contained up to 29 mass% Fe2O3 and 26 mass% Al2O3. The specific product flow reached 1700 and 2450 kg/(m2·day) with specific energy consumption 14–16 and 9–10 kW·h/kg, respectively. The crucibles did not undergo any appreciable corrosion during the period of the work performed and are reusable. The product consisted of a borosilicate matrix, containing up to 10 vol.% crystalline phase of spinel. The method of induction melting in a cold crucible is especially effective for crucibles with a large diameter, since the specific productivity increases and the specific energy consumption on the vitrification of high-level wastes decreases.

The Simulation of Thermal Field in Induction Melting with Cold-Crucible and Dual Frequencies

A study of thermal field in induction melting with cold crucible and dual induction frequencies is presented in this paper. Numerical simulation was done by finite element based software COMSOL with use of multi-physical modeling. A method of superposing of magnetic vector potential is proposed to solve the complexity of dual induction frequencies. Different conditions of high and low frequencies were given in the calculation to find the effects on distribution of thermal fields. The computed temperature distribution was compared with the macro etched section of solidified ingots and the results showed that the orientation of dendritic grain structures follow to the similar direction of heat flow. This indicates that proper set of induction coils could enable more uniform thermal field distribution and attaining controlled solidification morphology in the ingots.

Dynamic melting model for small samples in cold crucible

PurposeA small size cold crucible offers possibilities for melting various electrically conducting materials with a minimal wall contact. Such small samples can be used for express contamination analysis, preparing limited amounts of reactive alloys or experimental material analyses. Aims to present a model to follow the melting process.Design/methodology/approachThe presents a numerical model in which different types of axisymmetric coil configurations are analysed.FindingsThe presented numerical model permits dynamically to follow the melting process, the high‐frequency magnetic field distribution change, the free surface and the melting front evolution, and the associated turbulent fluid dynamics. The partially solidified skin on the contact to the cold crucible walls and bottom is dynamically predicted. The segmented crucible shape is either cylindrical, hemispherical or arbitrary shaped.Originality/valueThe model presented within the paper permits the analysis of melting times, melt shapes, electrical efficiency and particle tracks.

Numerical studies of the melting process in the induction furnace with cold crucible

PurposeAims to present recent activities in numerical modeling of cold crucible melting process.Design/methodology/approach3D numerical analysis was used for electromagnetic problem and 3D large eddy simulation (LES) method was applied for fluid flow modeling.FindingsThe comparative modeling shows, that higher H/D ratio of the melt is more efficient when total power consumption is considered, but this advantage is held back by higher heat losses through the crucible walls. Also, calculations reveal that lower frequencies, which are energetically less effective, provide better mixing of the melt.Originality/value3D electromagnetic model, which allows to take into account non‐symmetrical distribution of Joule heat sources, together with transient LES fluid flow simulation gives the opportunity of accurate prediction of temperature distribution in the melt.

The growth of ZnO crystals from the melt

The peculiar properties of zinc oxide (ZnO) make this material interesting for very different applications like light emitting diodes, lasers, and piezoelectric transducers. Most of these applications are based on epitaxial ZnO layers grown on suitable substrates, preferably bulk ZnO. Unfortunately the thermochemical properties of ZnO make the growth of single crystals difficult: the triple point 1975 ∘ C , 1.06 bar and the high oxygen fugacity at the melting point p O 2 = 0.35 bar lead to the prevailing opinion that ZnO crystals for technical applications can only be grown either by a hydrothermal method or from “cold crucibles” of solid ZnO. Both methods are known to have significant drawbacks. Our thermodynamic calculations and crystal growth experiments show, that in contrast to widely accepted assumptions, ZnO can be molten in metallic crucibles, if an atmosphere with “self-adjusting” p O 2 is used. This new result is believed to offer new perspectives for ZnO crystal growth by established standard techniques like the Bridgman method.

Numerical modeling of coupled phenomena in a mechanically stirred molten-glass bath heated by induction

A mechanically stirred molten-glass bath heated by direct induction in a cold crucible was numerically modeled. The aim of the study was to develop numerical tools to understand thermal, hydrodynamic and electromagnetic phenomena that occur in the bath. Models and coupling between these phenomena are described. This coupling and the high content of elements in the 3D mesh result in a long calculation time. The study demonstrates how to couple programs to yield the highest degree of accuracy in the shortest calculation time possible. Numerical studies are also used to characterize the fluid dynamic behavior and heat transfer in an industrial-size tank. Classical correlations commonly used to characterize stirrer efficiency and heat transfer for fluids with constant physical properties were adapted for molten glass. The power number N p and the Nusselt number Nu are used as macroscopic indicators. The results of these global studies will be useful for the operation and optimization of the vitrification facilities.

Dominant Factor for the Occurrence of a Steam Explosion

For the study of a steam explosion phenomenon in a nuclear reactor, prototypic corium, a mixture of UO2 and ZrO2 was melted in a cold crucible by applying an induction heating technique. The molten corium was then poured into cold water. It was fragmented into very small particles, so called debris, which enables a very rapid heat transfer to the water. Some cases led to steam explosions by thermal expansion of the water. After the tests, all the debris particles were dried and classified by their size. From the analysis by using EPMA, it was shown that the particles generated by a steam explosion had fine and irregular forms. It is known that real corium (including UO2) hardly leads to a steam explosion, different from pure ZrO2 or metal. A reason for this was previously suggested in that the corium generated hydrogen gas during melt-water interaction, and it enclosed the melt drops to prevent a direct contact of the corium and water. In order to confirm this fact, the debris particles were analyzed with ICP-AES for their typical element contents, EPMA for the homogeneity of the solid solution, XRD for the chemical compounds, and TGA and hydrogen reduction analysis for the percentage of the debris oxidation and reduction. These analyses showed that hydrogen was not directly related to steam explosion. Meanwhile, the material characteristics of the corium compositions are newly suggested to be the most probable reason for the occurrence of a steam explosion so far.

Cold Crucible Vitrification of Uranium-Bearing High Level Waste Surrogate

AbstractThree tests on vitrification of U-bearing DWPF SB2 and SB4 waste surrogates (40-50 wt% waste loading) in a lab-scale (56 mm inner diameter) copper cold crucible with melt surface area of 24.6 cm2 energized from a 5.28 MHz/10 kW generator were conducted. At the feeding of the air-dry batch (3-5 wt% moisture) glass productivity and specific melt productivity achieved under steady-state conditions were 0.64 kg/h and 256 kg/(m2×h) [6150 kg/(m2×day)], respectively. Similar to previously prepared U-free products, the U-bearing vitrified SB2 (highferrous) waste product was composed of U-enriched glassy matrix and high-ferrous spinel structure phase. As expected, the vitreous phase in the vitrified SB2 waste surrogate was enriched with Na, Al, Si, P, S, Ca, Sr, Pb, U oxides and Cl, whereas spinel preferentially accumulated transition metals (iron group elements: Mn, Fe, Ni) as well as Cu and Zn oxides. Iron oxide content in the spinel phase was ∼67–71 wt %. This result was consistent with XRD data and implies that this spinel was close to magnetite with respect to chemical composition and structure. This was consistent with our previous data on characterization of glass crystalline materials containing both U-bearing and U-free SB2 waste surrogate produced in resistive furnace and cold crucibles. The vitrified SB4 (high-alumina) waste products were predominantly amorphous at waste loading of up to 50 wt %. Traces of quartz occurred in the “skull” zone. In all the products, uranium was found to be uniformly distributed within the vitreous phase. Leach rates of Na, Si, Fe, and U from the vitrified SB2 waste surrogate (50 wt % waste loading) determined by a SPFT procedure were found to be 1.38, 1.42, 0.122, and 0.841 (g×m−2×d−1), respectively.

Thermal and Hydrodynamic Analysis of the Melting Process in the Cold Crucible Using 3D Modeling

Thermal and Hydrodynamic Analysis of the Melting Process in the Cold Crucible Using 3D Modeling

Nuclear Waste Immobilization by Vitrification in a Cold Crucible Melter: 3D Magnetic Model of a Melter

AbstractThe design and development of prototype cold crucible melters for waste vitrification are based on models of the basic physical phenomena, including electromagnetic induction and the thermal and hydraulic properties in natural or forced convection. The complexity of new nuclearized facilities results in significant errors on the results of predictive models based on 2D axisymmetric geometry that can only be resolved by modeling the device in 3D geometry. This document discusses the specification and electromagnetic design of a melter carried out using electromagnetic computation software, FLU3D, developed in 3D geometry by Cedrat. The principles and results of this study are directly applicable to nuclear facilities with allowance for the particular requirements of a nuclearized environment.

Glass Formulation Development in Support of Melter Testing to Demonstrate Enhanced High Level Waste Throughput

AbstractThe U.S. Department of Energy (DOE) is currently processing high-level waste (HLW) through a Joule-heated melter (JHM) at the Savannah River Site (SRS) and plans to vitrify HLW and Low activity waste (LAW) at the Hanford Site. Over the past few years at the Defense Waste Processing Facility (DWPF), work has concentrated on increasing waste throughput. These efforts are continuing with an emphasis on high alumina concentration feeds. High alumina feeds have presented specific challenges for the JHM technology regarding the ability to increase waste loading yet still maintain product quality and adequate throughput. Alternatively, vitrification technology innovations are also being investigated as a means to increase waste throughput. The Cold Crucible Induction Melter (CCIM) technology affords the opportunity for higher vitrification process temperatures as compared to the current reference JHM technology. Higher process temperatures may allow for higher waste loading and higher melt rate.Glass formulation testing to support melter demonstration testing was recently completed. This testing was specifically aimed at high alumina concentration wastes. Glass composition/property models developed for DWPF were utilized as a guide for formulation development. Both CCIM and JHM testing will be conducted so glass formulation testing was targeted at both technologies with a goal to significantly increase waste loading and maintain melt rate without compromising product quality.

Characterization of the glass-ceramic material prepared upon vitrification of an iron-containing surrogate of high-level wastes in a cold crucible

Vitreous materials are prepared by cold crucible induction melting of a surrogate of high-level wastes (from the Savannah River Site, United States) and a borosilicate glass frit taken in mass ratios from 45: 55 to 60: 40. According to the X-ray diffraction and electron microscopic data, the vitreous materials thus produced consist of a glass matrix and a magnetite-type spinel enriched in transition elements. The degree of crystallinity of the materials increases with an increase in the waste oxide content from 6 to 18–20 vol %. The vitreous materials are characterized by a high chemical durability, which decreases only at high contents of the waste oxides (55 wt % and higher) due to the formation of an additional nepheline phase.

Influence of oxygen on solidification behaviour of cast TiAl-based alloys

The influence of oxygen on solidification behaviour of cast TiAl-based alloys containing 40–48 at.% of Al was studied. Twelve alloys with fixed Ti:Al ratios ranging from 1.08 to 1.5 and oxygen content of 0.1, 0.8 and 1.5 at.% were prepared by induction melting and casting in a cold crucible under protective atmosphere. Increasing oxygen content affects significantly the macrostructure of the as-cast ingots, increases volume fraction of the α phase formed during peritectic solidification and leads to a change of the β primary solidification phase to the α phase in ternary Ti–44.2Al–1.4O, Ti–47.3Al–0.9O and Ti–47.2Al–1.5O (at.%) alloys. In the case of the alloy with the α primary solidification phase, the partition coefficients achieve values of k Al / α s / l = 0.9 and k O / α s / l = 1.29 . In alloys containing 1.5 at.% of oxygen, it has been observed that the β/α solidification phase transition is shifted to a lower aluminium contents when compared to that of binary systems. Oxygen extends the stability of single α phase region.

LPE growth of AlN single crystal using cold crucible under atmospheric nitrogen gas pressure

We have grown AlN single crystal on 6H-SiC substrate using Al-Cu solution of 1500-1600 °C. Nitrogen is supplied from gas phase under atmospheric pressure and the cold crucible technique is applied. Thickness of AlN reached 40 μm with the growth rate of 10 μm/h. Rather high growth rate in solution method is due to the increase in the melt surface area, dynamic convection of the melt and high temperature gradient along the seeded axis, which are easily realized in cold crucible technique. TEM observation reveals that the dislocation density beyond 10μm from the interface decreases by a factor of 10 to the power of -1 or -2. The melt included in between the substrate and AlN epi-layer eases stress in epi-layer and reduces cracks. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Vitrification of a surrogate for high-level wastes from the Savannah River facility (USA) in a commercial cold-crucible facility

Experiments on vitrifying a surrogate for SB2 pulp from the Savannah River facility (USA) in a cold crucible with inner diameter 418 mm in a commercial facility for vitrifying medium-level wastes at the Moscow Scientific and Industrial Association Radon are performed. Borosilicate glass materials, containing 50 mass % oxides of the wastes, including a magnetitic spinel phase in amounts not exceeding 15 vol. %, are obtained. The maximum mass average velocity of the slurry load and the melt output reach 40 and 16 kg/h, respectively. This corresponds to maximum specific vitrified wastes production capacity 2830 kg/(m2·day). The specific energy consumption for obtaining the glass product is about 10 kW·h/kg, which is approximately half the level for reprocessing slurries in crucibles with half the diameter. The chemical stability of the glass materials is 10–50 times higher than that of the materials made from glasses recommended by the US Environmental Protection Agency.