Hafnium solubility determination in soda-lime aluminosilicate glass

Abstract

The solubility of hafnium dioxide (HfO 2 ), used as an uranium surrogate, is measured in glass melts belonging to the CaO-Al 2 O 3 -SiO 2 and Na 2 O-CaO-Al 2 O 3 -SiO 2 systems, under oxidizing and reducing conditions. Two methods have been carried out to determine it and the kinetic factors controlling the HfO 2 dissolution in glass melt have been investigated in order to approach equilibrium. The solubility ranges from 3 to 6.5 mol% HfO 2 in aluminosilicate glasses at temperature between 1250°C and 1400°C, and is not affected by the redox conditions. Conversely, the solubility is modified by the melting temperature and the glass composition. The excess of alkalies or alkaline earths which are not involved in the charge balance of AlO 4 tetrahedrons in the silica network appears to play a significant role. Glass homogeneity is checked by scanning electron microscopy and X-ray diffraction. O 7 are metastable crystals observed in the glass melts. The stability of those crystalline phases mainly depends on the glass composition. KeywordsHafnium oxide, solubility, aluminosilicate glass melt, reducing conditions, crystallization.to optimize the Hf-doped glasses elaboration conditions in order to approach equilibrium. The second objective is to determine hafnium solubility as a function of the melting temperature, the glass composition and the redox conditions. Hafnium crystallization is also investigated. State of the artThe thermodynamic solubility of an element in the glass is the maximum concentration of this element that can be loaded in the glass network at a given temperature. To determine it, two methods are commonly used. The first one consists in gradually increasing the amount the element to incorporate in the glass until a heterogeneous phase appears in the system (crystallization or/and demixing) [12]. The second one is embodied in introducing the desired element in excess in the glass and measuring its concentration in the vitreous matrix [13,14]. That means the saturated glass melt shows lots of heterogeneities. To make a difference between the gradation and the saturation methods, we named the last one the saturation solubility (S S ). The issue of reaching the equilibrium is raised in both cases. To approach equilibrium, the system has to verify several conditions [15], and if not, a conditional solubility (S C ) can be introduced notably for the gradation method [2,10]. This solubility depends on the experimental conditions used by the authors and it may be lower than the thermodynamic solubility for the gradation method or higher for the saturation technique [16]. Most hafnium solubility studies are focused on borosilicate glasses. Lopez measured conditional solubilities in a borosilicate glass as function of the melting temperature [10]. At 1400°C, hafnium conditional solubility was between 1.2 and 1.5 mol% HfO 2 . Cachia [17] reported a higher conditional solubility at the same temperature, between 1.8 and 2.2 mol% HfO 2 in the same glass by modifying the experimental protocol. In the Na 2 O-B 2 O...

Uranium solubility and speciation in reductive soda-lime aluminosilicate glass melts

“…Two U-free starting aluminosilicate glasses were prepared with high purity component oxides and carbonates following previously described protocol [32]. The chemistry and homogeneity of glasses (herein named A and B) were assessed by energy and wavelength dispersive spectroscopy (EDS and WDS) analyses (Table 1).…”

“…Considering the redox conditions of our specific vitrification process (highly reducing conditions), uranium is expected to be mainly found in the U IV oxidation state in the glass melt. In those conditions, hafnium has been identified as a potential surrogate for uranium [32]. It has a single oxidation state, tetravalent hafnium (Hf IV ).…”

“…1 Introduction 15 atm. This aluminosilicate glass matrix was selected for the immobilization of intermediatelevel waste (ILW) including actinides such as uranium and plutonium, via an innovative vitrification process as already described in previous work [32]. In this process, the glass melt is in contact with molten metals at high temperatures (1250-1400 °C), imposing highly reducing conditions under which uranium solubility has to be assessed.…”

CHARACTERIZATION OF DIRECT ADDITIVE MANUFACTURED U3Si2 SURROGATE TO PREDICT U3Si2 MICROSTRUCTURES

“…Prior research studies show the feasibility of employing Zr3Si2 as a neutron reflector in the core of future gas cooled nuclear reactors [8] and ZrO2 has been employed as a surrogate material for uranium fuel in TRISO coated fuel [45]. Hafnium which has similar characteristics to those of zirconium [46] including crystal structure, enthalpy of formation, heat capacity, and thermal expansion coefficient was also evaluated to be incorporated in the experimental matrix ( [42].…”

“…Previous research work have successfully employed hafnium as a uranium surrogate to simulate uranium fuel debris for clean-up projects and high-level waste disposal [45] [46]. Hafnium and silicon powders were placed in a tungsten crucible to undergo laser synthesis.…”