Liquidus Temperature and Primary Crystallization Phases in High-Zirconia High-Level Waste Borosilicate Glasses
- PDF / 446,134 Bytes
- 6 Pages / 416.88 x 635.4 pts Page_size
- 13 Downloads / 214 Views
Pacific Northwest National Laboratory, Box 999, Richland, WA 99352 ABSTRACT
Liquidus temperature (TL) studies of high-Zr high-level waste (HLW) borosilicate glasses have identified three primary phases: baddelyite (ZrO2), zircon (ZrSiO 4), and alkali-zirconium silicates, such as parakeldyshite (Na 2ZrSi 2O 7). Using published TL data for HLW glasses with these primary phases, we have computed partial specific TLS for major glass components. On the Na2 O-SiO 2 -ZrO2 submixture, we have determined approximate positions of the boundaries between the baddelyite, zircon, and parakeldyshite primary phase fields. The maximum that can dissolve at 11 50'C in a borosilicate HLW glass subjected to common processability and acceptability constraints appears to be 16.5 mass% ZrO2 . INTRODUCTION
Several published composition variation studies (CVS) [1-8] provide databases linking the liquidus temperature (TL) associated with Zr-containing primary phases to high-level waste (HLW) glass composition. Three Zr-containing phases were identified. The zircon (ZrSiO4 ) primary phase field occupied most of the region covered by the published data. The rest was divided between baddelyite (ZrO 2) and alkali zirconium silicates, mainly parakeldyshite (Na 2ZrSi 20 7). Studies [2-5] express TL as a function of glass composition by the equation
N Ti=
where TLi is the i-th component partial specific TL (also called i-th component coefficient), gi is the i-th component mass fraction in glass, and N is the number of components (usually limited to N-I major components plus Others, the sum of all minor components; see the Table I a footnote). Equation (1) represents a nearly planar liquidus surface. This is a satisfactory approximation of the real TL surface on a constraint region within a multicomponent mixture. Such regions have narrow ranges of mass fractions of components. In this paper, we evaluated the following databases: Hanford CVS [1,2], simulated transuranic (TRU) waste CVS [3-5], and Idaho National Engineering Environmental Laboratory (INEEL) CVS [6-8]. To obtain TLi values, we sorted the glasses into three groups according to the primary phase (zircon, baddeleyite, and parakeldyshite) and fitted Equation (1) to TL versus gi data for each group. TL VERSUS COMPOSITION DATA
This section summarizes the published TL databases [1-8] that we used to compute the TLi values. In all studies [1-8], the primary phases were identified by x-ray diffraction and scanning electron microscopy. TL was measured by the gradient furnace method only in the earliest study, the Hanford CVS [1 ], that covers the glass-composition region envisaged for Hanford before 1995 [9]. A disadvantage of this method is that at T> 850'C, the surface-tension gradient induced by volatilization may generate melt convection that shifts the crystallization front. In later studies [3-8], this method was replaced by the uniform-temperature heat-treatment method. 709 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
Out of more than 100 glasses tested in
Data Loading...
