Metastable Liquid Immiscibility in Nuclear-Waste-Glasses

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METASTABLE LIQUID IMMISCIBILITY IN NUCLEAR-WASTE-GLASSES JOHN E. ENGELL AND GJERLOFF ROED Institute of Mineral Industry, Technical University of Denmark, DK-2800 Lyngby, Denmark ABSTRACT For glass systems related to two potential nuclear-wasteglasses, ABS-39 and ABS-41, the 600 0 C isotherm of the metastable immiscibility field has been located experimentally. Both glasses are in the one phase glass field down to temperatures below 600 0 C. ABS-39 and ABS-41 are Na2 0-B 2 0 3 -SiO2 glasses containing 1.66 wt% UO2 and 9 wt% of simulated fission products. Additional components in ABS-39 are A12 03 and Fe603. In ABS-41 these two oxides are partly substituted with ZnO and Li 2 0. This substitution causes an extension of the metastable immiscibility field relative to the ABS-39 system. INTRODUCTION The work presented here concerns glasses related to two potential nuclearwaste-glasses containing 1.66 wt% U02 and 9 wt% of simulated fission products. These glasses (ABS-39 and ABS-41, Table I) are presently under consideration by the Swedish Nuclear Energy Association (KBS/SKBF). Like most nuclear-waste-glasses considered in recent years, ABS-39 and ABS-41 have compositions closely related to the ternary system Na2 0-B 2 03 -SiO2 . In this system a broad spectrum of glass compositions show metastable phase separation upon heat treatment in the range 550 0 C to 755°C [1]. Addition of Mo0 3 [2,3J or ZnO [4] is known to cause an expansion of this immiscibility field. For waste-glasses the cooling rate after fabrication will be strongly limited in this temperature range, due to the heat generated by the fission products in the glasses. Changes in chemical durability of borate glasses upon phase separation is well-known [5,6,7] and is utilized in the production of silica-rich glasses by selective extraction of the alkali-borate-rich phase from the separated glasses [8,9]. A similar behavior of nuclear-waste-glasses is clearly unwanted. Neglecting the fission products and other modifying components a direct projection of ABS-39 and ABS-41 into the system Na2 0-B 2 O3 -SiO2 (Fig. 1) indicates, that ABS-41 is close to the 600 0 C immiscibility isotherm, whereas ABS-39 falls well outside the metastable immiscibility field. ABS-41 further differs from ABS-39 in that it contains Li 2 0 and ZnO (Table I). If the Li 2 0-content of ABS-41 is recalculated to the equivalent amount of Na2 0, the projection of this glass also falls well outside the critical field (Fig. 1). However, the combined influence of the fission products and other additional components on the immiscibility dome is unknown and impossible to predict theoretically. The purpose of this investigation was to locate the boundary of the immiscibility field experimentally for glass compositions relevant to ABS-39 and ABS-41. METHODS AND MATERIALS The method used is based on the observation, that HF-vapour etches the surface of a phase-separated glass much more rapidly than the surface of a non-separated glass of

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