Heat capacity of complex aluminoborosilicate glasses

Abstract

For nuclear waste vitrification in the Tank Waste Treatment and Immobilization Plant (WTP) at the Hanford site, waste feed is charged into a continuously fed electric melter and the molten glass is poured into stainless steel containers to solidify. The heat from the molten glass must be removed from the facility within plant operating constraints, making knowledge of thermal properties of the waste glasses critical. The heat capacity of Hanford low‐activity waste glasses with systematic variations in composition were studied using complementary thermal analyses – power‐compensated differential scanning calorimetry (pcDSC) at 50–630°C, heat‐flux differential scanning calorimetry (hfDSC) at 200–800°C, and drop calorimetry at 600–1200°C. Variations of the heat capacity with composition have been observed around the transition temperature (Tg) and above, particularly when B2O3 and Al2O3 is varied. Above Tg, all glasses exhibit decreasing heat capacity with increasing temperature. A correlation was found between this curve and the fraction B[4]2O3 in glasses, suggesting boron coordination plays a role in this anomalous behavior. These measurements will be used as critical input to validate the heat balance calculations used for facility design and thus the overall operation of the WTP.

Glass transition temperature of low‐activity waste nuclear glasses

“…The onset glass transition temperature of 10 simulated LAW samples measured by DSC was recently published by George et al 19 These values are combined with measurements of the present work to dataset D1 with a total of 32 T g values of simulated LAW glasses determined by DSC. A total of 121 T g values of simulated HLW glasses determined by dilatometry 18 constitute dataset D2.…”

“…The trends agree with melt viscosity models, which generally show that the viscosity (or more precisely the activation energy of the Arrhenius equation, B in Figure 4B) is decreased most by Li 2 O and Na 2 O and increased most by Al 2 O 3 and SiO 2. 12,48 The similarity in the trends is unsurprising as SiO 2 , Al 2 O 3 and ZrO 2 act as network formers that strengthen the glass structure, 49 whereas the alkali oxides create nonbridging oxygens, mostly around silica and boron, that weaken the glass structure 19,44 . Alkaline earth oxides and Fe 2 O 3 have a relatively minor effect on T g and viscosity 12,48 .…”

“…The onset glass transition temperature of 10 simulated LAW samples measured by DSC was recently published by George et al 19…”

“…Previously developed T g models for nuclear waste glasses were derived from HLW data 18 but were never tested if the models can be applied to LAW glasses. A small collection of LAW T g data with decent variation in glass composition has only recently become available 19 …”

“…A small collection of LAW 𝑇 g data with decent variation in glass composition has only recently become available. 19 A reliable 𝑇 g model can extend the validity of viscosity models developed from high-temperature data well below 950 • C. 6 Although the temperature of the melt in vitrification melters is typically kept around 1150 • C, low-temperature viscosity models are necessary for simulations of canister filling, melter idling, and loss-of-power scenarios. 20 Additionally, low-temperature viscosity models can be utilized to provide information on the viscosity of the low-temperature alkali-rich glass-forming melt produced when slurry feed is charged on top of a molten glass pool in the WTP melter.…”