The Impacts of Uranium and Thorium on the Defense Waste Processing Facility (DWPF) Viscosity Model
The Defense Waste Processing Facility (DWPF) at the Savannah River Site (SRS) vitrifies high level liquid waste (HLLW) into borosilicate glass for stabilization and permanent disposal. The viscosity of the borosilicate glass melt as a function of temperature is the single most important variable affecting the melt rate and pour ability of the glass. The viscosity determines the rate of melting of the raw feed, the rate of glass bubble release (foaming and fining), the rate of homogenization, the adequacy of heat transfer, the devitrification rate, and thus, the quality (in terms of glass homogeneity) of the final glass product. If the viscosity is too low, excessive convection currents can occur during melting, increasing corrosion/erosion of the melter materials of construction (refractory and electrodes) and making control of the melter more difficult. The lowest glass viscosities allowed in the DWPF melter have, therefore, been determined to be approximately 20 poise. DWPF glasses must pour continuously into a large steel canister for ultimate storage in a geologic repository, but glasses with a viscosity greater than or equal to 500 poise do not readily pour. Moreover, too high a viscosity can reduce product quality by causing voids in the final glass. A conservative range of 20-110 poise at a melt temperature, Tmelt or Tm, of 1150 degrees C was, therefore, established for DWPF production. In summary, a uranium term is not needed in the DWPF viscosity model as long as the U3O8 concentrations of the glasses being melted are less than or equal to 5.76 wt percent, the maximum value examined in this study. The fact that a U-plus-6 term is not needed in the DWPF viscosity model is consistent with the fact that U-plus-6 has four bridging and two non-bridging oxygen bonds. Therefore, the impact of the number of bridging and non-bridging oxygens is approximately equal at U3O8 concentrations of less than or equal to 5.76 wt percent. Uranium may not have an impact at higher U3O8 concentrations but this would have to be demonstrated since the effects of the 0.66:0.33 BO to NBO ratio may become more significant as the U3O8 content increases. While U-plus-6 appears to have little to no impact on glass viscosity, this may or may not be true for U-plus-4 and U-plus-5 in glass since these species were not examined in this study. This is of especial note since the DWPF is currently operating at a REDOX target of 0.2 where 45 percent of the uranium is U-plus-6, 45 percent is U-plus-5, and 10 percent is U-plus-4. An additional 26 glasses for which 98 viscosity-temperature measurements were available indicate disparate roles for ThO2 depending on the U3O8 concentration and the Al2O3 concentration of the glasses measured. For the data generated on three DWPF glasses at SRNL where the ThO2 content and U3O8 content were each in the 2.5-3.0 wt percent range, the presence of ThO2 made the melts more fluid. This is consistent with what is known from the literature about the coordination chemistry of Th-plus-4 in glass, e.g. that it may act as a weak network modifier. However, twenty two West Valley mixed uranium-thorium glasses with U3O8 approximately 0.6-0.7 wt percent and ThO2 of 3.5-3.6 wt percent, demonstrate a trend toward more polymerized melts (higher viscosities). The West Valley glasses are much higher in Al2O3 than the glasses measured at SRNL although they are in the range of the DWPF viscosity model. This indicates that there may be a synergistic interaction between ThO2, U3O8, and Al2O3 that needs further investigation.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- AC09-96SR18500
- OSTI ID:
- 839552
- Report Number(s):
- WSRC-TR-2004-00311; TRN: US0501752
- Resource Relation:
- Other Information: PBD: 28 Feb 2005
- Country of Publication:
- United States
- Language:
- English
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