Defense Waste Processing Facility (DWPF) Durability-Composition Models and the Applicability of the Associated Reduction of Constraints (ROC) Criteria for High TiO₂ Containing Glasses

Abstract

Radioactive high level waste (HLW) at the Savannah River Site (SRS) has successfully been vitrified into borosilicate glass in the DWPF since 1996. Vitrification requires stringent product/process (P/P) constraints since the glass cannot be reworked once it has been poured into ten foot tall by two foot diameter canisters. A unique "feed forward" statistical process control (SPC) was developed for this control rather than relying on statistical quality control (SQC). In SPC, the feed composition to the DWPF melter is controlled prior to vitrification. In SQC, the glass product would be sampled after it is vitrified. Individual glass property-composition models form the basis for the "feed forward" SPC. The models transform constraints on the melt and glass properties into constraints on the feed composition going to the melter in order to determine, at the 95% confidence level, that the feed will be processable and that the durability of the resulting waste form will be acceptable to a geologic repository.The DWPF SPC system is known as the Product Composition Control System (PCCS). One of the process models within PCCS is known as the Thermodynamic Hydration Energy Reaction MOdel (THERMO™), which was developed in 1995 and utilizes data from the short term (7-day) durability test given in the ASTM standard C1285A. The DWPF durability model is based on a free energy of hydration function calculated from the molar glass compositions. An individual component free energy, ∆G i , exists for each oxide in a HLW glass and the ∆G i 's are weighted by the molar concentration of each oxide in the glass. This gives an overall preliminary hydration free energy, ∆G p , for a given glass, which is predictive and independent of any leachate solution pH impacts. The less negative the ∆G p the more durable the glass; the more negative the ∆G p the less durable the glass.The DWPF PCCS models are parsimonious in that the oxide terms in each model are only those which are necessary and sufficient to describe the glass property of interest. This approach excludes composition terms that are unnecessary to the implementation of the DWPF flowsheets, and helps to minimize the sources of error in the PCCS models. These parsimonious models have successfully operated the DWPF vitrification process over the last 20 years.

SME Acceptability Determination For DWPF Process Control (U)

“…A ROC study was also deemed necessary before the coupled operation of DWPF with SWPF since TiO 2 concentrations in the resulting glass waste form are anticipated to exceed 2 wt% [14].…”

“…The efforts associated with support for the introduction of waste streams from SWPF into the DWPF flowsheet involved the study of glass systems at higher concentrations of TiO 2 , specifically, employing the glass test matrix defined in Reference 17. The results from that study led to the revision of the viscosity model [18], the durability models [14], and the liquidus temperature model [19] (these new models are discussed below). As a consequence of these results, the acceptability of DWPF's glass waste form at higher concentrations of TiO 2 is to be determined by the acceptability of the model predictions for viscosity, durability, and liquidus temperature.…”

“…As already mentioned the solubility constraint associated with P 2 O 5 was removed from the scope of the PCCS control system as part of Revision 4 of this report [10], and the limit for TiO 2 solubility is replaced by a modeling limit based upon the results from the viscosity studies associated with the introduction of SWPF into the DWPF flowsheet [18]. In addition, as discussed above, an assessment of the TiO 2 content relative to a trigger value of 2 wt% must be determined as part of the evaluation of homogeneity for the SME batch ( Figure 5-1) [14]; this is discussed in more detail in a following section.…”

“…There were changes to this approach that were ‡ In general, this equation (with m d being the estimated slope and b d the estimated intercept) represents common logarithm (log) of the B, Li, and Na normalized releases, NC i 's, from the PCT. Specifically, c d equals ∆G p , the free energy of hydration (in kcal/mole) and thus uses all oxides described in introduced as part of Revision 5 of this report, and the models themselves were modified slightly as a result of the studies to support the introduction of SWPF into the DWPF flowsheet [14].…”

“…Reference 3 provides the complete background on the development of the durability-composition models that have been utilized as part of PCCS since the startup of radioactive operations. However, these models were re-evaluated as part of the studies associated with the integration of waste streams from SWPF into the DWPF flowsheet [14]. The result of that evaluation was the modification of these models to support a broader range of TiO 2 content [17].…”