Evaluation and Testing of Noble Metals Surrogates [report]

S K Sundaram, Brett D Macisaac, Alan R Cooper, Candice H Tschauner, Jeremy J Holbrook
2002 unpublished
This report summarizes the results of testing and evaluation of noble metal surrogates that will support testing of noble metals in melters. The candidate system, potential surrogates, model glass systems, and test conditions have been chosen based on the existing reported data in literature as well as expertise existing at various sites, thermodynamics, physical properties, and cost of materials. The methodology involves testing interaction of RuO 2 and WO 3 and other surrogates in the test
more » ... ates in the test melt followed by characterization of the bulk glass and oxide-glass melt interfaces. The report explains different techniques used for studying settling, crystallization, rheology, and partitioning of noble metals or surrogates or spinel phases in the selected systems. The report presents and discusses how these results support the selection of the suitable surrogate for RuO 2 . Finally the report concludes with identification of the best-suited surrogate (solubility and redox surrogate WO 3 and conductivity surrogate Cr 2 O 3 ) proposed and makes suitable recommendations for the follow up melter test activities. iv Summary The formation and settling of noble metals in high-level waste (HLW) glass melts poses a major challenge to the vitrification technology. Precipitation of noble metals in these joule-heated melters can lead to operational difficulties and premature failure of the melter through electrical shorting and enhanced corrosion of the electrodes. Additionally, the noble metals are known to act as nucleation sites for the precipitation and growth of spinel (crystalline) phases, which in turn will settle to the bottom of the melter and cause the viscosity of the melt to increase in that region. Use of actual noble metals and compounds for melter testing is not cost effective. Pacific Northwest National Laboratory (PNNL) and Savannah River Technology Center (SRTC) have undertaken efforts to identify suitable surrogates for noble metals to facilitate extensive melter testing that will provide key processing data. This report summarizes the results of the experimental testing evaluation of noble metal surrogates performed at PNNL. Initial selection of model glass test systems, noble metals, potential surrogates, and test conditions was done based on the existing data and expertise at DOE various sites (West Valley, Savannah River, and Hanford), thermodynamics, physical properties, and cost of materials. A simplified version of HLW glass (MS-7) and a modified neutralized caustic acid waste (NCAW) glass compositions were used in this study. Overview of existing literature on noble metals in waste melts and melter testing indicated that RuO 2 was the most commonly detected phase present. Hence, the present testing was focused at RuO 2 and its potential suitable surrogates. The potential surrogates considered and studied were WO 3 , Cr 2 O 3 , NiCr 2 O 4 , and Inconel 600. From chemical stability and density perspectives, the W-O system showed the promise of serving as a surrogate for Ru as well as RuO 2 . WO 2 (12.11 g/cm 3 ) could act as a surrogate for Ru (12.30 g/cm 3 ) under reducing conditions. Alternatively, WO 3 (7.20 g/cm 3 ) could act as a surrogate for RuO 2 (6.97 g/cm 3 ) under oxidizing conditions. Therefore, the surrogate testing was focused at WO 3 . The testing included: 1) double-crucible settling study, 2) viscosity and rheology, 3) crystallization study, 4) high temperature optical microscopy, and 5) partition study.
doi:10.2172/15010037 fatcat:5i7ldnzbrva7pmhfiy5txtaoqu