Melter Testing for Technetium Removal Using Goethite Precipitation

Abstract

During vitrification of low activity waste (LAW) at the Hanford Tank Waste Treatment and Immobilization Plant (WTP), liquid effluents are produced in the primary off-gas treatment system that processes the exhaust gases from the LAW melters. That effluent stream, which contains captured hazardous and radioactive species from the melter exhaust, is then recycled and eventually becomes part of subsequent melter feed batches. Other constituents, such as sulfur and halides, are also recycled in this process and this can impact the LAW glass loadings that are achievable. Thus, breaking the melter recycle loop could have advantages with respect to reducing LAW glass volumes. However, this recycle stream cannot currently be diverted to liquid secondary waste treatment because of the levels of several constituents in the scrub solutions, particularly technetium. Researchers at Pacific Northwest National Laboratory (PNNL) have been investigating a process whereby the technetium could be precipitated from the off-gas recycle stream in the mineral goethite ((Tc, Fe)OOH). It could then be diverted to the high level waste (HLW) stream through the WTP pretreatment facility and the decontaminated liquid stream could be sent to secondary waste treatment. As part of the test program to develop the goethite precipitation process, it is necessary to assess the impacts of including this material in the HLW melter feed and the extent to which the captured technetium is volatilized during HLW vitrification. To support such testing, sufficiently large batches of material were prepared at PNNL using rhenium as a surrogate for technetium. These materials were shipped to the Vitreous State Laboratory (VSL) of the Catholic University of America (CUA) for testing on a DuraMelter 10 (DM10) small-scale joule-heated ceramic melter system. This report describes the testing that was performed with those materials.