SUMMARYThe Fuel Cycle Research and Development Program, sponsored by the U.S. Department of Energy Office of Nuclear Energy, is currently investigating alternative waste forms for wastes generated from the reprocessing of nuclear fuel. One such waste results from an electrochemical separations process, called the "Echem" process, in molten KCl-LiCl salt as a spent salt containing alkali, alkaline earth, lanthanide halides and a small quantities of actinide halides where the primary anion is chloride with small quantities of iodide. Pacific Northwest National Laboratory (PNNL) is investigating two candidate waste forms for the Echem spent-salt, high-halide minerals (i.e., sodalite and cancrinite) and tellurite (TeO 2 )-based glasses. Both of these candidates showed promise in FY2009 and FY2010 with a simplified nonradioactive simulant of the Echem waste. Further testing was performed on these waste forms in the fiscal (FY) and calendar (CY) year of 2011 to investigate the possibility of their use in a sustainable fuel cycle. This report summarizes the results of FY/CY2011 efforts.The chemical durability results for the tellurite glasses in FY2010 revealed a wide variety of waste form performance in these materials. The most promising glass studied was a lead-tellurite glass formulation with the Echem salt simulant. This glass showed very good chemical durability with a normalized release for sodium of 0.478 g/m 2 following a seven-day leaching test. With this in mind, the following studies were undertaken in FY/CY2011 to further investigate and optimize this type of glass as a potential waste form: 1) Expanded study on lead-tellurite glasses. The lead-tellurite glass system was further investigated in FY/CY2011 to determine the waste-loading limits and assess the tellurium-oxygen bonding structure as a function of waste loading with Raman spectroscopy and X-ray diffraction. Te-O bonding in tellurite glasses is quite complex; at least eight different structures have been presented in the literature and typically include α-TeO 4 , β-TeO 4 , TeO 3+1 , and TeO 3 structures (the α/β notation denotes the way that the TeO 4 species bond to one another, and the number after the "O" denotes the number of oxygens bonded to the central Te atom in that species). We also investigated some of the other waste-form related properties of the lead-tellurite glasses, such as the thermal diffusivity, glass transition temperature, crystallization temperature, melting temperature, and mass loss as a function of temperature. The structural investigation revealed that the medium-range order in the glass decreased and the tellurite network was depolymerized as the waste fraction was increased. It was apparent that glass-network breaking was predominantly caused by the additions of the waste cations and that the presence of chloride may have actually caused some repolymerization of the network by scavenging alkali and preventing it from acting as a network modifier. At the highest waste loadings, the glass became phaseseparated, resulting...