Ming Tang1
Clemson University1
Glass waste forms are currently used to stabilize legacy high-level waste (HLW) materials, and glass is the baseline technology for treatment of HLW that would result from reprocessing commercial used nuclear fuel (UNF). Electrochemical reprocessing is one option for The United States Department of Energy (DOE) Office of Nuclear Energy (NE) to reprocess used nuclear fuel in advanced nuclear cycles. During electrochemical reprocessing, used nuclear fuel is dissolved in molten salt (e.g., LiCl-KCl eutectic). Due to the low solubilities (< 1.5 mass% for silicate-based glass) of the chloride/fluoride ions and evolution of Cl2/F2 gas from the melt under operation temperature, it is not suitable to employ the borosilicate glasses as the host for the full-salt waste streams. Phosphate glass, especially iron phosphate glass, exhibited promising properties as waste form candidates including good chemical durability; high solubility for many heavy metals (e.g., uranium, cesium, molybdenum), noble metals, and rare earths. However, there are few studies on iron phosphate glass waste forms to immobilize salt waste streams.<br/>In this study, we intend to develop and optimize highly durable and easily processable iron phosphate glass waste form to immobilize the salt streams from advanced nuclear fuel cycle by tailoring the composition of the glasses at intermediate melting temperatures. A simulate salt recipe representing the salt waste stream in our study is 70LiCl-10Li2O-20SrCl2 which is based on previous DOE national laboratory reports. For the compositional variation study, different Fe:P ratio vs waste loading in iron phosphate glass waste forms are fabricated; various glass modifiers are added into iron phosphate glass compositions to improve chemical durability and reduce melting temperature. Further, thermal stability, chemical durability, radiation stability tests are performed on these iron phosphate glass and glass waste form samples. Specifically, DTA/TGA measurements are used to understand thermophysical and structure information of these glass waste forms. Monolithic leaching test is performed to study chemical durability of these glass waste forms. Ion beam radiation is used to simulate self-radiation in nuclear waste storage. Various characterization techniques (including XRD, SEM/EDS, TEM, Raman, X-ray CT) are used to characterize structural and chemical information. The study of composition-property-structure correlations of phosphate glasses is aimed to further optimize the waste forms by pushing the limit of salt loading while targeting high chemical durability and easily processing with present technologies. The preliminary result shows promising for iron phosphate glass waste form being an improvement over other candidate waste forms.