PNNL

Abstract

Glass-bonded sodalite composite waste forms have been developed for the immobilization of liquid radioactive wastes resulting from off-gas treatment during aqueous reprocessing of used nuclear fuel, with a particular focus on 129I. The proposed composite waste form is comprised of aluminosilicate ceramic phases containing volatile radionuclides bonded with a glassy matrix. In this work, a suite of ten candidate low-temperature glass binders (ZnO-Bi2O3-based glasses and a Na2O-B2O3-SiO2 glass) were examined. The choice of binder was down-selected according to specific design criteria: glasses with a low glass transition temperature (Tg) and a large sintering window, ?T (the difference between Tg and the onset temperature of crystallization, Tx), allowing for viscous flow without significant crystallization. Six down-selected glasses were mixed with caustic scrubber waste simulant previously converted into a sodalite-rich material (to provide glass fractions of 10 and 20 wt.%), uniaxially pressed into pellets, and sintered at 350 °C or 550 °C for 8 h in air. Iodine retention after heat treatment was assessed by neutron activation analysis. The aqueous durability of the resulting materials were then assessed, following the ASTM C1308 standard test, with iodine releases of 1 to 23 g·m-2 after 4 d. Iodine retention for the best performing system (a zinc-bismuth-borate glass) was 67%, and the cumulative iodine release was