PNNL

Abstract

Waste forms containing glassy and crystalline phosphate and silicate phases were produced to immobilize salt waste simulants from pyroprocessing. Pyroprocessing of used nuclear fuel results in fission product radionuclides in chloride form, which have low solubilities in silicate glasses and phosphate glasses. To improve waste loading in these glass systems, the salt waste can be dechlorinated prior to immobilization in a glassy waste form through vitrification. In this work, a phosphosilicate waste form is presented to leverage the benefits of both borosilicate glasses and iron phosphate glasses. Prior to immobilization, salt simulants were successfully dechlorinated using ammonium dihydrogen phosphate, were mixed with a borosilicate frit (5–30 mass%) and Fe2O3, and vitrified. Additions of 2.5-15 wt% borosilicate glass (NBS3) improved normalized release rates for Cs relative to iron-phosphates sans NBS3, resulting in a chemical durability similar to high-level waste borosilicate glasses. Alkali metals (Li, Na, K, Cs) normalized release rates were the lowest with the addition of 5 wt% NBS3. Although Sr was not specifically targeted in this study, there is evidence that it preferentially partitioned with Si to an amorphous droplet phase within the phosphate glass matrix.