Vitrification of High-Level Radioactive Waste by Sintering Under Pressure

Abstract

Sintered silicate glass waste forms with a continuous glass phase and embedded but not completely dissolved waste particles were prepared by powder technology. We used a typical defense waste compositions, containing 38 wt.% of ZrO2 and prepared sinter glass samples with 30, 35, and 40 wt.% waste loading at 830°C and 27.6 MPa, by hot isostatic pressing. The glass former was amorphous silica powder. Simulated waste was added as calcine. Samples were characterized using scanning and transmission electron microscopy. The results show that extensive sintering took place, particularly with the 40 wt.% samples. Waste components, such as Na2O, CaO, MnO2, La203, Fe2O3, NiO, Cr2O3, and P2O5, dissolved completely in the continuous glass phase. ZrO2 was partly dissolved and occurred as aggregates of tiny baddeleyite crystals. The porosity of the samples was 2-3%, indicating that the optimum sintering conditions have not been met to reach porosities <1%.Chemical durability tests were carried out with sintered glass with 35 wt.% waste loading in deionized water at 90°C under static conditions, showing that the glass is at least as chemically durable as a borosilicate glass with 31 wt.% melted at 1350°C.