Vitrification of radioactive waste by reaction sintering under pressure

Silicate nuclear waste glasses were synthesized by reaction sintering of po wdered precursors under pressure. The glass samples contained a glass matri x phase with embedded zirconia (baddeleyite) particles. A waste composition with 38 wt% of ZrO2 was prepared with a waste loading of 30-50 wt% at 800 degrees and 28 MPa, by hot isostatic pressing. The glass former was commerc ial amorphous silica powder to which simulated waste was added as calcined oxides. Phase compositions and microstructure of the sintered glass samples were characterized using scanning and analytical electron microscopy. The results show that extensive sintering took place and that a continuous glas s phase was formed, particularly at higher waste loading. Waste components such as Na2O, CaO, MnO2, La2O3, Fe2O3, Cr2O3, and P2O5 dissolved completely in the glass phase. ZrO2 was also dissolved but recrystallized from the gl ass as aggregates of baddeleyite crystallites surrounding the original sili ca particles. MCC-I type chemical durability tests showed that the glasses are durable with dissolution rates similar to or lower than that of the hig hly durable French R7T7 borosilicate glass. This glass contains 13 wt% high -level radioactive waste from light water reactor fuel reprocessing and has a melting temperature of 1150 degrees C. The long-term chemical durability of our sintered glasses is expected to be as high as that of rhyolitic gla sses, based on hydration energies of 3.7 and 3.3 kJ/mole, respectively. Rhy olitic glasses show little alteration over geological periods of time with a typical corrosion rate of 1 mu m/1000 yr. (C) 1999 Elsevier Science B.V. All rights reserved.