Immobilizing the calcined high level waste in glass and crystalline phases

The glass-ceramic waste form is among the options under consideration at the Idaho Chemical Processing Plant (ICPP) for immobilizing the calcined high level waste (HLW). This paper describes the development of waste forms that are predominantly composed of natural crystalline and glass analogues. The compositions of phases are based on the natural partitioning of the chemical components among the durable crystals and the coexisting liquids. The hot isostatic processing conditions emphasize nucleation and growth of the potential actinide bearing grains zircon and apatite, and other crystalline phases capable of accommodating the compatible elements. The crystalline phases occur as filter pressed residues as well as inclusions in glass, whose composition is typically enriched in silica and alumina. The role of glass in these waste forms is to render strength to the polycrystalline microstructure and host the incompatible solutes of the high level nuclear waste. The volatiles F, Cl, and S are incorporated in both crystals and glass under hot isostatic pressure. The microstructure and chemical composition of these waste forms are as complex as natural rocks and their high durability is judged by the total release of elements to the leachant and MCC-1 leach rates of less than 1 g/m2-day for the most mobile elements B, Cs, and Na. A high total waste loading of 75 weight% calcines results because of the spontaneous distribution of the chemical components, on the order of 5 weight% in glass and 95 weight% in crystalline matter