ABSTRACT

The SYNROC process proposes to immobilize high-level wastes as dilute solid solutions (i.e. as integral parts of crystal lattices) in the constituent minerals of a synthetic rock formed from a mixture of oxides, principally, TiO₂, BaO, ZrO₂, Al₂O₃ and CaO. A new modification of SYNROC, comprising the titanate mineral assemblage Ba-hollandite (BaAl₂Ti₆O₁₆), perovskite (CaTiO₃) and zirconolite (CaZrTi₂O₇) has been developed. Experiments show that the entire spectrum of high-level waste elements can be incorporated in the crystal lattices of these 3 phases and in a few minor accessory phases. This titanate assemblage has proved to be exceptionally resistant to hydrothermal leaching and in this respect, amongst others, is demonstrably superior to alternative ceramic waste forms and to borosilicate glasses. The relative stabilities of various waste forms were compared in hydrothermal leaching experiments using both pure water and lOwt.% NaCl solutions (temperature range 300-1, 000°C; pressure range 300-5, 000 bars). Borosilicate glasses are almost completely decomposed and disintegrated after only 24 hours at 350°C and 1, 000 bars, and extensive losses of hazardous high-level waste elements occurred. The phase pollucite (CsAlSi₂O₆), which provides the site for Cs-fixation in alternative ceramic waste forms, likewise begins to decompose at 400°C, with total loss of Cs by 600°C. On the other hand, the hollandite-perovskite-zirconolite SYNROC assemblage proved to be exceptionally resistant to leaching, surviving unaltered extreme conditions up to 900°C and 5, 000 bars. Hazardous species e.g. Cs, U and Sr, were quantitatively retained by the hollandite, zirconolite and perovskite phases respectively. Geochemical studies of naturally-occurring minerals containing radwaste elements are relevant to the problem of radiation damage to SYNROC phases. These imply that the a-particle flux in SYNROC is unlikely to be enough to impair the ability to immobilize radwaste for the required period. SYNROC can be produced by mixing about 10wt.% radwaste calcine with appropriate amounts of inert oxides. Subsolidus hot-pressing at 1, 200-1, 300°C in sealed Ni containers results in a dense, compact, mechanically-strong material. The production of SYNROC in its terminal state and final encapsulation and sealing are accomplished in a single step, and moreover, volatile species e.g. Cs, Ru, are quantitatively retained during hot-pressing. In contrast to borosilicate glass technology, expensive equipment for volatile recovery and recycling is not required. These advantages are believed to make SYNROC economically competitive with borosilicate glass. Moreover, the simplicity of the hot-pressing process makes it very suitable for remote operation in hot cells. SYNROC phases have structures analogous to natural minerals which have survived a variety of geological conditions for millions of years while retaining certain high-level waste elements in their crystal lattices. This fact, coupled with the exceptional resistance exhibited by SYNROC in accelerated leaching tests, leads to considerable confidence in the long-term stability of SYNROC, and in its capacity to isolate high-level wastes from the biosphere for periods sufficiently long to permit their safe decay.