Immobilization of high-level waste in ceramic waste forms

Abstract

High-level wastes (HLW) can be incorporated in the crystal lattices of coexisting phases in ceramic waste forms. The properties and performances of ceramic waste forms are largely determined by their phase chemistry, phase assemblage and microstructure. Currently, the best categorized advanced ceramic waste form is SYNROC, a titanate ceramic composed of ‘ hollandite ’ Bat 1(Al,Ti)2^Ti|]*"70 16, zirconolite CaZrTi 2 O 7 , perovskite CaTiO 3 , rutile TiO 2 and minor amounts of metal alloys microencapsulated by the titanate matrix. Two factors contribute to the capacity of synroc to accommodate high (e.g. 20% ) loadings of HLW, together with variations in waste-stream composition. Firstly, the constituent phases can accept, as solid solutions in their crystal lattices, a broad spectrum of cationic species of diverse charge and radius, either singly or by complex substitution mechanisms. Secondly, the phase assemblage itself spontaneously adjusts its modal mineralogy in response to waste stream fluctuations. The presence of both rutile and a source of trivalent titanium (from reaction of rutile with added Ti metal) in the synroc phase assemblage is largely responsible for this flexible and accommodating nature. The titanate minerals in synroc also occur in Nature, where they have survived for many millions of years in a wide range of geological environments. Experimental studies show that synroc is vastly more resistant to leaching by groundwater than borosilicate glass; moreover, its high leach resistance is maintained at elevated temperatures. Experimental and analogue studies indicate that the HLW immobilization properties of synroc are not significantly impaired by radiation damage. These properties show that synroc would provide an effective immobilization barrier for HLW when buried in suitable repositories. They also permit the use of a wider range of geological disposal options than are appropriate for borosilicate glass. In particular, synroc is well suited for disposal in deep drill-holes, both in continental and marine environments. The fact that synroc is composed of minerals that have demonstrated long-term geological stability is important in establishing public confidence in the ability of the nuclear industry to immobilize high-level wastes for the very long periods required.