The role of crystal chemistry in developing ceramic materials for high level radioactive waste immobilisation is reviewed. In nature certain minerals have shown the ability to retain radioactive elements such as 238U, 232Th, 40K, 87Rb and their progenies for hundreds of million of years. The original “SYNROC” (synthetic-rock) concept in the 1970s was developed utilising this philosophy. Here, it was possible to produce a ceramic material consisting of four main titanate phases, zirconolite (CaZrTi2O7), “hollandite” (Ba(Al,Ti)2Ti6O16), perovskite (CaTiO3) and titanium oxides (TinO2n-1) with the capacity to incorporate nearly all the elements present in high-level radioactive waste (HLW) into their crystal structures in solid solution. In the 1990s this concept was extended to tailor ceramic materials having predominantly zirconolite or its closely related pyrochlore structure to incorporate wastes rich in actinides, notably surplus US and Russian waste. Apatite (Ca5(PO4)3(F,OH,Cl)) is a naturally occurring mineral that accepts numerous substitutions of both cations and anions. One of its members is fluorapatite (Ca5(PO4)3F) which occurs naturally. By replacing Ca by Pb and P by V it was possible to incorporate I, a larger anion, at the F site. This ceramic material has the potential to incorporate 129I, which is a long-lived waste radionuclide. In this work we will discuss briefly the crystal chemistry of the main SYNROC phases, pyrochlore and apatite and their applications in developing wasteforms.
Keywords

Synroc, Radioactive Wasteforms, Crystal Chemistry, Zirconolite, Apatite.
Introduction

Before 1970, the nuclear power industry strategy for high-level radioactive waste (HLW) was to immobilise it in borosilicate glass, followed by deep burial in the Earth. However in the early 1970s, geochemists realised that borosilicate glasses might not be particularly stable with respect to contact by groundwater when buried in the ground, because a) radiogenic heating within the glass and/or the geothermal gradient could produce temperatures well in excess of 100°C, and b) groundwater could not be guaranteed to avoid contact with the glasses, even if a series of additional barriers such as metal containers and clay overpacks were introduced into a geological repository.

Alternative disposal matrices (wasteforms) were first put forward for HLW at the Pennsylvania State University, with the basic idea of incorporating the waste fission products and associated actinides in the crystalline lattices of synthetic minerals, which were known to be very long-lived (many millions of years) in nature [1]. These minerals included silicates (pollucite, CsAlSi2O6; Sr-feldspar, SrAl2Si2O8), phosphates (monazite, CePO4; apatite, Ca5(PO4)3(F,OH,Cl)) and oxides (fluorite-structured UO2). Mixtures (assemblages) of these phases were formed by adding calcia, phosphate, alumina and silica to the fission product wastes and sintering at ~ 1100°C. Waste loadings in these “Supercalcines” were as high as 70 mass%.

In 1979, Ringwood et al. [2] suggested assemblages of titanate minerals could be used to incorporate HLW, on the basis that in Nature, titanates are much more water-resistant than the supercalcine suite of minerals. In titanate assemblages waste ions are only dilutely incorporated into the phases, whereas in the supercalcines fission products/actinides were the basis of the phases. Because of the resemblance of the titanate phases to naturally occurring rocks, the alternate material was called SYNROC (synthetic-rock) Several forms of SYNROC have been named but the most extensively studied is SYNROC-C which was designed to incorporate Purex-type reprocessing waste from commercial nuclear power production [3]. SYNROC-C consists of four main titanate phases, ziconolite (CaZrTi2O7), “hollandite” (Ba(Al,Ti)2Ti6O16), perovskite (CaTiO3), titanium oxides (TinO2n-1) and ~ 2-5% metallic alloys. The first three phases provide various lattice sites in which nearly all the HLW ions are incorporated in solid solution. Titanium oxides are important - although they do not incorporate waste elements, they provide a chemical buffer that enables the relative proportion of SYNROC phases to vary in an in-situ manner. Some of these phases and their stability and the incorporation mechanisms of waste ions are discussed. Also another naturally occurring mineral, apatite is discussed with respect to immobilising niche waste 129I.