Murataite as a universal matrix for immobilization of actinides

Abstract

A new variety of matrices based on synthetic phases whose structure is close to that of murataite (a natural mineral) is proposed for immobilization of nuclear wastes. Murataite is Na, Ca, REE, Zn, and Nb titanate with a structure derived from the fluorite lattice. This very rare mineral was found in alkali pegmatites from Colorado in the United States and the Baikal region in Russia. The synthetic murataite-like phases contain manganese instead of zinc, as well as actinides and zirconium instead of sodium, calcium, and niobium. Varieties with threefold, as in the mineral, and five-, seven-, and eightfold repetition of the lattice relative to the fluorite cell have been established. Correspondingly, the structural varieties M3, M5, M7, and M8 are recognized among the synthetic murataites. A decrease in the contents of actinides, rare earth elements, and zirconium occurs in the series M7-M5-M8-M3, along with enrichment in Ti, Al, Fe, and Ga. Murataite-based ceramics are characterized by high chemical and radiation stability. The rate of U, Th, and Pu leaching with water at 90°C in static and dynamic tests is 10−6–10−5 g/m2 per day. These values are lower than the leaching rate of other actinide confinement matrices, for example, zirconolite-or pyrochlore-based. Murataite is close to other titanates in its radiation resistance. At 25°C, amorphization of its lattice is provided by a radiation dose of 2 × 1018 α decays/g, or 0.2 displacements/atom. Murataite-based matrices are synthesized within a few hours by cold compacting combined with sintering at 1300°C or by melting at 1500–1600°C and subsequent crystallization. The melting technology, including induction smelters with a cold crucible, makes it possible to produce samples with zonal murataite grains. The inner zone of such grains is composed of structural variety M5 or M7; the intermediate zone, of M8; and the outer zone, of M3. The contents of actinides, zirconium, and rare earth elements reach a maximum in the inner zone and drop to a minimum in the outer zone, while the amounts of nonradioactive elements—Ti, Al, Fe, and Ga—vary conversely. The U, Th, and Pu contents in the inner and outer zones differ by three to five times. Such a distribution precludes removal of actinides by interaction of the matrix with solution after its underground disposal. Individual actinides (Np, Pu, Am); the actinide-zirconium-rare earth fraction of high-level radioactive wastes (HLW); Am-Ga residues of weapons plutonium reprocessing with its conversion into U-Pu mixed oxide (MOX) fuel; and other sorts of HLW enriched in actinides, REE, and products of corrosion (Mn, Fe, Al, Zr) can be incorporated into a murataite-based matrix. As much as 350 kg of HLW components can be included in 1 t of such a ceramic. An actinide matrix that is composed of titanates with a pyrochlore structure is its nearest analogue. The advantage of murataite in comparison with pyrochlore consists in its universal character; i.e., a murataite-based matrix can be used for utilization of a wider range of actinide-bearing highly radioactive wastes.