A pyrochlore-based matrix for isolation of the REE-actinide fraction of wastes from spent nuclear fuel reprocessing

Abstract

The closure of the fuel cycle for more effective usage of uranium, plutonium, and other potentially useful components of spent nuclear fuel (SNF) is one of the prerequisites for stable development of nuclear power engineering. In this case, the spent nuclear fuel is forwarded to reprocessing for extracting uranium and plutonium and producing a new portion of fuel. This process is accompanied by generation of high� level radioactive wastes (HLRW). The wastes require stable materials for their isolation, or the socalled confinement matrices, which should be characterized by a high capacity relative to HLRW as well as their high chemical and radiation stability. The right selec� tion of such materials simplifies separation of HLRW into groups of elements with similar properties. One such group is represented by the fraction of trivalent cations of REE fission products and actinides. It is composed of approximately 80-90 and 10-20 wt % of REE and actinides, respectively, and REE are domi� nated by light lanthanides (La, Ce, Pr, Nd, Sm), while actinides are largely represented by Am and Cm (1). The presence of stable natural minerals containing REE and radioactive elements, which include com� plex oxides with a fluoritetype structure (zirconolite, murataite, pyrochlore) and some minerals such as monazite, zircon, britholite, and others (1-3) with low solubility in water, implies the possibility for isola� tion of this fraction in artificial matrixes. The behavior of such minerals in water is influenced by destruction of phase structures during the decay of actinides. The phase amorphization usually increases the leaching rate of actinides by less than an order of magnitude; for the titanate matrix alone, it becomes >50 times higher as compared with the initial one (1-5). Taking into consideration the data on the contents of waste components and their hydrolithic and radia� tion stability (1-6), zirconates with the pyrochlore structure are suitable for isolation of actinides. At the same time, their synthesis is complicated by low reac� tion velocities, which is responsible for preservation of the charge residue, or a high melting temperature, when they are crystallized from melts. The addition of titanium accelerates the solid phase synthesis (7) and decreases the melting temperature; therefore, we sug� gest using pyrochlore A2(ZrTi)O7 (where A are ele� ments of the fraction) for isolation of the REE actinide fraction. In this communication, we discuss new data on samples of matrixes with imitators of the HLRW REE actinide fraction. These samples were obtained by cold pressing-sintering (CPS) or induction melt� ing in the "cold" crucible (IMCC). Unlike previous works, we have investigated compositionally more complex samples, one of which was produced using the IMCC technology, which is more promising for the industrial production of refractory crystalline matrixes for isolation of actinides. The charge for synthesis by the CPS method (Sam� ple P�1) was prepared from TiO 2, ZrO2, and REE2О3