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Spent nuclear fuel in a closed nuclear fuel cycle is reprocessed using the purex process. This technology has certain drawbacks, including the following: large amounts of radioactive wastes are produced; flammable extracting agents are used; and, radiolysis of water and organic phases with release of explosive substances occurs. In contrast to the purex process, water-free (pyrochemical or pyrometallurgical) methods are distinguished by the small size of the technological apparatus, the small volume of liquid wastes which are produced during fuel reprocessing, and radiation-resistant reagents and therefore short holding times for the fuel before reprocessing.The pyrochemical reprocessing of spent oxide fuel using molybdate melts is being studied at the A. A. Bochvar All-Russia Research Institute of Standardization in Machine Engineering [1,2]. Binary and ternary phase diagrams have been studied: oxides of the spent-fuel components -molybdenum trioxide -sodium molybdate [3][4][5]. The presence of molydenum trioxide gives rise to oxidation of the fission products which are in a metallc state; sodium molybdate dissolves the oxides obtained, which has been confirmed experimentally for the fission product 106 Ru [6]. The isothermal section of a ternary phase diagram UO 2 -MoO 3 -Na 2 MoO 4 is shown in Fig. 1. x-Ray phase analysis of melt samples obtained by heating mixtures of UO 2 , MoO 3 , and Na 2 MoO 4 to 1000°C in an inert atmosphere and then cooling at a rate of 300 deg/h was performed. The mixtures were prepared in accordance with the composition indicated at the nodal points of the diagram. A special feature of the system UO 2 -MoO 3 -Na 2 MoO 4 is crystallization in one region of the uranium dioxide diagram UO 2 (in the figure the crystallization region is indicated by a circle) and crystallization of uranium molybdate U(MoO 4 ) 2 (B) and other compounds in another region. The diagrams with PuO 2 and the oxides of fission products have a similar form. The feature mentioned above is used for developing methods for reprocessing by recrystallization purification of uranium and plutonium dioxides and uranium molybdates in molybdate melts with the appropriate compositions.

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