ИСТИНА

Ammonium polyuranates play an important role in the nuclear power, as they are actively involved in the nuclear fuel cycle in the course of UO2 pellets preparation. However, neither the chemical composition, nor structure of such compounds have not been uniquely determined. Though properties of ammonium polyuranates have been actively investigated in the previous century[1–3], its nature is still an open question. It is still debated, whether ammonium polyuranates are compounds of individual phases with certain stoichiometry or a homogeneous system in which NH3:U (and H2O:U) ratio can be varied continuously. Fortunately, the advancement of the characterization techniques, including synchrotron-based methods of X-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS) spectroscopy, makes it possible to investigate structure of ammonium uranates even though only polycrystalline samples can be synthetically obtained. It is not surprising, that technical achievements resumed scientific interest to this problem[4,5]. This contribution will show the results of structural characterization of ammonia uranate system. All samples were synthesized by adding ammonia aqueous solution to solid UO3 with various ratio of UO3/NH3. The mixture has been stirred at 40 oC for 28 and 166 days. The precipitates were washed and investigated with a bunch of methods, including XRD, transition electron microscopy (TEM), elemental analysis, infrared, Raman and EXAFS spectroscopies. Table 1. Results of the structure solution for ammonium polyuranates. 3UO3·NH3·5H2O 2UO3·NH3·3H2O Crystal system Orthorombic Orthorombic Space group Pnnn I212121 Formula units 6 6 a, Å 12.229(1) 12.175(1) b, Å 15.012(1) 14.421(1) c, Å 7.1327(8) 7.0691(7) V, Å3 1309.43 1241.16 Rp 0.0280 0.0441 Rwp 0.0366 0.0498 It was found, that UO¬3/NH3 ratio and synthesis time have a significant effect on the product properties, resulting in monophasic or polyphasic samples. The chemical composition of monophasic samples have been revealed and found to be 3UO3·NH3·5H2O and 2UO3·NH3·3H2O. All samples possess a layer structure similar to uranium oxohydroxide (schoepite), with interlayer water partially substituted with ammonium NH4+. Diffraction data have been used for structure solution, additional crystallographic information have been obtained (Table 1). It is worth to note that despite ammonia polyuranates have similar to schoepite local structure, the layer topology of each compound is different and has some peculiarities.