ИСТИНА

Uranium carbide and nitride matrix are considered as promising nuclear fuel for fast reactors of IV generation. It is quite clear from the comparison of different literature sources that both leaching rates of the U carbide or nitride spent nuclear fuel (SNF) itself and the possible options and special features for this SNF processing are strongly dependent on the nature of the fuel matrix, the burn-up, carbon precursors and many other factors [1-4]. Of special interest are some metal fission products as their final chemical forms could have drastic influence on the chemical activity of SNF. For Zr and Mo the carbide speciation and the formation conditions are well described elsewhere and recently confirmed in [5]. The behavior of Tc, Ru and Pd is more problematic due to many confusing data present in the literature [5-6]. The final vision of Tc behavior is well described recently in [5]. Accordinc to this work, the formation of Tc carbides can be viewed as a process of inserting carbon atoms into Tc lattice, resulting in a lattice expansion.Compared with hcp Tc (14.59 °A3), the increments of specific cell volume (normalized per Tc atom) are 4.66%, 5.55% and 7.74% for Tc10C (15.27 °A3), Tc8C (15.40 °A3) and Tc6C (15.72°A3), respectively. All three these forms could be formed on the rection of very small amount of carbon elibirated on uranium fissionin the reactor. Thermodynamic modeling of the phase composition of fuel irradiated with fast neutrons requires knowledge not only of the temperature and pressure, but also of the concentration of fission products and of major and impuritycomponents at the given burn-up values. Our model calculations of changes in the chemical and phase composition of uranium–plutonium nitride containing 0.25 wt % C (U0.8Pu0.2N0.9475· C0.0525) were based on data from [7] for U0.8Pu0.2N and ASTRA-4 program complex [8]. Some preliminary results are presented in [9]. Finally, concerning Tc our calculation predicts that at high temperature the cubic structure will be elemental technetium. This can be directly checked, and we performed quasiharmonic free energy calculations to investigate relative stability of the fcc and roomtemperature hexagonal (hcp) forms of technetium. As shown in [5] above 1775 K, cubic (fcc) Tc is more stable than the hcp phase of Tc due to its larger vibration entropy.