| [Solid and liquid wastes from industrial processes: Innovations in material recovery and environmental protection] Initial stages of nitride spent nuclear fuel (SNF) processing Thermodynamic modeling and experimental results Initial stages of nitride spent nuclear fuel (SNF) processing Thermodynamic modeling and experimental results Alexei Potapov1; Kirill Karimov1; Vladimir Shishkin1; Mikhail Mazannikov1; Yurii Zaikov1; 1INSTITUTE OF HIGH TEMPERATURE ELECTROCHEMISTRY, Ekaterinburg, Russian Federation; PAPER: 292/Recycling/Regular (Oral) SCHEDULED: 17:50/Sat. 26 Oct. 2019/Adonis ABSTRACT: Currently, a number of countries are developing new nitride (UN-PuN) fuel for fast-neutron nuclear reactors [1]. At the same time, methods for nitride SNF processing are developed. These methods are proposed to abandon water technologies in favor of pyrochemical methods, which use molten salts [2, 3]. As nitrides are electronically conductive, an initial opening of nitride SNF was suggested to be performed by electrochemical dissolution with the transfer of SNF components into the molten LiCl-KCl eutectic. It turned out, however, that the reaction UN - 3e = U<sup>3+</sup> + 0.5N<sub>2</sub>↑ is accompanied by the UNCl formation reaction. UNCl is an insulator, it covers the anode and dissolution stops. Currently, the research is suspended. Another method is “soft” chlorination (i.e. without chlorine gas). In the (LiCl-KCl) <sub>eut.</sub> melt, the following reactions are possible: UN + 1.5CdCl<sub>2</sub> = UCl<sub>3</sub> + 1.5Cd + 0.5N<sub>2</sub> ΔG = -60.6 kJ/mol at 500°C (1) PuN + 1.5CdCl<sub>2</sub> = PuCl<sub>3</sub> + 1.5Cd + 0.5N<sub>2</sub> ΔG = -149 kJ/mol at 500°C (2) Experimental verification, however, revealed that at 500 0C the reactions of formation of a number of other stoichiometric and non-stoichiometric nitrides - UN<sub>1.5</sub>, UN<sub>1.55</sub>, UN<sub>1.5</sub>, UN<sub>1.55</sub>, UN<sub>1.69</sub>, UN<sub><sub>1.74</sub></sub>, UN<sub>2</sub> and UNCl proceed along with reaction (1). The UN → UCl<sub>3</sub> conversion degree is ~ 30%. The rest of the uranium was found as a black precipitate that consisted of a mixture of UNCl and various nitrides. Using thermodynamic modeling, it was determined that, at a temperature of 750°C and above all nitrides, including UNCl, dissolve in an excess of CdCl<sub>2</sub> to form UCl<sub>3</sub>. The experiment confirmed the calculation results completely. It was also found that the CdCl<sub>2</sub> substitution by PbCl<sub>2</sub> allows reduction of the process temperature to 650°C, but in this case, a UCl<sub>3</sub> + UCl<sub>4</sub> mixture forms. Another likely the first stage of the nitride SNF processing can be voloxidation, i.e. conversion of all nitrides to oxides by heating in air. For example, the uranium oxidation chain is as follows: UN → (U<sub>2</sub>N<sub>3</sub>+UO<sub>2</sub>) → UO<sub>2</sub> → U<sub>3</sub>O<sub>7</sub> → U<sub>3</sub>O<sub>8</sub> → U<sub>2</sub>O<sub>6</sub> (gas) (slowly) An important advantage of this approach is that we obtain a universal process suitable for processing both nitride and oxide SNF. References: [1] R. Thetford, M. Mignanelli, J.Nucl. Materials 320 (2003) 44-53.<br />[2] Yu.P Zaykov, Yu.S. Mochalov, V.A. Khokhlov, V.Yu. Shishkin, A.M. Potapov, V.A. Kovrov, M.N. Gerasimenko, A.S. Zhidkov, S.G. Terentev, Pyrochemical recycling of the nitride SNF of fast neutron reactors in molten salts as a part of the short-circuited nuclear fuel cycle, in: Int. conf. on Fast Reactors and Related Fuel Cycles: (FR17), 26-29 June 2017, Ekaterinburg, Russia, (CD) IAEA-CN245, Rep.259. <br />[3] V.Yu. Shishkin, A.M. Potapov, V. Kovrov, Yu.P. Zaikov, The peculiarities of pyrochemical reprocessing of spent nuclear fuel, in: Int. nuclear fuel cycle conf. Nuclear energy innovation to the carbon-free world (GLOBAL 2017), September 24-29, 2017, Seoul (Korea). Proceedings. Paper EA-104-PD3. |
