The sustainable progress of nuclear power relies on a safe and secure management of radioactive waste and on the implementation of processes fulfilling the enhanced safety and security principles of an advanced nuclear fuel cycle. To this end, a pyrochemical electrorefining process for the recovery of actinides from metallic nuclear fuel based on An-Zr alloys in a molten salt medium is investigated at ITU. It consists of anodic dissolution of the fuel in a molten salt bath and selective electrochemical reduction of the actinides that are then collected in the form of An-Al alloys on solid aluminium cathodes. For the separation of the actinides from the aluminium a chlorination route is proposed at ITU. It consists of three steps: (I) distillation of the adhered salt; (II) chlorination of the An-Al alloys with either HCl(g) or Cl2(g) under conditions ensuring a full chlorination, forming AnClx(s) and AlCl3(g) avoiding formation of volatile actinide chlorides. A subsequent sublimation step (III) to remove any remaining AlCl3 is also investigated.
Prior to laboratory experiments, thermodynamic equilibrium calculations were performed to evaluate the optimal reaction conditions for U, Pu and Np. Using the equilibrium module of the software HSC-Chemistry 6.0, the conditions for different temperatures and ratios of chlorination gas to An-Al alloys were calculated. Based on these calculations the temperature range to be tested experimentally was limited to 300-400 °C in the case of HCl(g) and 150-170 °C in the case of Cl2(g) to exclude the formation of volatile actinide chloride species such as UCl6(g).
As starting materials, samples prepared by electrorefining of U-Pu-Zr alloys, as well as An-Al alloys prepared by arc melting of the pure elements, were used. The experiments were carried out in a glove box under nitrogen atmosphere. The box is equipped with a Ar/HCl/Cl2 gas line, a vacuum pump and a vertical furnace in which a reactor of 1l in volume made of quartz glass is placed. A boron nitride crucible was used as the reaction vessel. After inserting the An-Al alloys into it, it was placed in the quartz reactor. The reactor was filled with Ar, heated to the desired temperature, filled with the chlorination gas and isolated for a specific time.
The evaluation of the process efficiency is based on XRD and ICP-OES analysis of samples taken from the reaction products and from the cold parts of the reactor, where evaporated material such as AlCl3 is condensed. The formation of actinide chlorides with high efficiencies was proved without An losses by volatilisation.