Development of a Solvent Extraction Process for Group Actinide Recovery from Used Nuclear Fuel

Doctoral thesis, 2012

When uranium is used as fuel in nuclear reactors it both undergoes neutron induced fission as well as neutron capture. Through successive neutron capture and beta decay transuranic elements such as neptunium, plutonium, americium and curium are produced in substantial amounts. These radioactive elements are mostly long-lived and contribute to a large portion of the long term radiotoxicity of the used nuclear fuel. This radiotoxicity is what makes it necessary to isolate the used fuel for more than 100 000 years in a final repository in order to avoid harm to the biosphere. To diminish this long-term radiotoxicity of the waste, to further increase the energy utilization and to decrease the heat load of the final repository there is an advanced reprocessing option called Partitioning and Transmutation (P&T). Within P&T the transuranic elements are separated from the fission products in the used fuel and transmuted using a fast neutron spectrum. During transmutation these long lived elements are transformed to short lived or even stable ones. The partitioning for transmutation can be realised using liquid-liquid extraction. Within this work a liquid-liquid extraction process of GANEX type has been developed and studied. The GANEX (Group ActiNide EXtraction) concept consists of two cycles; a first cycle where the uranium bulk is removed from the fuel dissolution liquor and a second cycle (the actual GANEX extraction) where the transuranic elements as well as residual uranium are extracted together as a group. Here, only the second cycle has been studied. The GANEX solvent developed comprises of the extractants CyMe4-BTBP and TBP in cyclohexanone. This solvent composition was found to be able to efficiently extract the actinides as a group from nitric acid. The actinides could also be separated from most of the fission products (including the trivalent lanthanides) with high separation factors. The few co-extracted fission products could to a large extent be managed by the addition of water soluble suppressing agents and scrubbing reagents. The solvent was found to be stable towards both hydrolysis as well as γ-radiolysis in the presence of nitric acid. The process was also shown to work under fission product loading conditions and after acid scrubbing of the solvent the actinides could be recovered as group. In addition a single stage continuous test was performed to test the solvents suitability for process implementation.