Bascone, Davide; (2018) Modelling and optimisation of intensified extraction in small channels for spent nuclear fuel reprocessing. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Nuclear energy is considered an option for future power supply. Spent Nuclear Fuel (SNF) reprocessing is essential to reduce the volume of nuclear wastes and to recover reusable materials, such as uranium and plutonium. Nowadays, all the commercial plants rely on the Plutonium Uranium Reduction Extraction (PUREX) process, an over 60-year old process. In the present work, a mathematical model for liquid-liquid extraction in small channels has been developed. The model is suitable for SNF reprocessing. Calculations of thermodynamics, hydrodynamics, pressure drop and nuclear criticality are included in the model. Several components and redox reactions, between the various oxidation states of U, Pu and Np, have been considered. Also, to increase the throughput and provide a good flow distribution within the channels, the design of a comb-like manifold has been included into the calculations. The resulting model, posed as optimisation problem, is a mixed-integer differential optimisation problem. The goal is to develop a methodology that allows to explore alternative flowsheets for the nuclear fuel cycle, using the small-scale extractor. Different case studies have been investigated. Firstly, the “codecontamination” section of the PUREX process has been investigated to demonstrate the applicability of the model. Secondly, a novel codecontamination section has been investigated to compare the small-scale extractor and the two main conventional technologies, i.e. pulsed column and mixer-settler. Finally, an alternative flowsheet has been proposed, using the small-scale extractors. This process has been obtained using a superstructure optimisation approach. The flowsheet produces a mixed uranium/plutonium oxide, to preclude the risk of nuclear proliferation. The mathematical model, despite its size and complexity, has been successfully solved in a short computational time. Results have shown that intensified extraction in small channel can provide to several benefits over the conventional technologies, in particular in terms of solvent degradation and mass transfer.

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