Mathematical Modelling of Intensified Extraction for Spent Nuclear Fuel Reprocessing

Abstract

Small scale extractors seem to be a promising intensified alternative to the conventional solvent extraction technologies, because of the well described hydrodynamics, enhanced mass transfer, and good phase separation at the end. One of the most interesting applications of intensified extractions is the reprocessing of spent nuclear fuel. Operating in small channels can reduce the volumes of involved hazardous materials and the residence times, thus minimising the degradation of the solvent and its regeneration cost. Finally, nuclear criticality safety may be easily achieved. In this paper, the application of small channels on spent nuclear fuel reprocessing has been investigated. A mathematical model of a multi-component liquid-liquid extraction has been developed. The multi-component system consists of U, Pu, HNO 3 , HNO 2 , Zr, Ru, Tc, Np(IV), Np(V) and Np(VI), the organic solvent is a mixture of 30% (v/v) Tri-Butyl Phosphate (TBP) and a paraffinic diluent. A segmented flow pattern, with the aqueous phase dispersed in a continuous organic phase, has been assumed. Calculations for the estimation of mass transfer, redox reactions, pressure drop, nuclear criticality and TBP hydrolysis have been included in the model. To increase the flow rates, the number of small channels was increased (scale out) and a comb-like manifold was considered to ensure good flow distribution in each channel. The problem is formulated as a mixed integer nonlinear programming problem and is implemented in the General Algebraic Modeling System (GAMS). The results show that this alternative technology for liquid-liquid extraction offers advantages, especially in terms of solvent degradation and low holdup volume. 4 3 3 4 (2) The reactions may be carried out in pulsed columns, mixer-settlers or centrifugal extractors. These technologies, however, have drawbacks including the need for large amounts of head space for pulsed columns, large floor space and poor geometry for criticality control for mixersettlers. Also, long solvent residence times are necessary, leading to solvent degradation. Centrifugal contactors are the most promising as a result of the short residence time and low holdup volume. However, their use is limited in industry since they are the least reliable of the three technologies, because of their poor tolerance to solids and need of periodic replacement of the motor and the rotor (Law and Todd, 2008). Volume and solvent residence times may be significantly reduced if small scale extractors are used instead of columns and mixer settlers. The large surface area to volume ratio of small scale extractors can also address criticality issues. There are no moving parts and their diameter may be large enough to avoid occlusion; therefore, they can sufficiently