## 2 The iMAGINE System and Its Optimization  
In the IMAGINE system, the approach to sustainable and efficient operation is to concentrate on separating out the elements which prevent the reactor system from long term operation instead of reprocessing the entire fuel. This avoids the separation of fissile and fertile material, such as uranium, plutonium and the minor actinides with the consequent demand for fresh fuel production and the arising proliferation concerns. The new approach of cleaning the fuel in operation creates two further positive outcomes. First, by avoiding the proliferation issues of established reprocessing methods through avoiding the separation of fissile material, it also removes the potential opportunity for misuse and theft and the consequent requirement for a massive and expensive safeguarding activity. Secondly, it has the potential to remove one of the most challenging requirements of waste management; the demand when reprocessing and partitioning an extremely high separation rate of the actinides from the waste stream to avoid the loss of TRU material and its carry over into future final disposal. This problem is further aggravated in the current approach by the accumulation of fissile material in the waste stream when recycling multiple times (multi-recycling) in closed fuel cycle operations and P&T [22]. To underline the serious consequences of the resulting radiotoxicity from such carryover, the influence of the separation efficiency of the TRU elements on the waste stream and the time scale of its radiotoxicity is given in Figure 1. This observation makes it obvious that the losses result in an incomplete/insufficient result of the P&T process, not leading to the promised sharp decrease in the radiotoxicity but leading to a time delay of up to more than 1000 years before crossing the radiotoxicity line of the inserted fuel. The development of the new, innovative iMAGINE approach to the separation of fission product elements will not only enable radically different thinking in transmutation but also in nuclear and separation chemistry with the demand to deliver breakthroughs not only in molten salt reactor design and operation but also in the development of salt clean-up processes; the iMAGINE approach will require developments in partitioning to deliver an integrated closed fuel cycle solution. In particular, the use of liquid fuelled systems, such as molten salt reactors, offers the opportunity to reshape the reprocessing process completely [24] by concentrating on those elements which have the largest impact on maintaining criticality during reactor operation. This breakthrough approach would concentrate on those elemental species which through neutron absorption have the strongest effect in reducing criticality, and therefore, reducing reactor fuel usage efficiency. This avoids the established approach of the complete removal/separation of fission products to regenerate the new, rigorously clean fuel required for solid fuel reactors. This new approach and these new processes offer significant optimization potential [24] around such key factors as, the throughput of salt through the chemical cleaning process, the selection of elements/species for separation, and the required process efficiencies of these specific separation processes. In addition, there is the potential for the significant reduction or removal of the pre- and post-processing steps required for a solid fuel-based fuel cycle, such as the storage and transport of the spent fuel, its cooling and dissolution, the precipitation of select materials after separation as well as the subsequent solid fuel production. Some of these steps are very costly (both  
Figure 1: Radiotoxicity of nuclear waste over time and the effect of different efficiencies of the separation of plutonium and minor actinides, [23] intechopen, CC BY 3.0.  
economically and in terms of resources) whilst others are very time consuming and can lead potentially to significant radiation exposure for workers [7].