## 5 Conclusions and Outlook  
Molten salt reactors, such as the proposed IMAGINE system provide new opportunities for large energy production, as well as for nuclear waste management in an integrated system. Ideally, this technology development will help to link these two objectives effectively for the first time since industrial energy production in fast reactors ceased in western countries. Molten salt reactor systems offer this opportunity due to a liquid phase core and absence of the requirement for pre- and post- processing of the fissile material compared to alternative classical solid fuelled reactor systems. There is a clear opportunity and demand for a new type of reprocessing, which separates off those elements that reduce the efficiency of the neutron economy, thereby preventing long term reactor operation, in contrast to the established current approach of separating fissile and fertile materials. This new approach will deliver two major advantages; it will help to avoid proliferation concerns due to the absence of separated fissile material and it will avoid a so-called 'plutonium economy'. This is due to Pu staying inside the reactor system at all times. Here, the Pu acts as a kind of catalyst enabling the use of U-238 as an almost unlimited energy resource through a breed and burn system, which should create a factor of up to 100 times more energy out of the given Uranium resource. This seems an attractive alternative to expensive and proliferation enhancing separation and reinsertion steps in the closed fuel cycle, as well as to solid fuel fabrication. The proposed new approach also could improve financial sustainability since it eliminates most of the costly and complex fuel cycle steps required in other reactor systems. Furthermore, this proposed, innovative approach could further reduce costs and complexity by avoiding the traditional requirements of extremely high separation rates and efficiencies for classical reprocessing. These demanding traditional requirements are a consequence of the impact of any residual TRUs into the final disposal stream with consequent implications for the waste management strategy and the required observation and active cooling times required in final disposal. For this new, innovative approach an important first step outlined here is to identify the most harmful elements which poison the neutron chain reaction and affect core criticality since these are the elements for which the separation processes are likely to be most effective. A series of calculations of the amounts of the specific elements appearing in the core after a burnup of 100 GWd/tHM have identified Ruthenium and Molybdenum as elements with the highest influence on criticality, each formed after 100 GWd/tHM burnup at the reasonably high concentration of 2500 to almost 4000 ppm. The next elements identified were Caesium, Neodymium, and Palladium. This work is an important first step; however, to be really successful in not only revolutionising the reactor design but also in creating effective fuel cycle integration, it is essential to create a much closer link between reactor physics, reactor design and operation, reactor chemistry and the chemistry of processing. Additional challenges include the requirement for a coupled modelling and simulation strategy which deals with changes in the chemical composition of the salt during burnup and the resulting effects on the physicochemical properties. For example, changes in chemical composition could lead to changes in physical properties, redox states, e.g., the amount of free and bound chloride in the salt, which needs to be monitored and potentially addressed through selected chemical addition, as well as targeted chemical extraction in such a system.