## 4 Modelling and Simulation Results
This is discussed in [24] in terms of the new optimization approaches made possible by integrated molten salt reactor systems, for which no pre- and post-processing of the fuel is required, as distinct from current technologies which use solid fuelled reactors and aqueous reprocessing. Another application will be in operational sensing and monitoring, which will be one of the most essential components of molten salt reactor systems in the short as well as the long term. This is because the fission process will result in a continuous evolution of the composition and potentially the chemical and physical properties of the salt through breeding, the fission process itself and fission product formation. Thus, monitoring and specific adjustment of this salt composition will likely be required for stable operation in addition to extraction for clean-up. A key challenge will be to keep the thermochemical, thermodynamic and redox environments as stable as possible for reliable operation. One example of the consequences of fission on reactor redox chemistry is the fission of one Uranium atom present as a U(IV) species in chloride, which results in Ba(II) and Kr(0) and the oxidation of 2 chloride ligands to produce free chlorine gas and/or the further conversion of U(III) to U(IV). Without control of such reactions, the operation could lead to significant deviation from the eutectic composition of 45%NaCl-17%UCl\({}_{3}\)-38%UCl\({}_{4}\), with the consequent change in the thermo-physical and thermochemical properties of the molten salt. A worst-case scenario could include dramatic changes in heat transfer and fluid dynamic properties induced by solidification or segregation, as a result of the consequent increase in the melting point of the salt when deviating from the eutectic. Another aspect where coupled chemical consideration is required is in fission product solubility and the release of gaseous fission products (like the noble gases). As well as for all fission products, which could be released in the form of aerosols due to the high temperature of the molten salt pool. This can be relevant since the melting point of the eutectic is much lower than for other compositions, which means in the case the reactor is not operated at the eutectic, higher temperatures will be required. This just demonstrates the importance of close inter-disciplinary interactions and coupled chemical and physical considerations for the successful design and operation of such a molten salt reactor system. The chlorine balance alone will require a coupled modelling and simulation campaign, delivering fission product composition, as well as the number of fissions for reactor physical models into the chemistry model and its feedback from chemistry to physics, e.g., due to the solubility and release effect for gaseous fission products; systems based on fluorine molten salts would produce different chemistries but offer similar process development challenges. A molten salt reactor system, such as iMAGINE, operating on a closed fuel cycle on spent nuclear fuel without prior reprocessing should be seen as one approach to integrating a whole fuel without the requirement for storage, transport, or external solid fuel production. However, this approach will still require further essential chemical stepsdirectly linked to reactor operation; the separation of fission products, their immobilization and the handling and control of the salt phase.