## 3 Results and Discussion  
_Criticality Effect of Fission Products_  
Generally, the criticality of a molten salt reactor is characterised by two different effects, the number of fissile materials, which can be changed through burning and breeding effects or feeding, and the number of neutron absorbers--fission products, which will accumulate due to burnup and can be reduced through salt clean-up. Figure 3 indicates the difference in the opportunities given through the use of a molten salt system. The red curve indicates the criticality behaviour, as would be the case in a classical, solid-fuelled reactor. In this calculation, all fission products are kept in the system; the calculation ran as one normal continuous calculation over the whole burnup period. In contrast, the blue curve is calculated using the above-described script with a cycle length of 10 GWd/tHM. After each cycle, the gaseous and volatile fission products, the elements 18, 35, 36, 53, 54, 85, are eliminated from the system, and the fuel is uniformly redistributed in the core. This first comparison already demonstrates the role of dynamic simulation in the investigation of long-term operations. While the continuous run, including all fission products produced and no redistribution of the materials, leads to improved breeding of materials at the beginning as well as a sharp drop in the criticality caused by the fission products (red curve), the dynamic simulation behaves much more stably. This difference is caused by the use of all fissile material in a comprehensive way (fuel does not stay in outer areas where it does not receive sufficient neutron flux) as well as due to the removal of the gaseous and volatile fission products through the script (blue curve). In the blue curve, each result of each HELIOS burnup step is shown, indicated by the increase in criticality through the five steps in one cycle and the followed drop in criticality caused through the redistribution of the material when the new input is generated through the PYTHON script. As a starting point for the dynamic analysis, a continuous run to a burnup of 300 GWd/tHM has been performed to obtain an overview of the accumulation of fission products during burnup, which is used to rank the fission products based on their concentration in ppm. The results have been produced on the full isotope set available in HELIOS, leading to the ranking given below, given in Tables 1 and 2. The results of this calculation coincide well with earlier simulations based on the use of SCALE/Polaris [16], leading to the same ranking of the concentrations, even when considering that the initial enrichment is slightly different and the POLARIS model itself is much more limited than the HELIOS model. However, the evaluation of the differences is only for some elements in very good agreement, while some elements show reasonably large differences of up to almost 30% in the worst case. Anyway, we should keep in mind that the results are for 100 GWd/tHM. This level of burnup is already almost double the standard LWR burnup, which is the normal level for the validation of such codes. The deviations show the clear demand for experimental investigation, essential for the validation of the codes for molten salt reactor application in the future when higher precision M&S results are required for detailed design considerations. For the current level of the study to support the development of separation technologies with the required basic data, the quality of the results and the coincidence of the ranking of the elements with the highest concentrations are very meaningful, even if the absolute number may deviate.