## 4 Modelling & Simulation Results  
### Shutdown System Study  
The currently proposed configuration for a molten salt fast reactor experiment is based on a homogeneous core composition with a heavy metal-rich salt composition containing 20% NaCl-56.35%UCL\({}_{4}\)-23.65%UCL\({}_{3}\) as it has been investigated for the breeding [28] and feedback [15] studies for iMAGINE. Such a homogenous system is in strong contrast to the classical zero-power reactor studies which are, in most cases, designed to investigate heterogeneous configurations, having the aim of understanding the effect of the heterogeneity. In a homogeneous system, the investigation of the undisturbed system will be of high interest, and thus a shutdown system which introduces a perturbation in the homogeneous structure would be a penalty. Looking into the study of homogeneous critical systems, these systems are often controlled via neutron leakage. In addition, the system should ideally be applicable in the solid state at room temperature for a first operational stage as well as for the liquid core in a higher temperature operation at a later stage of the experiments. A first guess of such a shutdown system is presented in Figure 5. In this approach, the bottom reflector of the core will be removed. Two different approaches are investigated: in case 1, only the bottom reflector is removed, while the side reflector is not changed, and thus only a gap is created, while in case 2, the side reflector will be limited to the bottom of the core, and thus a streaming path for neutrons to leave the system will be opened. Case 3 replicates case 1 but with an opening of 50 cm compared to the 30 cm opening in case 1. Case 4 follows the same principle, where it replicates case 2 but with a 50 cm opening instead of 30 cm. All 4 cases were calculated based on identical Monte Carlo settings and delivered the following outcome for the shutdown margin: case 1, \(-\)1855 \(\pm\) 33 pcm; case 2, \(-\)2737 \(\pm\) 33 pcm; case  
Figure 4: 2D SCALE/TRITON/NEWT model (molten salt fuel core—blue, stainless steel—light green, NaCl reflector—yellow, vacuum—gray) used for the analysis of the neutron flux distribution. 3, \(-2596\pm 30\) pcm; and case 4, \(-3181\pm 33\) pcm, all with a 95% confidence interval, which indicates that the statistics of the Monte Carlo calculation do not have a significant influence. The change from case 1 to case 2 aims to limit the influence of the side reflector and increase the shutdown margin by almost 50%. Increasing the opening from 30 to 50 cm increases the shutdown margin by \(\sim\)40% when the side reflector is not reduced, but only by a bit more than 15% when the side reflector is cut. Thus, creating a wider opening is only of interest when it creates an additional streaming path for neutrons, as in case 3. However, all cases demonstrate that the achievable shutdown margin is only in the area of 2000 to 3000 pcm, which is perhaps insufficient. Based on the above achieved results, it would be of interest to enhance the effect of the removal of the bottom reflector. A promising approach could be to remove not only the reflector but also a limited layer of the core. A comparable strategy was implemented in the teaching reactor SUR-100, which is based on a structure of polyethylene plates with homogeneously distributed fuel. However, while the SUR-100 is only a very small core with a very limited mass, the system for a zero-power reactor will have a substantial mass. Thus, specific solutions have to be developed to deal with this challenge. The investigation of this strategy is presented in the models created for the analysis and shown in Figure 6. For illustration, the base case is shown in the left top corner, and it is identical to case 2 of Figure 5. In the following steps, slices of 10 cm (top right), 20 cm (bottom left), and  
Figure 5: Different cases investigated for the study of the shutdown system moving the bottom reflector away from the core (top left—case 1 (reference), top right—case 2, bottom left—case 3, bottom right—case 4).