## 4 Test of Different Materials  
The study of the optimization of the reflector was, in the first step, based on the 2D critical reference system with the following dimensions: a core radius of 49.4 cm in a 2 cm thick SS304 vessel surrounded by a 30 cm thick NaCl reflector (see the arrangement given in Figure 2) and calculated using TRITON/NEWT. The reflector material choice for the investigation was driven, in a first step, by looking into traditional materials used in nuclear reactors and critical experiments; in the second in the second step, by looking into materials which came under discussion in the last years. Following this first insight, the driver was to create a deeper understanding of the effects of different alloying elements of stainless steel with the view of leveraging this knowledge for potential optimization. The results of all steps will be finally used for a down-selection for deeper studies. In the first analysis, typical materials used in nuclear reactors and experiments were investigated, and the change in criticality is shown in Figure 4. The strong influence of the reflector on the system's criticality is visible between the helium case (the reference for a non-reflected system) with k\({}_{\rm eff}\) = 0.7434 and the graphite case with k\({}_{\rm eff}\) = 1.1903, which would, in the end, reflect a massive difference in the required amount of fissile material for the experiment, leading ultimately to massive cost savings. Beside graphite, the investigated high-density metals led to a clear increase in criticality, while the low-density light metal sodium, as well as water and the shielding material ferroboron [12], led to a reduction of criticality compared with the reference material, NaCl. In the next step (Figure 5), the study was widened, on the one hand, to more exotic materials for reactor use, which are sometimes proposed for future reactor systems or for very specific issues in reactors. On the other hand, a more in-depth analysis was performed to understand the large difference between carbon steel containing 99% iron and 1% carbon (k\({}_{\rm eff}\) = 1.1271) and stainless steel SS304 (k\({}_{\rm eff}\) = 1.155). This was achieved by systematically studying the separate effects of the main alloying elements: chrome, nickel, manganese, and silicon, each singled out in a composition with 20% of the alloying element in 80% iron. The results of this single-effect study are shown in Figure 5. The study of the alloying elements indicated a strong influence appearing in the case of the addition of silicon. In addition, the study of alternative reflector materials indicated a very high k\({}_{\rm eff}\) for using BeO as the reflector material. Two other often-proposed hydrogen rich reflector materials, polyethylene and ZrH\({}_{2}\), do not deliver the expected improvement in criticality when used as reflector materials. Figure 3: General 2D (**left** and **centre**) Monte Carlo model of the configuration (**left**, top view x-y; centre, front view x-z) and 3D (**left** and **right**) Monte Carlo model of the configuration (**left**, top view x-y; right, front view x-z) with a molten salt fuel core (blue), a stainless steel vessel (light green), the reflector (yellow), and a vacuum (grey) used for the validation of the results and the 3D studies.