## 1 Introduction  
Zero or very low power experiments are recognized as the first step into a new reactor technology, historically [1] as well as in the recently published new development process [2] and in recently initiated programs [3]. The major point in the past has been to test new configurations in safe settings and to use the opportunity of these highly flexible experiments to create an accelerated learning curve at the beginning of a new technology. In new programs, there is the additional demand to provide safety demonstrations and to validate code systems to assure the quality of the predictions for the next step, while using this multi-fold opportunity for learning in manufacturing and educating future reactor physics experts [4]. The first part of this current series of publications on zero-power studies delivers on the importance of a zero-power reactor experiment for the process of developing a new, innovative reactor concept in the form of a molten salt fast reactor, and the challenges due to the homogeneous core composition (unity of coolant and fuel) are worked out [5]. The second part delivers on new approaches for the control and shutdown of a homogeneous core experiment [6]. This will be complemented by this study of the choice of the neutron reflector as an essential part of efficiently operating small reactor cores, especially in the case of the fast neutron spectrum. The reflector is an essential part of all operating reactors, as well as for a wide variety of reactor experiments. However, the reflector sometimes fulfils completely different demands in different reactors (e.g., protecting essential components versus improving the neutron economy) and can be formed easily out of coolant, normal structural materials of the reactor, or from a special material. In light water reactors, the reflector is just provided by the light water coolant and the core barrel; partly, the outer row of the low leakage loading could also be seen as a kind of reflector. The major function of the reflector materials in LWRs is to protect the pressure vessel from high-energy neutron flux to keep the fluence on the vessel within an acceptable limit over the lifetime [7]. In high-temperature reactors such as the AVR [8] and the THTR [9], the major aim of the reflector is to improve the neutron economy; thus, graphite is used as the reflector and it is also used as a structural material. It is interesting to recognize that the relative size of the reflector significantly decreases with increasing core size. This demonstrates that the relevance of the reflector decreases with a decreasing surface to volume ratio. An interesting and relatively new approach is the proposed use of BeO reflectors. In the micro-reactor U-Battery concept, this approach has been proposed to reduce the core size. In comparison with the classical graphite reflector, a clear size reduction can be achieved; however, this size reduction comes with a significant cost due to the more exotic reflector material [10]. Sodium-cooled fast reactors are significantly different in the design of the reflector, due to the closed system and the high-energy fast neutron flux, resulting in different approaches. The reflector in a SFR such as the pool type PFBR [11] comprises several rings, starting with the breeding blanket to improve neutron economy while breeding fresh fissile material, followed by a steel reflector and a B4C shield, which can be seen as the core. This area is followed by the internal vessel storage for the fuel, required due to the high decay power of the fuel assemblies when unloaded from the core. The internal storage is surrounded again by a stainless steel reflector and, finally, a large B4C shielding to prevent the heat exchanger containing secondary sodium from the high energy neutron flux, which could otherwise lead to activation of this secondary coolant.