## 1 Introduction
It is impressive to see the dimensions: while the core consists of approximately nine inner rings, the outer structure consists of 14 rings leading to an increase in the size of the rings to \(\sim\)200 core assemblies compared with \(\sim\)1300 assemblies in the outer structure. New investigations have recently been delivered with the aim of replacing a part of the reflector with a more advanced material like ferroboron [12]. Other recent studies have assessed the impact of reflector material on the core design for SFR. Delivering a mixed spectrum core design [13] or a compact sodium-cooled breed-and-burn reactor design [14] would demonstrate the opportunities and the influence which can be created through innovative reflector applications. The very significant role of the reflector in critical experiments is maybe best demonstrated by the last documented criticality accident at Sarov (Arzamas-16) on 17 June 1997 [15]. This accident was caused when reproducing an experimental setup which had been already used in the 1970s. \"He [the experimenter] had taken the dimensions for all of the system components from the original 1972 logbook. However, when he copied down the inside and outside dimensions of the copper reflector (167 and 205 mm, respectively), he incorrectly recorded the outside dimension as 265 mm. When the experimental setup had been assembled, this change in the reflector led to a prompt critical spike and the \(\sim\)6.5 day excursion. In general, in most nuclear experiments, the reflector is an essential piece for improving the neutron economy, leading to a reduced demand for fissile material and thus reducing the cost. However, the reflector has the potential not only to reduce costs but also to influence the results of future experiments, which can lead to significant problems; e.g., in YALINA experiments, the idea of supporting the fast reactor's configuration by a decoupled thermal system with the aim of boosting criticality has led to some really surprising and hard to explain results [16] caused by the penetration of thermal neutrons into the fast region [17]. Thus an improper decoupling of the fast system from the thermal system has finally taken place in a real experiment. An additional challenge for future projects is the required and requested flexibility of a multi-purpose facility for reactor physics of advanced molten salt-based reactor systems which may need different reflectors for each technology considered, which, in turn, may have contrasting neutron spectra. The primary focus of this work is on a fast system, but with a clear outlook into flexible multi-purpose operations for the long term. This formed a secondary focus, keeping in mind the view and the requirements of the reflectors that are suitable for a potential future experimental investigation of a thermal core. The investigation started from a generic reflector which has been used in the previous studies [5; 6] and proceeded into an investigation to support the right choice of the ideal reflector for a molten salt zero-power reactor answering the research question \"Which reflector material will assure the best possible performance in a fast reactor experimental configuration?\". The investigation started with a discussion of the different roles of the reflector, followed by a broad investigation of different potential materials and a refined investigation of a down-selected number of attractive materials. It finished with a verification of the results with alternative calculation methods and a test of the stability of the chosen material for the control and shutdown proposals of [6].