## 2 The Role of the Reflector  
Some of the different roles a reflector can play in a nuclear reactor and in nuclear experiments have already been discussed in the review of existing solutions in the introduction. However, it is important to collect all the functions of the reflector and all the requirements of the reflector in the case of a zero-power experiment; both points will be delivered in this section. Theoretically, the main aims of a reflector that surrounds a nuclear reactor or experiment are:  
* Saving neutrons by improving neutron economy,
* Protecting the environment from radiation,
* Protecting the experiments from the environmental influence,
* Potential control of the moving reflector, carrying the control system via rods or drums,
* Possibility for massive cost savings. The potential savings of adding a reflector to a zero-power reactor core has been investigated based on SCALE/POLARIS calculations for different enrichments and the related critical radius. Both different systems use a NaCl reflector; while the salt system is slightly different, one system is based on a eutectic NaCl-UCl\({}_{\text{X}}\) fuel, 42.5% NaCl-40.5 UCl\({}_{\text{4}}\)-17% UCl\({}_{\text{3}}\) (denoted 42.5% NaCl), and a second heavy metal rich system on 20% NaCl-56.35% UCl\({}_{\text{4}}\)-23.65% UCl\({}_{\text{3}}\) (denoted 20% NaCl) [5]. For both salt systems, the volume gain achieved through adding a 30 cm thick NaCl reflector has been determined and is given in Figure 1. There is a clear trend that the gain will increase with decreasing core size and increasing enrichment, which seems to be natural, while the gain for the heavy metal rich system is not very pronounced and may even be explained by the slightly reduced core size. However, it is clear from the figure that the volume of the fuel required for the experiment can be reduced by between 30 and 45% when a 30 cm thick NaCl reflector is applied. In addition to the potential gain and the protective effect of the reflector in nuclear reactors, some very specific demands on the reflector have to be kept in mind for a zero-power experiment, with some additional specifics for a molten salt reactor experiment:  
* The reflector should save as many neutrons as possible--reducing costs and reducing the core size or enrichment. * The reflector should only have a limited effect on the neutron core spectrum, which will have an influence on the power production/neutron flux at the core boundary in the spatial, as well as the energy distribution. * The reflector should be usable for all temperature ranges of the experiment. * The reflector should not disturb potential kinetic experiments--the long life-time of neutrons (e.g., in graphite or neutron production)--in the YALINA experiment. * The reflector should cause only limited disturbance of the natural neutron flux distribution of a homogeneous fast system. * Through the use of the reflector, the zero-power core will deviate from the future power-producing core. However, the issue of bias/reproducibility factors should be as limited as possible. * The reflector could act as a moderator for the thermal system or could provide additional neutrons, e.g., a hybrid system as in the YALINA experiments. The core study will focus on the experiments for the fast system as envisaged for iMAGINE [2], which ultimately will be a large-scale system operating on spent nuclear fuel without prior reprocessing [18]. Thus, the design of the zero-power reactor, as well as the experimental equipment and the experiment itself, will require some substantial innovation to allow as much reproducibility/transferability of the experiments through special approaches [19] to draw conclusions for the larger-scale systems foreseen in the process of developing an advanced reactor system as described in [2].