###### Abstract
Technically, the option of draining the heavy-water moderator for shutdown would be another option which has been discussed. Sodium-cooled fast reactors are typically controlled via different sets of rod systems, safety (shutdown) rods, regulating rods, rapid shutdown rods, and sometimes additional shutdown rods [9] with different types of drives, and even flexible chains have been investigated in some projects. The sodium-cooled fast reactor community has also been working on passively activated safety shutdown systems [10]. The German pebble bed HTGR used 36 absorber rods for power control and reactor scram, which are inserted or dropped into the side reflector via gravity, and for long-term shutdown, 42 absorber rods with pneumatic drives could be inserted directly into the pebble bed [11]. However, other approaches such as withdrawing a certain amount of pebbles from the pebble bed reactor have been investigated after problems occurred with the insertion of the shutdown rods into the pebble bed. The use of a zero-power reactor system, KAHTER [12], developed for the design of the industrial demonstrator THTR created the potential to investigate, on the one hand, the design of the industrial demonstrator through experiments to support the future system understanding. On the other hand, a zero-power facility allows understanding and testing new approaches, such as the effect of withdrawing pebbles in this case, which will help in the system optimization. For future molten salt reactors, control rods [13] and core drainage are proposed for shutdown for MSR systems and have been used in MSRE [14]. Initial studies indicate that burnup control could be achieved through control of the core temperature, due to the strong thermal feedback effect [15] which allows compensating a significant amount of reactivity with only a moderate change in the salt temperature. The approach of draining the salt will not work in an MSFR zero-power experiment, which will most probably take place in the first step in a solid arrangement in order to reduce the complexity. Looking into more exotic approaches in control systems, tests have been performed on \"Critical-Assembly Experiments on a Reflector Control System for a Boiling Reactor\" [16], reflector manipulations have already been used very early on in reactor physics experiments [17], and reflector manipulation drums are often proposed in very compact space reactors [18] or training, teaching, or research reactors [19; 20]. For critical experiments, moderator-level manipulation has been used for criticality determination [21], and changing or manipulating the reflector conditions is also used in critical assemblies [17]. Splittingof the spherical core into two half spheres is another approach for shutdown which has been used in some very small educational reactors. In the most recent zero-power experiments for accelerator-driven systems, YALINA [22] and GUINEVERE [23], the control and shutdown function was provided through accelerator beam control. Based on this literature overview on control and shutdown mechanisms, the work we provide here will aim to develop and evaluate diverse control and shutdown approaches as well as their interplay on the control and shutdown of a homogeneous critical zero-power experiment for a potential molten salt fast reactor through the use of a multitude of modeling and simulation results using different tools of the SCALE package.