## 2 Basic Considerations for Control and Shutdown  
Typically, a reactor is controlled through a set of active control systems based on the basic principles of reactor control (see Figure 1) and system-inherent physical processes.  
These active principles are supported in almost every well-designed reactor system by inherent control mechanisms based on the feedback effects due to temperature changes and the resulting effects (e.g., fuel temperature effect, moderator temperature effect, coolant density effect), even if these effects are not always all negative, and the overall balance of all feedback effects must be negative to ensure stable reactor operation.  
From the basic list above, it is already apparent that some control and shutdown approaches can act in parallel on different principles, e.g., control rods in the absorber to reduce the number of neutrons and change, through this, the neutron leakage from the core. Interestingly, the described inherent control mechanisms of reactors act on the same underlying basic principles, e.g., the increase in the fuel temperature leads to an increase in  
Figure 1: Basic principles of the control of reactivity in a reactor.  
the resonance absorption of neutrons, or the moderator density effect affects the fission probability of the neutrons due to a change in their energy.  
To understand the specific demand for the control and shutdown system to be designed, it is important to understand the specific boundary conditions. Two major points have to be mentioned here: Molten salt fast reactors typically consist of a homogeneous core configuration in contrast to typical solid fueled reactors with a heterogeneous fuel assembly and core composition. A zero-power experiment will have the opportunity/requirement to operate under room temperature with a solid salt, as well as a molten salt system at a higher temperature. Thus, the control system should be able to cope with both conditions, leading to a system with weak feedback effects at the solid state and the potential for a much stronger feedback in the molten state. In addition, in a reactor physics experiment, the control system has an additional function, where the system is typically used for the exact determination of criticality which has a strong role in the validation of codes on criticality, e.g., via the determination of the control rod position in both the experiment and the calculation.  
The described demand specifies the following requirements:  
* The shutdown and control system should be diverse;
* The systems should be able to operate independently;
* Both systems should allow a safe shutdown;
* The control system should not create a major inhomogeneity to avoid any impact on the experimental investigation;
* The control system should deliver sufficient sensitivity for the validation of criticality, e.g., measurable and reproducible movement following a temperature change in the core;
* The interaction of the systems should not impinge on their individual purpose;
* Both systems should be applicable in a solid as well as in a liquid core.  
Based on these requirements, two different systems are proposed:  
* Shutdown via splitting the reflector, or a part of the core with the reflector, from the main part of the core downwards. This can provide a fail-safe solution for the ultimate shutdown driven by gravity.
* Control via a vertically movable radial reflector as a movable sleeve around the reactor.  
Both systems will be usable for a zero-power system with a liquid as well as a solid core, and even more importantly, both systems should somehow work at the integral system level without disturbing the central part of the core, which will be the essential area for the experimental measurements. Both approaches should also be able to act as a singular system, as well as together. Their interaction and the sensitivity of the control system will be investigated in the following through the use of a multitude of modeling and simulation results using deterministic and Monte Carlo tools of the SCALE package.