# On the Dimensions Required for a Molten Salt Zero Power Reactor Operating on Chloride Salts
## 1 Introduction
In contrast to this, a major point in the zero power experiments is the significantly larger operational envelope of a zero-power facility and the higher flexibility than in power producing systems, while not requiring an energy removal system due to low or at least moderate operational temperatures and very low power. The low flux and very low power levels allows comparably easy access for the experimenters due to the reduced radiation level caused by the absence of large amounts of fission products. These properties widen the space of available instrumentation significantly and allow rapid changes of the setup to investigate different configurations. The properties also allow for almost immediate accessibility to the experiments after shut down and assure extremely fast take-up of a relevant set of experimental results. Following these arguments, the last, newly established technology, accelerator driven systems, and lead cooled reactors have been accompanied by two different zero power experiments, YALINA [5,6] and GUINEVERE [7,8], to create an understanding of the interaction between the accelerator and the sub-critical core as well as to deliver validation for lead-cooled reactor systems. Looking into molten salt reactor technologies, completely new approaches for operating reactors directly on spent fuel have come up through the proposed iMAGINE technology [3,9], and many companies like TERRESTRIAL Energy [10], TERRAPOWER [11], Elysium Industries [12], and others are planning reactors which will have demands for code and design validations. However, due to the homogeneous core composition (unity of coolant and fuel) of a molten salt reactor, a zero power reactor experiment is significantly different to existing, heterogeneous experiments. Thus, novel approaches for design, operation, control, and experimental setup must be scoped out, developed, and invented beginning with this series of publications. Meanwhile, Russia has already announced plans to develop their own research reference facility for their partitioning and transmutation molten salt reactor programme [13] and Terrapower has announced their approach using a Molten Chloride Reactor Experiment with low power (300 kW) to be located at the Idaho National Laboratory [14]. The general choice for the NaCl-UCI salt system has already been investigated in earlier works [15,16]. The major point for this decision is on the good potential for breeding (high heavy metal loading in the eutectic), which is required for the envisaged long-term self-sustained operation based on spent nuclear fuel, which is the core of the iMAGINE  
Figure 2: Process steps for the development of a new, innovative reactor system following the approach of [3]. technology. The focus of this publication will be on answering the questions on the salt compositions (i.e., potential enrichment and heavy metal loading) and the dimensions required of a potential very low power experimental setup. This will narrow down the choices for a zero power experiment for a molten salt fast reactor based on a multitude of modelling and simulation results using different tools of the SCALE package.