# On the Dimensions Required for a Molten Salt Zero Power Reactor Operating on Chloride Salts
## 4 Summary and Conclusions  
The investigated topic is introduced by making the case for the importance of a zero power reactor for the process of developing a new, innovative vector concept, such as a molten salt fast reactor. This is based on historical developments as well as the current demand for experimental results and relevant key factors for the success of the next step in the development process. The focus of the modelling and simulation work in this publication is on answering the questions on salt compositions and the required dimensions of a potential experimental setup to narrow down the choices for a zero power experiment for a molten salt fast reactor. A multitude of modelling and simulation results using different tools of the SCALE package is delivered studying two different salt systems, the eutectic and the heavy metal rich systems. The studies on the 2-D system radius show the volume reduction to be strongest in the step from 20% enrichment to 35% enrichment and the reflector effect to be  
Figure 14: Expected core volumes for the different studied cases using multi-group and continuous energy Monte-Carlo simulations. continuously increasing with enrichment. This results in reductions of the system radius, while the heavy metal rich system requires a smaller volume in all cases. The study of the thermal feedback effects (salt density and fuel temperature) shows a strong increase in the feedback effect with increasing enrichment and decreasing system radius with some indication of saturation at very high uranium enrichment. The study of the neutron spectrum indicates that the systems have a slightly harder neutron spectrum than in a typical sodium cooled fast reactor with some limited spectral hardening in the heavy metal rich system, increased spectral hardening in the case of the absence of the reflector, and significant spectral hardening in the cases with very high uranium enrichment. The analysis of the spatial neutron flux distribution of the reflected system using five specifically defined energy groups demonstrates that a major part of the lower energy flux is created in the reflector through collisions, which will lead to a slight increase of the power production in the outermost core region while the majority of the fast neutron flux follows a distribution with a strong central neutron flux peak. Based on the results and considerations on cost (required amount of fuel and potential cost increase as a function of core size) and non-proliferation/safeguarding requirements (potential use of high enriched Uranium >20%), three configurations have been chosen for the follow up 3-D analysis--the eutectic composition with 35% uranium-235 enrichment and the heavy metal rich compositions with 20% and 35% enrichment. The expected core volumes for these configurations have been studied using multi-group and continuous energy Monte-Carlo simulations, identifying the 35% enriched systems as the most attractive ones, but always with the caveat of the increased safeguarding requirements. Finally, if adding the potential facility size resulting from core size, the choice is disposed to the heavy metal rich composition of 35% enrichment as the reference system for future studies on zero power reactor investigation. However, if core size is not a limiting criterion, the eutectic system could be attractive too, since the required amount of UCI does not differ significantly between both compositions. The extra volume of the eutectic system is almost completely made up by the comparably cheap NaCl. The inter-comparison of the different applied codes and approaches available in the SCALE package has delivered a very good agreement between the results, creating trust in the developed and applied models and methods. However, this will not be sufficient to avoid the need for validation experiments to assure the best possible results for future modelling and simulation of molten salt reactor systems.