This work aims at evaluating the performance of certain selected moderators in a Space Nuclear Reactor (SNR); the analysis is carried out in the framework of the SELENE project, a feasibility study of a 350 kWe reactor foreseen to operate on the lunar surface, involving, among the partners, also ENEA and Agenzia Spaziale Italiana (ASI). After contextualizing the importance of nuclear power in space and giving a brief historical review of the main technological milestones reached in this field, the focus will shift on explaining both the nuclear fission physics pillars of neutron moderation in thermal reactors and a description of the SELENE reactor design baseline from which the core of the work will take place.