[en] There has been a renewed interest to develop new nuclear power systems for commercial deployment. Of the design categories receiving significant attention is molten salts reactors (MSRs). There are many MSR design configurations under development, this paper focuses on the thermal-spectrum, pebble-bed, salt-cooled concept, often referred to as the Fluoride High-temperature Reactor (FHR). Constructing a geometrically accurate model of a pebble bed reactor with double heterogeneity is addressed before any further analysis is conducted. Within the FHR design concept geometric and material parameters can vary considerably. The purpose of the paper is to examine two fundamental design parameters; the level of moderation and the coolant salt selection. Results indicate that additional moderation beyond the graphite in the pebbles is beneficial and alternative salts to 2⁷LiF-BeF₂, despite inferior results, do generate an adequately high multiplication factor. (author)