The present paper aims to compare the calculated and measured spectra after insertion of candidate materials for the MSR/FHR reactor system concept into the LR-0 reactor. The calculation is realized with MCNP6 code using ENDF/B VII.0, JEFF 3.1, JENDL 3.3, JENDL 4, ROSFOND and CENDL 3.1 nuclear data libraries. Additionally, comparisons between the slowing down power of each media were performed. The slowing down properties are important parameters affecting the thickness of moderator media in a reactor.