The 10 MW Indonesia’s Experimental Power Reactor (RDE) is a High Temperature Gas Cooled Reactor (HTGR)-type and planned to be operational in the Puspiptek Serpong area in 2022/2023. The reactor applies helium gas coolants, graphite moderator and 17% 235U enrichment fuels and has 8 control fuel rods. To design a nuclear reactor, there are many safety aspects which should be taken into account and one of them is neutronic safety analysis dealing with an accident of reactivity core change due to increases of fuel and moderator temperatures. To begin with the safety analysis, the RDE core should be modeled utilizing the combination of nuclear data libraries and a computer code. Prior to estimate the reactivity core change in the RDE core, the calculation of neutron effective multiplication factors during the accident was accomplished using the MCNPX computer code. In this paper, the neutronic analysis of RDE core has been focused on how the fuel temperature increase implies the reactivity core change in the RDE reactor. By combining the three major world nuclear data libraries and the MCNPX code, the all calculated results showed that all core reactivity changes in the RDE core due to fuel temperature increases starting from 26.85 °C (300 K) to 2,726.85 °C (3,000 K) are all negatives except those in the reflector zone which principally accumulates generated neutrons always coming from the central of the core during reactor operation. Therefore, those do not affect at all the safety of the reactor core during the accident. Indeed, the RDE reactor core is totally safe in the event of fuel-temperature-increase during its reactor operation and hence the RDE reactor is in a steady, safe operation.
