Summary This paper discusses the selection of reflector materials for the core design of modular gas‐cooled fast reactor (GFR). Ideally, the reflector materials can be recycled, compatible with helium as the primary coolant, and resistant to high working temperature in the modular GFR. The modular GFR is a reactor design concept that maintains the criticality to achieve a longer operation time. However, maintaining the neutron population in the fast reactor core is still a challenge due to the high neutron leakage. In this research, the main objective is selecting potential reflectors materials for a modular GFR using open‐source Monte Carlo, OpenMC code. The OpenMC code applied the Monte Carlo method that provides high fidelity three‐dimensional (3D) geometry modeling, implementing continuous energy cross‐sections, and enabling to use of the nuclear data of Evaluated Nuclear Data File (ENDF/B‐VII.b5). The various candidate reflector materials, such as pure nickel, pure magnesium, pure lead, PbO, Ba2Pb, BeO, SiC, and Zr3Si2, are calculated from the neutron physics phenomenon to determine a good neutron reflector. The considered parameters of neutron physics included the multiplication factor profile, neutron energy distribution, flux and fission rate distribution, neutron leakage, core lifetime, mass evolution of fissile and fertile material, and thickness of reflector. The most crucial parameter of GFR core design is determined based on the conditions at the beginning of life (BOL) and the end of life (EOL). Finally, we concluded that the BeO material and silicon‐based materials are good reflector candidates for the core design of modular GFR.

Modular Gas-cooled Fast Reactor (GFR) is one of six advanced reactor concepts set by the generation IV international forum. Modular GFR has the potential for use actinide recycling and closed fuel cycle as well as applying fast reactor, using helium gas as the main coolant, high working temperature and low void reactivity effect. The neutronic analysis of nuclear reactor means behavior study of subatomic particles that interact with matter. In this paper, the feasibility of plutonium fuel in modular Gas-cooled Fast Reactor (GFR) was investigated. The Monte Carlo method has advantages in full-scale and heterogeneous three-dimensional (3D) geometry modeling using Evaluated Nuclear Data File (ENDF/B-VIII.b5) nuclear data but requires a highly computation time. Since the progress of high performance computing, the reactor physicist community began proposing to use Monte Carlo method for nuclear reactor simulation through the parallelization of calculations. The GFR feasibility design study will carried out with plutonium fuel as fuel cycle inputs with 5-25% of fissile contain. The most important neutronic parameters characterizing of GFR core are determined for beginning of life (BOL) and during burnup calculation conditions. The results of calculation series in parallel computing give the good agreement will be faster of calculation time when more threads. Materials of (U-Pu)C and (U-Pu)N fuel are the good candidates to be chosen in GFR research that give keff more than 1.2 in fissile contain 20%Pu. The variation fissile contain gives the linearity with keff. In depletion simulation, the core reactor still in critical during 20 years operation, burn up values linear with operation time and mass evolution of plutonium and uranium from start-up core to equilibrium core.

Indonesia has many small regions and islands with low electrification ratio that needs small power plants. Gas-cooled Fast Reactors (GCFR) is one of the fourth generations of an advanced nuclear power plant with an improved safety system and optimum electricity production that match Indonesia’s energy needs. The neutronic analysis study and optimization of small modular GCFR has been performed with Monte Carlo method OpenMC code that use computational parallelization to speed up calculation time. The full core reactor design is cylindrical with a radius 100 cm and height is 120 cm. Additional 50 cm of radius and 40 cm for the height for the reflector. The core using several types of fuel composed of natural Uranium mixed with spent fuel Plutonium. The variation in fuel fraction pin and percentages of Plutonium in fuel to achieved optimum core design. The design of core reactor has flattened flux and power distribution.

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