Computational Analysis of Neutron Moderation in Graphite Reflectors for Advanced Small Modular Reactors (High Temperature)

Abstract: With growing interest in compact modular reactor concepts, new design challenges have emerged for the nuclear industry. A key economic issue is the ability of small reactors to compete with large-scale plants. This study explores a novel approach to reduce dependence on fixed burnable poisons during high-reactivity periods in a high-temperature graphite-moderated design. The proposed method involves modifying the core’s neutron energy spectrum over the fuel cycle to utilize bred plutonium. Removing part of the central reflector hardens the spectrum, increasing plutonium breeding. Later reinserting the reflector softens the spectrum to fission more plutonium. This provides a neutronic storage effect in 238U while breeding plutonium. The small annular core depends heavily on the central reflector for thermal neutrons. Removing it reduces thermal neutron fluence near the center, shifting fission outwards. Reinserting it then shifts fission back to the center to utilize bred plutonium and 235U there. Simulations show removing the reflector provides a 320 pcm reactivity drop over the cycle. The plutonium buildup offers additional fissile material until reflector reinsertion. This twofold benefit increased full-power days by ~31 days from extra fissility and reduced peak pin power by 30% during reflector removal.