Evaluation of reflector design of ALLEGRO refractory core

The core design concept for Gas-cooled Fast Reactors (GFR) is currently under development. In this paper, the reflector design for the 75 MW refractory core of GFR demonstrator ALLEGRO has been improved. SiC, ZrC and Zr3Si2 are considered as potential reflector materials, along with stainless steel (15-15Ti) for comparison. With the aid of Monte Carlo calculations, the neutronic performance of each material is evaluated with respect to criticality, power distribution, neutron spectrum, Doppler reactivity coefficient, burn-up, dpa of SiC cladding, and fast neutron flux on the reactor pressure vessel (RPV) wall. The results suggest that ZrC is the most suitable reflector material, offering the highest reserve reactivity, the flattest power distribution, relatively higher negative Doppler coefficients, the lowest reactivity loss rate of the core with burnup, and lower peak dpa damage. SiC is also an effective reflector material, but it causes very high pin power generation in the fuel rods facing the reflector due to significant thermalization of neutrons, challenging the reactor safety margin. Further analysis of the number of reflector rings and coolant volume fraction reveals that a four-ring reflector design appears to be a reasonable compromise between the core neutronic efficiency and minimizing the RPV dimensions. A coolant ratio of 20% is recommended based on conservative estimates of gamma heating and maximum temperature tolerable in the reflector. •Analyze the potential reflector materials for GFR refractory core design, including SiC, ZrC, and Zr3Si2, and 15-15Ti.•Present the model description of the ALLEGRO reactor core and perform neutronic calculations to examine reflector properties.•Investigate the influence of the number of reflector rings and cooling requirements arising from neutron and gamma heating.