Reactivity Feedback Effects of Fuel and Grid Plate Expansion in Small Gas-Cooled Fast Reactors

Abstract Small gas-cooled fast reactor is considered as an important candidate for micro nuclear power, with the lightweight, compact structure, and high thermal conversion efficiency. To achieve an ultra power-to-mass ratio, the reactor typically employs high-enriched uranium fuel designs. However, as the relative enrichment of 238U in the fuel decreases, the fuel temperature Doppler reactivity feedback effect in reactors may transport from negative to zero or positive. Furthermore, the fuel expansion reactivity feedback coefficient is small and instable, because of the low thermal expansion coefficient of UO2 and the fragmentation of fuel pellets after high burnup. Discussing the negative reactivity feedback by changes in the core structure is crucial to ensure the negative reactivity feedback for the entire core and system operation stability. The large temperature difference (∼300°C) at the inlet and outlet of small gas-cooled fast reactor leads to increased differential thermal expansion between the fuel restraining holes in the top and bottom grid plates, which result in the expansion named as free-flowering of fuel rod bundles. This phenomenon will lead to a decrease in the effective fuel density and an increase in the neutron leakage. Ultimately, the expansion reduces the excess reactivity of the core and manifests as a grid plate expansion negative reactivity feedback effect. In this paper, the reactor Monte Carlo neutron transport code RMC and the finite element analysis software ANSYS Mechanical are used to model and analyze the Prometheus space reactor. The focus was primarily on discussing the effect of grid plates expansion on reactivity feedback and power distribution. The results showed that grid plates expansion had a minimal impact on the axial power distribution in the core, as the difference of axial distribution between before and after free-flowering expansion is 0.05% in average, and the difference of radial distribution is less than 0.81%. Furthermore, the reactivity feedback induced by the free-flowering expansion of the grid plates is at the same magnitude of that caused by the fuel expansion, which ranges between −0.3 ∼ −0.4 pcm/K. Thus, the grid plates expansion reactivity feedback coefficient should not be neglected in small gas-cooled fast reactors.