Numerical investigation of hydraulic instability of circular arc shell-type fuel elements under axial flow conditions

Shell-type fuel elements in nuclear assemblies can collapse under high-speed coolant flows and impede channels, which is known as hydraulic instability. Stiffer than flat plates, curved shells are commonly in involute shape and circular arc shape. In this study, two parallel circular arc shells with three channels are numerically simulated for hydraulic instability prediction. Fluid-structure interaction between shells and fluid is achieved, and results show that the two arc shells are asymmetrically deformed in opposite directions. A “valley morph” pops near the leading edge at a channel velocity and becomes increasingly visible as the flow rate rises. The maximum deflection occurs at the leading edge, which is linearly related to the squared channel velocity below a value but nonlinearly steeply ascends otherwise. The two characteristic velocities are congruent and thus can be deemed as the “critical velocity” at which the response of fuel elements changes although shells remain elastic all through. The derived critical velocity is about 80% of Miller's theoretical estimate.