Fluidelastic instability of a fuel rod bundle subjected to combined axial flow and jet cross-flow

Abstract

Nuclear fuel bundles are subjected to axial coolant flow in addition to jet cross-flow in a specific design of pressurized water reactors (PWRs). The combined axial flow and jet cross-flow was found to induce fretting wear of the fuel rods at the spacer grid position. Investigating a reduced fuel rod bundle under jet in transverse flow (JITF) condition is necessary to determine safe vibration free operation conditions for these reactors.This paper presents an experimental and theoretical framework to understand the fluidelastic behavior of a rod bundle subjected to axial flow and localized jet cross-flow. A 6x5 axisymmetric flexible single-span rod bundle is used to simulate a part of a fuel assembly under combined flow. Two working flow conditions, pure axial flow, and JITF are studied on a mock-up PWR assembly. The experiments show that the arrays are stable under pure axial flow as expected. Then, the responses of the rod bundles under combined flows (i.e. JITF) are measured at three test axial flow velocities, VAxial= 1.0 m/s, 1.5 m/s, and 2.0 m/s. The results show that the critical jet cross-flow velocity increases linearly with the axial flow velocity with a velocity ratio (VR = VJet/VAxial) of 1.6.The second part of the work is the development of a fluidelastic instability model to predict the stability boundary for rod bundles under combined axial flow and jet cross-flow. The effect of axial flow on the jet cross-flow is introduced by implementing an incidence angle (θ) of jet flow with respect to the rods. The fluid-added damping terms in the developed model are shown to be functions of θ. The fluidelastic forces are expressed as functions of the projected rod area derivative to account for rod vibration. The stability analysis shows the capability of the model to predict the trend of critical jet cross-flow velocity with the axial flow velocity. The model validation shows the predicted critical velocities to be within an absolute error range from 7% to 12.5% when compared to experiments. The theoretical analysis highlights the importance of the cross-coupling fluidelastic forces in predicting the instability of the mock-up PWR assembly.