Dynamic Simulation Modeling and Analysis of Under-sodium Fuel during In-vessel Transfer Motions

The refueling—a procedure to replace a spent or damaged nuclear fuel with fresh fuel—requires fuel handling in a reactor vessel. Most Sodium-cooled Fast Reactors (SFRs) employ an in-vessel transfer machine (IVTM) [1]. Because the sodium used as a coolant for SFRs reacts with air and water, refueling is performed with the reactor lid closed except early experimental or small reactors. Therefore, IVTM carries a fuel assembly (FA) under sodium, and motions in sodium cause adverse effects on the system due to viscous hydrodynamic forces on an FA. An excessive viscous force due to a fast motion is not desirable in considerations of position accuracy and structural damage on both the IVTM gripper and FA. In this paper, we deal with the dynamic behavior of an FA transferred by an IVTM under the sodium fluid. To this end, an efficient dynamic simulation model of an FA attached to the gripper in a hydrodynamic environment, reflecting fluid drag forces without applying the fluid volume (or fluid domain) in the simulation, was proposed. A commercial dynamic simulator was used for demonstration, and the test result is summarized in the end of the paper with closing remarks.