Investigation on fuel cracking induced relocation behavior of dual-cooled annular fuel based on finite element simulation

Fuel relocation caused by fuel cracking plays a key role in the thermo-mechanical response of nuclear fuel elements. In this study, the Finite Element Method (FEM) was applied to simulate the relocation behavior of cracked fuel. Based on benchmarking with a well-established empirical model for solid fuel rods derived from large amounts of experimental data, it was proved that FEM is a reasonable and feasible methodology for simulation of fuel relocation. Based on the real morphology images of fuel fragments after in-pile irradiation, investigation on the fuel relocation of a dual-cooled annular fuel rod was carried out with consideration of fuel swelling. A bidirectional (inward and outward) relocation was observed in the simulation results, which can be divided into three different phases as burnup increases, including a relocation development phase, a relocation recovery phase, and a fuel-cladding hard contact phase. By calculating displacements of all the grid nodes on the fuel surface, the statistical fuel dimensions and gas gap widths were obtained. The maximum total gap reduction due to the bidirectional relocation was determined to be 78 µm, while the maximum allowable recovery fraction of the gap reduction was determined to be 55.1%. Based on the simulation results, a relocation model was then proposed for the dual-cooled annular fuel rod.