Research on Cracked Fuel Relocation of Dual-Cooled Annular Fuel Element Under Normal and LOCA Conditions

The application of the dual-cooled annular fuel element can significantly improve the safety and economy of current pressurized water reactors. Due to the geometric differences, the fuel relocation caused by the fuel cracking in annular rods was much more complex and influential on thermal-hydraulic performance than that in solid rods. In this research, the Discrete Element Method (DEM) was applied to perform the simulation of fuel fragment relocation in the annular fuel rod under both long-term normal operation and LOCA conditions, in order to get deep insights into the fuel relocation mechanisms and develop the mathematical relocation model. Under normal operations, it was found that the radial fuel relocation was bidirectional of both inward and outward, resulting in the radial size reduction of both internal and external gas gaps. The maximum reduction of total gaps was about 58% of the as-fabricated value, and the maximum allowable recovery fraction of fuel relocation was about 55%. Under LOCA conditions, the ballooning of the cladding was full considered in the fuel relocation. As a consequence, the substantial axial fuel relocation was found, which resulting a size reduction of active fuel length and a local fuel accumulation at the position of ballooning.