Investigation on axial relocation of fragmented annular fuel under loss of coolant accident conditions with discrete element method

Axial fuel relocation of conventional cylindrical fuel rods under Loss of Coolant Accident (LOCA) conditions has been intensively studied. However, characteristics of cladding ballooning, fuel fragmentation and relocation of a dual-cooled annular fuel rod could be quite different due to its innovative geometry. In this study, based on fragmentation images of annular fuels after in-pile irradiation, discrete geometry for fuel fragments was established considering the effect of burnup. With the ballooning cladding as constraint boundary and the gravity as driving force, axial relocation of an annular fuel at different burnup stages was simulated with discrete element method (DEM). The employed DEM methodology was first validated through a benchmark against a theoretical model and a reference simulation. Simulation results indicate that relocation is determined by the fuel fragmentation morphology and cladding ballooning shape. At low and medium burnup stages, it was found that although there exists obvious mass accumulation in the ballooning region, many fuel fragments above the ballooning region are stuck between the internal and external claddings, resulting in almost no missing length of the fuel stack. At high burnup stages, because of the presence of finer fuel particles at High Burnup Structure (HBS) regions, fuel jam can be mitigated or avoided, resulting in a higher peak mass fraction and an obvious missing fuel length. The critical HBS volume fraction that can totally avoid the fuel jam was determined as about 15%. In addition, the sensitivity study indicates that the higher the volume proportion of HBS is, the more obvious the axial fuel relocation is. In the absence of sufficient irradiation data, this study provides a reliable modeling methodology and valuable understandings of the axial fuel relocation in dual-cooled annular fuel rods.