Modeling Axial Relocation of Fragmented Fuel during Loss of Coolant Conditions Using the BISON Fuel Performance Code

Fuel rods subjected to a Loss of Coolant Accident (LOCA) in a nuclear reactor can undergo a complex process known as fuel fragmentation, relocation, and dispersal (FFRD). The loss of coolant results in a severe degradation of the thermal heat transfer coefficient on the waterside surface of the cladding. This, coupled with the loss of coolant pressure causes the cladding to distend (balloon) outward. If the cladding distention is large enough, fuel fragmented during irradiation, may relocate from upper regions of the rod into the ballooned region. This mass relocation results in increased localized heating due to the presence of more fuel within the balloon resulting in further clad distention leading to potential rupture of the cladding. In the event that cladding rupture occurs the relocated fragmented fuel may disperse outside of the rupture opening if the size of the fuel fragments is small enough. Fuel performance modeling can be used to help understand the behavior of fuel rods during the LOCA transient. Existing models are particularly limited in regards to FFRD. This report briefly describes the existing state of the art for fuel performance modeling of fragmentation and fuel axial relocation. A more in depth description the axial relocation model implemented into the Bison fuel performance code in FY17 is provided. In this report, the extension of this existing model from a layered 1D framework to a 2D framework that allows for the introduction of azimuthal variation in the boundary conditions as experienced during a LOCA is discussed. The 2D framework utilized has been developed by the author of this report as part of his dissertation research ongoing at the University of South Carolina under the direction of Professor Travis W. Knight. The report briefly describes the 2D framework used and highlights in more detail the required developments in Bison to facilitate the extension of the axial relocation model to work within this framework. Finally, a few demonstration cases are provided that illustrate the benefit of using the new extended axial relocation model for the simulation of fuel rods under LOCA conditions.