Investigation on radial fuel relocation and its influence on heat split phenomenon of dual-cooled annular fuel element

Radial fuel relocation due to fuel fragmentation can significantly change the heat split, further affecting the thermal performance and safety of dual-cooled annular fuels. Considering the rarity of in-pile irradiation data for annular fuels, three different fuel relocation and its recovery models were developed based on modifications of relocation models for solid fuel rods or the mechanistic relocation simulation of pre-fragmented, interacting and deformable fuel fragments. The three relocation models were implanted into the fuel performance analysis code FROBA-ANNULAR, and thermo-mechanical simulation was carried out to investigate the influence of fuel relocation on gap size variation and heat split variation in an annular fuel. The results indicate that different relocation models lead to significant difference on the prediction of gap size and heat split. Relocation models developed through mechanistic simulation of pre-fragmented fragments seems to be more reasonable, compared with the other two models developed based on unreasonable and extreme assumptions. Mechanistic simulation results indicate that radial relocation of fragmented annular fuels can be bidirectional even at the initial burnup stages, resulting in the reduction of gap sizes of both internal and external fuel-cladding gaps. In addition, it was found that the gap size variation due to radial fuel relocation significantly changed the heat split and matching-rate between heat split and flow flux distribution, exerting an influence on fuel temperature and MDNBR.