A model of the nitride pellet – cladding axial interaction in a fast reactor due to jamming

In the process of a post-irradiation examination of nitride pins irradiated as part of experimental BN-600 reactor fuel assemblies, a noticeable elongation and a higher ovality of nitride pins cladding were observed as compared with oxidefuel pins. The analysis results suggest that this takes place due to a local mechanical effect fuel pellets have on the cladding. The potential cause for the fuel-cladding thermomechanical interaction is the fuel pellet misalignment, the pellet jamming and cracking as a result of thermal stresses under reactor power rise, and subsequent jamming of large fuel fragments in the cladding. The paper presents the fuel jamming model which assumes the fuel-cladding interaction via an axial force in each calculated radial layer. The axial force is determined from the condition of equilibrium in the radial cladding and fuel cross-section and ensures that the jamming condition (the same growth in axial deformation) is complied with. Jamming is assumed along the entire core height in the presented model version. The presented results of the jamming model verification, based on post-irradiation examination results of the BN-600 EFA-11 (experimental fuel assembly), show that the model allows simulating the fuel pin cladding elongation. The EFA-11 fuel assembly with fuel pins of the BREST-OD-300 reactor type with the cladding made on EP823-Sh ferritic-martensitic steel and nitride uranium-plutonium fuel was irradiated in the BN-600 reactor to the maximum burnup of 9 at.% and the maximum damage dose of 107.6 dpa.