Improved stress analysis of fuel rod with $UO_2$ crack using finite element method

Abstract

The cladding tube is the first barrier against the release of radioactive fission products to the surrounding coolant. The cladding local deformation due to the pellet-cladding mechanical interaction is considered as the principal cause of cladding failure. A computer code is developed for the analysis of mechanical behavior of the fuel rod under steady state operation using finite element method with radialtangential system. The cracking phenomena in the fuel pellet are induced by the steep temperature gradient. During the pelletcladding interaction process, the foremost contact point is the crack tip of the pellet. Therefore, the stress is concentrated on that point. It is important that the stress distribution is analyzed in detail centering around this crack tip. The existing codes can not analyze the stress distribution aroudn crack tip. Thus, the stress distribution is analyzed at clad centering around the crack tip during PCMI. For this analysis, the crack tip of radialtangential system is treated as an independent node of the finite element. From the analysis, the following phenomena are observed; (1) earlier contact than embedded crack in node and model of axisymmetric assumption. In Kori-2 fuel rods, when the crack tip contact and exert point pressure to the cladding, the linear power rate and burnup are equal to 350 W/cm and 23000 burnup, respectively. (2) more detailed stress concentration and distribution centering around crack tip than embedded crack in node and model of axisymmetric assumption. (3) The radial-tangential system is proved to be more suitable than earlier analysis.