2015 年 81 巻 824 号 p. 14-00676
Spent nuclear fuel assemblies are stored in the cask with the necessary protective function during the spent fuel transportation or storage. The structural integrity of the fuel rods for the impact load caused by cask mishandling drop accident has to be evaluated to realize more economical and safe operation. However, the experiments for the spent fuel rods accompany many of difficulties and the available experimental data is limited. Hence, the numerical simulation is useful method to predict and evaluate the dynamic response of the fuel rods. This paper deals with the dynamic response of the spent fuel rods under side drop condition. One spacer span of the fuel rod of the undermost layer during side drop is analyzed by finite element method (LS-DYNA). The fuel rod was held between the upper and lower spacers. In the numerical model, the fixed rigid base and the drop weight were connected to the lower and upper spacers, respectively. The numerical analyses were carried out by varying the impact speed of the drop weight and the spacer plate size. Further, the cases of entire drop, in which both the drop weight and fuel rod have the same impact speed, and weight drop, in which only the weight collides with the spent fuel rod placed on the rigid wall were also analyzed. It is found that the fuel pellet and cladding tube reached their yield stresses near the impact point even for the lowest impact speed (2.4m/s) regardless of the spacer size and drop conditions. The plastic region of the pellets is limited within one pellet and the corner part of the neighboring pellet even for the highest impact speed (13.3m/s). The deflection of the fuel rod increased from impact point to the end of the fuel rod in the entire drop condition, but it was limited near the impact point in the weight drop condition. Therefore, the weight drop condition may underestimate the deformation or damage of the spent fuel rod compared to the entire drop condition.
