چكيده لاتين
Abstract
When making fuel rods, its handling and transportation and loading of fuel pellets are done horizontally. If the fuel pellets are not restrained, there is a possibility of a gap between them. When installing the fuel rod, the pressure inside the fuel rod is about 2 MPa, and if there is a gap between the pills, it is placed in this gap and does not allow the pills to return to their place. This causes the fuel pellet to deviate and stick to the wall of the sheath and distance itself from the adjacent fuel pellets. In such a case, since the pressure inside the reactor is around 15 MPa, the fuel sheath reacts to the reactor pressure and causes the diameter of the sheath to decrease in the separation area. The irregularity of the column of tablets cannot be corrected after installation and brings negative consequences and effects. To solve these problems and keep the column of fuel pellets dense, it is common to use a spring in the plenum space to apply an axial force to the fuel pellets and prevent the fuel pellets from moving during loading, storage and transportation. For this reason, the analysis and investigation of the plenum spring is particularly important for the operation of nuclear reactors.
In this thesis, FRAPCON and COMSOL software are used for the thermomechanical analysis of plenum spring. FRAPCON software is a thermomechanical analysis code of the fuel rod and specifications such as thermal expansion of the pellet and fuel sheath, temperature of the pellet, sheath, fluid, plenum and other information are given as output, but obviously no effect and data are given about the plenum spring. At first, the FRAPCON code was written using neutronic data obtained from the employer, and its results were used as input in COMSOL software.
In this project, the temperature gradient of the spring and the constant changes of the spring and the applied forces of the spring were investigated and evaluated over time in the normal working state of the reactor core caused by temperature stresses. For this purpose, after writing the FRAPCON code and getting the outputs, simulation was followed in COMSOL software with two-dimensional and three-dimensional modes. In the two-dimensional model, we considered the axial power distribution of the fuel in different time intervals as a heat source, and the boundary conditions such as the low fluid velocity of the plenium were obtained for the three-dimensional model. In the three-dimensional mode, the plenum space, the spring and the fluid passing around it were modeled by applying the boundary conditions taken in the two-dimensional model and the boundary conditions taken from FRAPCON. In this part, for each time period, the model was simulated and the temperature gradient of the spring and then the amount of change in the length of the spring due to temperature effects was obtained. In the next step, the change in the overall length of the spring, the force applied to the spring and the constant changes of the spring were obtained by taking into account the axial expansion of the fuel pellets, casing and spring over time in the normal operating mode of the reactor core.
