Our firm was asked to calculate the stresses on two types of nuclear fuel rods due to a crushing load of an accidental side drop. The fuel rods are assembled in a basket containing several stacks of fuel rods. The fuel rods are separated by spacer grids, which is where maximum load transfer occurs. Additional stress is imparted on the fuel rod due to internal pressure caused by gas release during fuel burn-up.
Our engineering team first modeled the stack of fuel rods to determine the maximum reaction of the fuel rod stack. The model took into consideration any reduction in load because of contact at mid-span of the fuel rods due to deflection. Once the maximum load was retrieved, a separate finite element model of the fuel rod was created. The fuel rod, uranium fuel, and spacer grids were included as shown above. Various material properties and modeling options were considered to test the sensitivity of our model. After reasonable, yet conservative, analysis assumptions were established, the fuel rod hoop stress was compared to the allowable yield strength of the zircaloy fuel rod material.