Numerical Analyses for Derivation of New Control Rod Replacement Criteria

The control rod is essential for controlling reactivity and ensuring reactor safety. However, various defects may compromise its integrity due to the increasing trend toward load-following operation, which involves frequent insertion and withdrawal of the rod control cluster assembly. The fully withdrawn condition of the control rod can also contribute to initiating and propagating these defects. In this study, a series of numerical analyses was conducted focusing on the typical wear defects in control rods of Korean standard nuclear power plants. Firstly, parametric finite element (FE) analyses were performed by changing the defect types, lengths, depths and wrapping angles. Subsequently, the influence of defect geometry was analyzed using response surface methodology. Finally, based on the FE analysis results, replacement criteria were derived to prevent the failure of the control rod with wear defect. The criteria were presented as ranges for each defect type and geometric parameter. The key findings obtained from FE analyses results and comparisons on current replacement criterion were discussed.