Research on control rod cusping effect correction methods in the 2D/1D transport calculations

The cusping effect is one of the critical challenges that must be addressed in the reactor core physics simulations involving control rod movement. This study proposes two improved methods for correcting the control rod cusping effect in the 2D/1D transport calculations. First, the conventional Both Axial and Radial Homogenization (BARH) method in NECP-X is enhanced by subdividing partially rodded axial planes into sub-planes, each undergoing independent radial homogenization. The radial leakage term of each sub-plane was then approximated using the leakage terms of its adjacent layers. Second, an Axial-Only Homogenization (AOH) method was developed, building on the material-region-based 2D/1D transport method. This method leverages high-precision axial fine-mesh fluxes from material regions to perform flux-volume-weighted axial homogenization. It initially approximates the radial leakage terms of sub-planes using those of adjacent layers, and further refines these terms via pre-calculated radial leakage correction factors. Validation of the proposed methods was performed on the VERA #4 and VERA #5 benchmarks, and on the SPERT 3 × 3 assembly and full-core problems. Results show that, compared with the conventional BARH method, the improved BARH method significantly improves the correction accuracy of the cusping effect and demonstrates strong potential for application to typical commercial PWR control rod clusters. For problems involving large, strong-absorber control rods in the research reactors, the AOH method combined with radial leakage correction achieves the highest accuracy, meeting the precision requirements for correcting the cusping effect in research reactors.