Validation of Axpo’s SIMULATE-3K Model Against Stability Measurements of Leibstadt Nuclear Power Plant

The main objective of this research is to validate Axpo’s core dynamics models—based on CASMO-5, SIMULATE5 (S5), and SIMULATE-3K (S3K)—against the stability tests performed at the Leibstadt Nuclear Power Plant (KKL). The validation effort began with benchmarking the model against the stability measurements of cycle 19, as well as against solutions developed by multiple organizations that participated in an earlier benchmark organized by KKL. The validation was subsequently extended to include stability measurements from cycles 10, 13, and 38, providing a broader basis for assessing model performance. To ensure the reliability of the stability analysis, the steady-state results of S3K were systematically compared with the already validated S5 model, confirming accurate core initialization. Various parameters of interest were evaluated, and overall good agreement was observed for most parameters across the majority of tests. Further validation was performed by comparing S3K predictions with traversing in-core probe (TIP) measurements for cycles 19 and 38, demonstrating excellent consistency with plant data. In particular, the Axpo S3K model shows very good agreement at the bottom of the core and ranks among the best-performing models submitted in the KKL benchmark. Validation of key stability parameters—decay ratio and resonance frequency—confirmed the robustness of the model. Decay ratio predictions agree with measurements for all cycle 13 and cycle 19 tests and for most cycle 10 and cycle 38 tests, with remaining cases showing a systematic overprediction likely attributable to mixed-core configurations. Resonance frequency results show very good agreement, with only a slight underestimation observed for a few cycle 19 tests, consistent with trends reported in the benchmark.