Benchmarking of the CMS5 Code System for the NEA/OECD Watts Bar Unit 1 Multi-Cycle Depletion Problem

The Studsvik Core Management System CMS5, consisting of the CASMO5 lattice physics code and the SIMULATE5 three-dimensional nodal simulator, was evaluated against the Nuclear Enersy Agency/Organisation for Economic Co-operation and Development TVA (Tennessee Valley Authority) Watts Bar Unit 1 Multi-Physics Multi-Cycle Depletion benchmark. The assessment included cycle 1 zero-power physics tests and full-power boron letdown and flux map comparisons for cycles 1, 2, and 3, using plant measurements as the primary validation basis and continuous-energy Monte Carlo reference solutions for complementary verification.The impact of the evaluated nuclear data was investigated using CASMO5 libraries based on ENDF/B-VII.0, ENDF/B-VII.1, and ENDF/B-VIII.1. Transitioning from the ENDF/B-VII.1- to the ENDF/B-VIII.1-based library resulted in a systematic increase in the predicted critical boron concentration, corresponding to a reactivity shift of approximately 114 pcm. In contrast, the control bank worth, reactivity coefficients, boron letdown, and flux map root-mean-square deviations showed only limited sensitivity to the selected library.Overall, the CMS5 predictions showed consistent agreement with both the plant measurements and continuous-energy Monte Carlo reference solutions, indicating reliable multicycle core predictions, with the primary impact of the evaluated nuclear data confined to absolute reactivity predictions.