Load-Follow Operation Capability of Soluble Boron-Free Small Modular Reactor ATOM

Yunseok JeongDongju ChoiTaesuk OhYonghee Kim^*

• Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea

This study investigates the feasibility of Daily Load-Follow Operation (DLFO) for the Autonomous Transportable On-demand Reactor Module (ATOM), a Soluble Boron-Free (SBF) small modular reactor (SMR). The ATOM core was selected as a reference model due to its adoption of key SBF-compatible design features, including Centrally-Shielded Burnable Absorbers (CSBAs)-burnable absorbers with controlled self-shielding-and a Truly-Optimized Pressurized Water Reactor (TOP) lattice, which employs enhanced moderation to ensure favorable neutron economy and temperature feedback. Together, these features provide stable excess reactivity and favorable Moderator Temperature Coefficient (MTC) characteristics across the reactor cycle. To enable effective reactivity and axial power distribution control in such an environment, the Mode-Y control logic was applied. Mode-Y is a newly developed control strategy that relies solely on Control Element Assembly (CEA) movements and allow independent insertion of gray banks by eliminating conventional overlap constraints. A challenging DLFO scenario was simulated at three representative burnup conditions-Beginning-of-Cycle (BOC), Middle-of-Cycle (MOC), and approximately 90% End-of-Cycle (EOC)-to evaluate the performance of Mode-Y control logic. The scenario involved rapid power ramps with 50%p changes within 3 hours, followed by irregular hold periods, to test the control logic under highly dynamic conditions. The analysis employed a conventional two-step approach: multigroup crosssections were generated using the SERPENT2 Monte Carlo code with ENDF/B-VII.1 library, and whole-core transient simulations were performed using KANT nodal diffusion code. Results confirm accurate power tracking, stable Axial Shape Index (ASI) control, acceptable coolant temperature management, and sufficient nodal and pin-wise power peaking margins throughout all burnup stages.