Application and analysis of Cr-coated GdN-CBA to i-SMR core with two control rod patterns for load-following operations

A new burnable absorber called GdN-CBA with Cr-coating is applied to the i-SMR (Innovative Small Modular Reactor) for boron-free operation. The burnable absorber considered employs a gadolinium nitride coating burnable absorber with a 20 μm chromium protective layer to preserve coating integrity and reactivity control over long cycles. Furthermore, an optimized control rod (CR) pattern consisting of 20 Inconel-625 fingers and 4 Ag-In-Cd fingers (24 fingers in total), is introduced to facilitate the load following operation and compared against a previously established CR configuration. Core performances and safety characteristics were evaluated under both hot full power and load-following operations were simulated to show its feasibility. The results show that the proposed design satisfies all key design criteria, including axial offset, power peaking factors, and shutdown margin under cold zero power conditions. Notably, the optimized CR pattern (Pattern 1) achieved extended cycle length, reduced CR movements, and improved axial power distribution stability during load following operation. These findings demonstrate that the i-SMR cores using the Cr-coated GdN-CBA combined with the new CR strategies have an efficient load following capability.