Loading pattern design of the soluble boron-free SMR using LEU+ fuel and multitype burnable absorbers

Small Modular Reactors (SMRs) represent a promising technology for addressing global energy demands. South Korea’s innovative SMR (i-SMR) is targeting soluble boron-free (SBF) operation to simplify reactor systems and eliminate boric acid corrosion. However, the elimination of soluble boron necessitates alternative reactivity control strategies to manage excess reactivity throughout extended fuel cycles, particularly for 30+ month operational periods that require higher initial fuel enrichment levels. This study evaluates the integration of three distinct burnable absorber (BA) approaches to achieve optimal reactivity control in LEU+ fuel configurations for extended cycle operation. The first approach employs Highly Intensive Gadolinia burnable Absorber (HIGA) rods containing concentrated Gd2O3 embedded in an alumina matrix, which achieves strong spatial self-shielding to extend reactivity control duration throughout the cycle. The second approach utilizes erbia (Er2O3) at natural isotopic composition uniformly admixed with fuel, providing smoother reactivity depletion characteristics through resonance-dominated burnup that supports long-cycle operation. The third approach involves enriched gadolinia mixed directly into fuel pins, leveraging high fractions of Gd-155 and Gd-157 isotopes to extend reactivity suppression while utilizing spatial self-shielding effects to slow absorber burnout. The synergistic combination of these three burnable absorber strategies enables effective management of the substantial excess reactivity inherent in LEU+ fuel while achieving cycle lengths exceeding 30 months. Core analysis is performed using the validated PRAGMA/SPHINCS nuclear design code system, where PRAGMA generates pin-level multigroup cross sections and depletion data, while SPHINCS performs three-dimensional pin-by-pin diffusion calculations. This approach to burnable absorber deployment maintains near-critical conditions throughout the extended cycle while avoiding the limitations of conventional single-absorber strategies that deplete too quickly in SBF operation. The successful demonstration of 35+ month fuel cycles through optimized burnable absorber integration significantly enhances SMR economic competitiveness by reducing refueling frequency, minimizing operational downtime, and improving overall plant capacity factors. These results provide a pathway for commercially viable SMR deployment with enhanced economic performance while maintaining the safety and operational flexibility advantages of soluble boron-free reactor designs.