Optimization of spent fuel reduction and reactivity control using LEU+ fuel and hybrid burnable absorbers in innovative small modular reactors

This study presents an advanced core design for the innovative small modular reactor (i-SMR), enabling fully soluble boron-free operation through optimized integration of hybrid burnable absorbers (BAs)—gadolinia (Gd 2 O 3 ) and erbia (Er 2 O 3 )—with LEU + fuel. The design addresses the challenges of excess reactivity control and spent fuel minimization, both critical in boron-free SMR environments. Gadolinia provides strong initial reactivity suppression, while Er 2 O 3 ensures gradual, sustained absorption, enabling long-cycle operation. LEU + fuel increases average discharge burnup from approximately 50,000 to 68,500 MWd/MTU, extending the cycle by over 33 % and reducing spent fuel assemblies by about 25 % over an 80-year lifetime— demonstrating significant improvements within the i-SMR design framework. The design employs the PRAGMA/SPHINCS coupled code system, where PRAGMA performs continuous-energy Monte Carlo lattice calculations to generate homogenized few-group cross-sections, and SPHINCS carries out high-fidelity pin-by-pin diffusion simulations for full-core analysis. This coupled modeling framework accurately captures neutron flux distributions and burnup behavior inherent in hybrid burnable absorbers and high-enrichment LEU + fuel configurations. Evaluations confirm that key core safety parameters—including fuel temperature coefficient (FTC), moderator temperature coefficient (MTC), integrated temperature coefficient (ITC), and subcriticality margin—are satisfied throughout the cycle. The results show the i-SMR core achieves operational safety and sustainable spent fuel management, offering a robust, environmentally competitive solution for next-generation nuclear plants.