Enabling long-cycle, high-burnup light water SMRs with LEU+ fuel: A coupled core and fuel performance comparison of boron free and borated designs

Recent development of the LEU+ fuel supply chain enables new pathways to extend discharge burnup in light water SMRs, whose burnup is often reactivity-limited due to high neutron leakage. This study investigates LEU+ fueled core concepts for an i-SMR-based SMR with the objective of reducing annual spent fuel assembly discharge by approximately 30 % relative to the reference i-SMR with U-235 enrichment below 5 %, which corresponds to extending the cycle length from 730 to 1043 EFPD while maintaining the same number of feed fuel assemblies. Two variants are designed and assessed: a soluble-boron-free (SBF) core and a soluble-boron-used (SBU) core. For the SBF design, increased reliance on burnable absorbers and control rods motivates adoption of a 17 × 17 assembly with 28 guide tubes and 1 instrumentation tube, together with multiple IGD absorber options including enriched gadolinium configurations. Although a carefully designed shuffling scheme in the SBF core can effectively mitigate hoop stresses, control rod-driven axial depletion non-uniformity increases the peak nodal burnup to 80 MWd/kgU at rod average burnup below 65 MWd/kgU, accelerating fission gas release and rod internal pressure at high local burnup. In the SBU design, soluble boron provides cycle-long global reactivity control, reducing reliance on control rod insertion, yielding more uniform depletion behavior with moderated peak nodal burnup and fuel performance trends. Overall, the results support the technical viability of LEU+ for long-cycle SMR from the standpoint of neutronics and fuel performance under normal operation; however, the SBF implementation may induce increasing control rod-induced depletion non-uniformity when pursuing additional burnup extension beyond the burnup levels evaluated in this study (65 MWd/kgU).