Comparative neutronic analysis of diverse seed blanket unit (SBU) concepts for the ACP-100 small modular reactor

This study investigates the neutronic feasibility of diverse seed blanket unit (SBU) concepts for the ACP-100 small modular reactor using the Monte Carlo code SERPENT. Pairing four different seed fuels (UO₂, UC, UN, and MOX) with three blanket materials (natural UO₂, depleted UO₂, and mixed ThO₂-UO₂), a total of twelve different SBU-utilizing core models were developed and evaluated. The results indicate that MOX fuels with depleted/natural UO₂ blankets are unsuitable for SBU cores as they cannot sustain prolonged core operation. Of the twelve models, eight successfully remained critical for more than one effective full power year (1 EFPY). The best performing seed/blanket compositions were UC/Th and UO₂/Th with cycle lengths of 1.88 EFPY and 1.75 EFPY, and discharge burnups of 15.85 MWd/kg and 15.39 MWd/kg respectively. The discharge burnup and cycle length of UC seed paired with thorium blanket surpassed those of the reference by 7.9% and 1.6% respectively. All SBU cores achieved high conversion ratios, maintained negative fuel and moderator temperature coefficients (FTC and MTC) and had lower power peaking factors compared to the reference, indicating a more uniform power distribution. The SBU cores utilizing thorium had more negative temperature coefficients than the reference and produced less fissile plutonium (with the exception of the UN-seeded core) at the end of one year. Although a degradation in beta effective was observed, all SBU cores had sufficient control rod reactivity worth to render them subcritical at any instant during operation.