Small modular reactors (SMRs) are increasingly considered for flexible and low-carbon nuclear energy deployment. However, robust economic assessment remains challenging because conventional plant-level scaling from large nuclear reactors may obscure SMR-specific differences in component count, integral system architecture, equipment duty, and fixed-cost effects. This study develops a design-informed, component-level bottom-up cost modelling framework for the ACP100, a 385 MWth/125 MWe integral pressurised-water SMR. The model reconstructs the plant cost structure using approximately 105 cost items and replaces uniform plant-level scaling with account-specific estimation relationships selected according to the dominant physical and engineering cost drivers. Component-count scaling is applied to discrete equipment, heat-transfer-area scaling to integrated steam generators, hydraulic-duty scaling to reactor coolant pumps, mass- and size-based scaling to major pressure vessels, thermal-duty scaling to reactor auxiliary systems, and capacity-based relationships to electrical-plant and turbine-cycle equipment. Parent accounts are recalculated from their underlying child accounts to maintain cost-account consistency and avoid double counting. The resulting estimated direct cost of the ACP100 is US$638.879 million. Applying a 20% indirect and owner's cost allowance gives an estimated overnight capital cost of approximately US$766.7 million, corresponding to approximately US$6,134/kWe at a net electrical capacity of 125 MWe. The analysis demonstrates how integral reactor architecture, equipment quantities, component sizing, and system-specific engineering characteristics can be explicitly incorporated into SMR cost estimation rather than being represented through a single plant-capacity scaling exponent. The proposed framework provides a transparent and extensible approach for design-informed economic assessment of integral SMRs and establishes a basis for subsequent uncertainty, sensitivity, and comparative analyses of advanced small-reactor concepts.

Small Modular Reactor (SMR); ACP100; Bottom-Up Cost Modelling; Overnight Capital Cost; Techno-Economic Assessment