Smoke Evolution and Evacuation Dynamics in an Underground Powerhouse: Fire Safety Margin Evaluation Coupled with Sprinkler Intervention and Exit Blockage

To address the challenge of quantitatively coupling fire smoke evolution and occupant evacuation in the underground powerhouses of large-scale pumped-storage power stations, full-scale three-dimensional numerical models were developed using PyroSim for fire dynamics and Pathfinder for egress behavior. A multi-factor coupled ASET–RSET safety margin matrix was constructed, integrating fire heat release rate (HRR), sprinkler intervention, egress passage blockage, and occupant load. The results show that visibility continuously dropping below 10 m at a characteristic hazardous height of 4.5 m serves as the governing criterion determining the available safe egress time (ASET). Under a 10 MW fire without sprinklers, the ASET is 343.0 s, whereas activating the sprinkler system effectively suppresses smoke and reduces temperatures, extending the ASET to 380.9 s. With both exits unobstructed, the required safe egress time (RSET) for the baseline 130 occupants is 303 s and increases marginally to only 304 s as the crowd scales to 250 occupants, thereby maintaining a positive safety margin under all scenarios. Conversely, the blockage of a single-sided passage induces a pronounced bottleneck shift. Specifically, blockage of the main access tunnel exhibits acute sensitivity to crowd enlargement: the RSET surges to 441.8 s for 250 occupants, resulting in a severe safety margin deficit that cannot be fully rectified even with sprinkler intervention. Accordingly, two distinct risk boundaries—management-sensitive and structure-constrained regimes—are identified. An optimal egress routing scheme characterized by “hierarchical zoning, lateral priority, and nearest-ascending egress” is proposed, accompanied by a two-dimensional cooperative prevention and control strategy: “source control to delay ASET and capacity expansion to reduce RSET.” This work provides quantitative support for performance-based fire safety design and emergency management in deep-buried underground powerhouses.