Local Enhanced Ventilation for Radon Mitigation in a Main Drift Using Air Amplifiers: A Case Study of Beishan Underground Laboratory for High-Level Radioactive Waste Disposal

Localized radon accumulation can persist near tunnel walls, in floor corners, and around the outlets of blind drifts even when a continuous main airflow has been established. This study develops an integrated framework that links granite radon-release tests, diagnosis of the baseline ventilation field, and air-amplifier-assisted local enhancement. Uniaxial compression and closed-chamber accumulation tests were conducted on intact and fractured Beishan granite to determine radon exhalation and diffusion parameters, which were then converted into near-wall source terms for computational fluid dynamics (CFD). A three-dimensional model of the −280 m test level was used to identify 12 m/s as the baseline supply velocity that best balanced radon dilution, flow-field stability, and energy demand. An L9(33) orthogonal design was subsequently used to examine the effects of outlet diameter, compressed-air pressure, and installation pattern on tunnel airflow, wall radon distribution, and radon concentration in the 1.5 m breathing zone. The geometric-mean radon exhalation rate increased from 0.00139 to 0.00432 Bq/(m2·s) after fracturing, an increase of about 3.1 times. Conventional ventilation generated a stable axial stream, but low-velocity regions close to the wall and in floor corners still retained radon. The CFD results indicate that air amplifiers can disrupt continuous high-concentration bands through enhanced jet entrainment, local mixing, and momentum transfer, while improving air renewal in the breathing zone. Within the investigated parameter range, the L9 orthogonal analysis indicated that supply pressure had the greatest influence on radon mitigation performance, followed by installation pattern and air-amplifier geometry. The factor-level analysis predicted A2B3C3, corresponding to a 440 mm outlet configuration, 0.6 MPa supply pressure, and a top–bottom–bottom arrangement, as the preferred combination for the present tunnel geometry and operating conditions. The key lesson is practical: local radon control is not simply a matter of adding more air. It depends on delivering additional momentum to places that the main ventilation stream does not reach effectively, shortening radon residence time and moving it into the axial flow for downstream exhaust. A source–flow–control interpretation is developed to show how rock damage sets the radon load, the primary system provides axial transport, and the air amplifiers reconnect stagnant near-wall regions to that transport route.