Rarely. Sealing helps but typically reduces radon by only 10-20%. Active sub-slab depressurization is the EPA-recommended primary strategy because it actively removes radon from beneath the foundation before it can enter.
More detail
EPA and AARST/ANSI guidance is consistent: sealing alone is inadequate as a primary mitigation strategy. The reason is that no amount of visible-crack sealing can fully close every soil-gas entry path; concrete itself is porous, mortar joints in stone basements are not realistically sealable to soil-gas-tight tolerance, and stack effect plus negative interior pressure will find any remaining path. Sealing IS however valuable as a complement to active depressurization: a sealed sump-pit lid, sealed slab cracks, and sealed plumbing rough-ins increase the effectiveness of the suction-pit fan because there are fewer competing paths for the negative pressure to dissipate through. Every Cincinnati-area install includes sealing as part of the standard scope, not as an alternative to fan-driven depressurization. Cincinnati pre-war housing reality: stone basement walls in Hyde Park, Norwood, Mt. Lookout, and Walnut Hills have hundreds of mortar joints, each a potential soil-gas path. Sealing every joint individually is impractical and ineffective. Active sub-slab depressurization plus targeted sealing of the highest-flow paths (sump pits, visible cracks, plumbing penetrations) is the only practical approach for these homes.