This draws in more radon from the ground by concentration-driven diffusion, until equilibrium is established on all floors. Any openings and cracks have to be sealed off or caulked to stop radon, as well as moisture and soil gas. You need a self-leveling polyurethane caulk. Instead of just bridging a radon-filled gap, it sinks in and fills it. Such deep caulking also withstands hydrostatic pressure.
Polyurethane remains flexible for years, as the house settles and concrete moves.
It is best to open up such joints with a grinder and properly fill the gap with the self-leveling caulk. The RadonSeal solution RadonSeal Penetrating Concrete Sealer seals microcrevices and minute pores inside the concrete against radon atoms, as well as the much larger water molecules. Instead of removing radon from underneath the slab by a fan, it simply seals the concrete. Sealing the pores stops radon diffusion and advection through the slab, as well as the walls. And in addition, it blocks the other radon pathways - water migration and emanation from concrete.