Sealing the cracks in the foundation and other openings, such as drainage and sump pump sinks, is a basic part of most approaches to radon reduction. This technique reduces the flow of radon into your home thereby making other radon reduction techniques more effective and cost-efficient, and also reduces the loss of conditioned air. This technique, however, has its limitations. It is difficult to identify and permanently seal all the places where radon can enter, and if you have a finished basement, this is almost impossible without extensive and possibly expensive work. Furthermore, even if you were able to do so in an unfinished basement, normal settling of your house will eventually open new entry routes and reopen old ones.
You can try a special floor drain adapter for your drainage holes in your basement. I myself have purchased couple of these Dranjer D-R2 floor drains. They allow for water to flow down the drain but prevent any gas/air from coming up through it into the living space. According to the website I purchased these from it does the following: “Dranjer seals permit unrestricted flow of water into floor drains or sump pits while sealing out the entry of mold spores, insects and radon gas from the sub-slabe floor area.” To be completely honest, I am not sure these worked very well, as my radon detector did not register any significant changes.
As a side note: EPA does not recommend the use of sealing alone to reduce radon because, by itself, sealing has not been shown to lower radon levels significantly or consistently