My house has slab heat and the pipes come out of the dirt and go to the heating unit. This is located in my basement and I'm wondering how to seal it to help reduce radon entering the house.
Unless you have had your house professionallly tested for radon and got a positive (i.e bad) result, do nothing.
If you do have high levels of radon, it's much simpler to install an exhaust fan system, either in the basement or (more properly) by cutting a hole in the basement floor and dropping a 6-inch pvc tube down into the dirt. That acts as the source for a closed-system exhaust fan that should be ducted to the outside.
Most hydronic heating lines have a vapor barrier and insulation below the concrete slab with these layers I doubt you would have a radon problem from the ground but it is always a good idea to test.
A few years back I installed a radon abatement system because of a positive test. The company did the test again and it came back positive, I asked to see the results, no radon in the basement, high levels on the main floor and acceptable levels in the up stairs. The radon was coming from the natural stone counter tops, so if you have a positive report ask to see the test results. I don’t remember where they got the granite but it was a pale rose and they had ~40’ or more in the kitchen. I wish I remembered where they got it from, or if it was because of the color. I have installed darker granite since then and had it tested (negative). I started doing concrete counter tops after that but that is off topic.