My basement floor has control joints and some have opened up. Floor was poured 5 years ago and the cracks are stable now.
The worst section is 3/16ths wide.
What caulk should I use to seal this? I want something safe for indoor use.
I've used this stuff in many a basement or garage. It is designed to bond to the concrete, the color blends into undyed concrete, and it can be applied with a caulking gun (much easier than the squeezy tubes for doing long cracks).
You don't really have to worry about Radon infiltration if you already have a mitigation system, even if the vapor barrier isn't perfectly sealed. It's not under pressure or anything so that system of sub-slab pipes and vent stack already provides a path of least resistance for the gas to bypass you and escape to atmosphere. In high-Radon areas like where I used to live, builders install a fan to pull a slight vacuum as well.
Note: this is not a specific product recommendation FWIW. Quite a few brands produce this type of sealer.