If those holes connect to a chimney that's in use, yes they should definitely be sealed up. You could get a backdraft. I'd combine mortar and a few bricks. It doesn't have to be pretty.

If you are concerned about radon, do a radon test rather than speculating and worrying...possibly for no reason.

Radon tests at the hardware store are cheap. You can also look up your county's rate. In my county, nearly half of homes are in areas with radon above the recmmended amount.

I'm pretty sure radon seeps through cement basements. No one suggested to us our chimmeys were an issue for radon.

Thanks, I should’ve added more details. I’ve been testing for radon and we see spikes whenever it gets cold out, so I assume the chimney might be contributing to the stack effect of air being sucked out of the basement, which in turn pulls in radon through any cracks.