I think all of the suggestions here are valid in cases of “normal diarrhea” but radiation really changes the ballgame. Even my own suggestion for Kaopectate is probably not gonna work.
I think something stronger to reduce intestinal motility is what is needed - an RX for something like Lomotil. All the probiotics are going to do is produce more gas and yogurt, kefir, etc were expressly forbidden on my diet from Sloan as they contain live cultures which normally is a GOOD thing but not during this treatment.
Talk to your RO about getting a prescription for something stronger than OTC remedies; you’re not the first person with this issue!