RT is required by customers - not by Codes. Customers require RT, usually on a random basis, after welding. There have been many queries regarding this subject; I suggest that you run a search on the subject.
IF There is no requirement for a gap after welding, and weld shrinkage can close a gap that was present prior to welding. Then is bether determine if there was a gap prior to welding by radiography, that was a question.