1. You put the radiation source, machine or isotope, on one side of the piece and the recording medium, film or digitizing plate, on the other. Since a fillet weld is usually used to join two materials at 90 degrees or so, you can't get the radiation and film where they need to be. (The film must be placed in a light sealing cassette which results in the film not extending to the very edge of the casette and the isotope is some distance from the outside of the guide tube.)
2. Rule of thumb for the size defect you can detect is 2% of the thickness the radiation passes through. Even if you could get the radiation source and recording medium in position, the weld would probably represent a small percentage of the material the radiation would have to pass through; resulting in a significant decrease in sensitivity.
