They x-ray (or even gamma-ray) every cm of welds on the natural gas pipelines that crisscross the country. These pipelines can be over a meter in diameter and

They x-ray (or even gamma-ray) every cm of welds on the natural gas pipelines that crisscross the country. These pipelines can be over a meter in diameter and perhaps (from my recollection) a couple of cm thickness. Consequently, the x-ray machines are not machines one wants to be near when operating, they are quite powerful. I learned this during a summer job in college at the research labs for a large gas pipeline company in the early 70's. At the time a "pig" was used to provide the x-rays for checking the welds of under-sea gas pipelines while they were being constructed. This pig was a remotely controlled wheeled device that moved from weld to weld and x-ray film would be put on the outside of the steel pipe for exposure. The pipe segments were welded together on a barge that floated in perhaps the Gulf of Mexico. Because the welding took several separate passes around the pipe it was done in a continuous operation with the pig x-ray as the final stage before the pipe slid off the back of the barge into the sea. Normally, the pig motored toward the open, working end, of the pipe as new sections where being assembled on the barge. Once, the pig malfunctioned and took off in the wrong direction; it rolled down to the sea-floor. A guy from the pig company had to be flown to the gulf. He was rolled down the pipe lying on something like a skateboard (attached to a rope) to retrieve the pig!!!