For 10 years, Gaetan Geoffray worked as a plasterer and painter, before learning metalwork at a company that made cranes. Arnaud Dupuy was a policeman. A third colleague at their factory in the depths of rural Burgundy used to be a baker.

The factory is owned by Framatome, a subsidiary of state-controlled power utility EDF, and the trio are hoping to qualify for one of the most sought-after jobs in France, as nuclear-grade welders. If all goes well, they’ll one day be allowed to work on the most intricate features of the steel parts assembled in the plant, where the all-important 24-metre-long casings protecting the core of atomic reactors are made.

For now, that goal is at least three to four years off, so exacting are the demands in a field in which imperfect finishes can delay a project by months and cost millions, if not billions, of dollars.