[en] Reprocessing operations on gas cooled French reactor spent fuel included a mechanical operation called decladding, (i.e., stripping the fuel rod from its magnesium metal cladding). After this operation shut down, COGEMA started a D and D project on the decladding building where specific equipment was stored under water. COGEMA then developed and operated a process to decontaminate and cut metallic structures remotely, using ultra-high-pressure water mixed with sand (3600 bar). This paper describes the experience gained, since the start up of this operation in 1994, discusses resulting dosimetry and waste produced, during decontamination and underwater cutting of high active large metallic structures including some with 200 mm thick steel plates. This process results in significantly lower exposures to workers involved in the D and D operations. In addition, the work was carried out in an environmentally safe manner with reasonable financial costs. (author)