Transport and storage of nuclear fuel assemblies may require double confinement depending on the circumstances. A device and a method are described to perform this double conditioning without the need of a hot containment, and in which the loading and pre-positioning steps can take place in a pool.The device (10) includes a metallic inner leak tight conditioning receptacle (20) and a metallic outer leak tight receptacle (30). When the inner receptacle (20) is located in the outer receptacle (30), a passage (15, 25) remains free between the two receptacles, from the open end to the bottom of the outer receptacle. The outer receptacle (30) can be drained through this passage, particularly by a dip tube (33).