Abstract: A storage system with 100% secondary containment includes a rigid outer vessel, a flexible cell inner vessel and a spacer layer therebetween. The region between the outer and inner vessels is monitored for leakage through either the rigid outer vessel or the flexible inner vessel. A number of expandable positioning rings keep the inner vessel substantially inflated regardless of the amount of fluid within the inner vessel. Outlet openings are formed in the outer and inner vessels to permit the passage of fluid into and out of the inner vessel. To retrofit a rigid vessel in use, the vessel is emptied, a manhole opening is cut, the vessel is inspected and conditioned as necessary, and the inside surface is covered with a coating layer. A spacer layer is mounted to the coating layer to cover the inside surface of the outer vessel. Fluid detectors and a suction tube are mounted within channels formed in the spacer layer. The flexible inner vessel is placed into position within the rigid vessel and is inflated.