Patent Number: 
Section: claims

1. A method for securing and confining a gasket plane of a vessel of a nuclear reactor during an operation for closing the vessel in order to prevent a migration of solid material particles towards an inside of the vessel, the gasket plane being formed by a ring-shaped recess on a flange of the vessel and by a ring shaped shoulder on a lid of the vessel, the ring shaped recess on the flange and the shoulder facing each other and each including a vertical wall delimiting between them a gap after laying the lid on the vessel flange, the shoulder including a horizontal wall provided with two O-ring seal gaskets wherein after having disassembled the lid and laying the lid on a support, the method comprises the following steps:placing around the shoulder of the lid a ring-shaped gasket formed by a metal strip maintained on the vertical wall of the shoulder by self-maintaining members firmly attached to the metal strip;placing the lid bearing the ring-shaped gasket above the vessel by positioning it at a determined height and performing a cleanliness inspection of the gasket plane;lowering and laying the lid on the vessel so that end side edges of the metal strip come into contact with respective horizontal walls of the lid and of the vessel flange;attaching the lid on the vessel flange; andleaving the ring-shaped gasket in place during a whole operating cycle of the reactor. 2. A ring-shaped gasket for securing and confining a gasket plane of a vessel of a nuclear reactor during an operation for closing the vessel in order to prevent a migration of solid material particles towards the inside of the vessel, the gasket plane being formed by a ring-shaped recess made on a flange of the vessel and by a ring-shaped shoulder made on a vessel lid, the recess and the shoulder facing each other and each including a vertical wall delimiting between the recess and the shoulder a gap, the shoulder including a horizontal wall provided with two O-ring seal gaskets, the gasket comprising:a metal strip including two end side edges, each edge capable of bearing upon respective horizontal walls of the lid and of the vessel flange, and self-maintaining members of the ring-shaped gasket bearing upon the vertical wall of the shoulder of the lid. 3. The ring-shaped gasket according to claim 2 wherein the self-maintaining members include at least two opposite metal tabs attached on an outer face of the metal strip, the metal tabs facing the vertical wall of the shoulder of the lid. 4. The ring-shaped gasket according to claim 3 wherein each metal tab is attached on the outer face of the strip by welding and has a thickness between 0.2 mm and 0.5mm. 5. The ring shaped gasket according to claim 4 wherein the metal tab thickness is 0.25 mm. 6. The ring shaped gasket according to claim 4 wherein the metal tab thickness is 0.5 mm. 7. The ring-shaped gasket according to claim 2 wherein the self-maintaining members include several metal tabs uniformly distributed over a perimeter of the metal strip and attached on an outer face of the metal strip facing the vertical wall of the shoulder of the lid. 8. The ring-shaped gasket according to claim 2 wherein the metal strip of the ring-shaped gasket has the shape of a flexible ring with a thickness between 0.3 mm and 1 mm. 9. The ring-shaped gasket according to claim 2 wherein the metal strip of the ring-shaped gasket has an L-shaped cross-section including two walls, an upper wall and a lower wall, the upper wall and the lower wall forming an angle between them, the upper wall having an end side edge bearing upon the horizontal wall of the lid and the lower wall having a smaller width than the upper wall, the lower wall having an end side edge bearing upon the horizontal wall of the vessel flange. 10. The ring-shaped gasket according to claim 9 wherein the self-maintaining members include metal tabs attached on an outer face of the upper wall of the metal strip. 11. The ring-shaped gasket according to claim 9 wherein the self-maintaining members include metal tabs attached on an outer face of the lower wall of the strip.