Method for securing and confining the gasket plane of a vessel of a nuclear reactor and a gasket for applying this method

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;
• and
• leaving the ring-shaped gasket in place during a whole operating cycle of the reactor.