Annealing the reactor pressure vessel (RPV) of the VVER-440 reactor has begun at unit 2 of Russia’s Kola NPP. Annealing restores physical and mechanical properties of the RPV metal, which undergoes changes due to radiation during operation. The annealing procedure has been developed by NRC Kurchatov Institute. It is carried out in the reactor hall, with the reactor vessel in place. The special annealing furnace is placed on the RPV and inserted inside it. Metal is slowly heated up to the annealing temperature (475 degrees C) and then maintained for 150 hours before being gradually cooled down.
In total, it will require about 15 days to prepare and conduct the annealing of the Kola reactor. Then, it will be necessary to justify the feasibility of extended service life of the RPV, The Kurchatov Institute then will issue a statement on the feasibility of life extension, based on the results of metallographic studies of post-annealing vessel metal specimens.
In 1989 units 1 and 2 of Kola plant were annealed. The process was first carried out at unit 3 of the Novovoronezh NPP also in 1989. Similar work has been undertaken for reactors at the Rovno NPP (Ukraine), Armenian NPP, Greifswald NPP (Germany) and Kozloduy NPP (Bulgaria).