The water gap between the wall and the core of the RPV (Reactor Pressure Vessel) in a VVER-440 plant is small compared with typical Western type LWR5. The neutron fluence on the RPV wall is, consequently, much higher in a VVER-440 plant. In older VVER-440 plants the material of the RPV, especially the horizontal core weld, contains so much impurities (P- and Cu-content) that the irradiation embrittlement has become a problem. On bases of fracture mechanics analyses in Loviisa, IVO has been forced to make several measures to ensure safe operation of the plants. According to IVO's current understanding, both plants may be in operation for the design life without annealing of the RPVs.
Ahlstrand, R., & Valo, M. (1992). Experience of irradiation embrittlement in the Loviisa reactor pressure vessels. Nuclear Engineering and Design, 137(3), 433-436. https://doi.org/10.1016/0029-5493(92)90265-W