In this article, it is described how the safety features of modern VVER designs are based on redundant passive and active systems to ensure the corresponding safety functions and thus meet the safety requirements of the regulations of different countries.
The passive systems incorporated in these designs include: hydro-accumulators of different designs (HA-1, HA-2 and HA-3) to mitigate active injection systems failure sequences; passive secondary cooling systems, water-cooled or air-cooled; the passive containment cooling system and the core-catcher system. All these passive systems, together with the active primary, secondary and containment systems, are described in detail throughout the paper.
Finally, in the article a comparison of the results of the level1/level2 internal events PSAs for different designs of PWR and VVER reactors of Gen-II, Gen-III and Gen-III+ is made to show the evolution of risk reduction in the latest designs in operation or under construction worldwide. Overall, it can be seen that the safety of the new designs has been improved with the incorporation of passive systems.