Leveraging Specific Plant Features To Manage Internal Hazards

Internal hazards and accident progression are often determined by plant-specific features. A typical feature of internal hazards and a special concern of accident progression are that they challenge simultaneously more than one functional level of the defence-in-depth concept or penetrate more than one of the physical barriers against fission product releases. Internal hazards can themselves be initiating events, such as common cause failures, internal fires or floods, missiles, inhomogeneous boron dilution, or primary-to-secondary leakage accident. Internal hazards can also be created during the accident progression such as pressurized thermal shock, loop seal issue, boron crystallization, containment sump clogging, or inherent boron dilution mechanisms. The aim of this paper is to discuss, how various internal hazards are managed at the Loviisa VVER-440 units. The Loviisa plant configuration is quite unique, since the original VVER-440 design has been amended with ice condenser containment, specific reactor coolant pumps and many other features.