Inspection and Replacement of Baffle Former Bolts in VVER-440 Reactor Type

Reactor vessel internals are structures located within the reactor vessel that support and orient the reactor fuel assemblies and direct coolant flow through the core. The core basket is part of the internals structure, which consists of vertical plates that surround the outer faces of the hexagonal peripheral fuel assemblies. The vertical plates are bolted to the edges of horizontal former plates that are bolted to the inside surface of the core basket. There are typically four levels of former plates located at various elevations within the core basket of a VVER-440 reactor. The bolts that secure the baffle plates to the former plates are referred to as "baffle former bolts. Nuclear power plants identified the cracking of baffle former bolts as early as 1988 and this problem continues to occur. Although this cracking is not fully understood, testing of cracked bolts suggests an age-related intergranular stress-corrosion cracking process influenced by bolt material, fluence, stress, and temperature. Inspection and replacement of baffle former bolts have been carried out successfully at the Loviisa NPP Unit 2 in early September 2006. In particular, this is the first time that bolts replacement is performed in a VVER-440 reactor type. Ultrasonic testing was used to assess the integrity of the baffle former bolts. Subsequently some defective bolts were replaced with a remotely controlled underwater manipulator system, as a preventive maintenance measure. This work is a further step in the application of a complex Western PWR technology to VVER reactors type.