Design, fabrication and qualification of an NDT system for core vessels of pressurized water reactors

In late 1995, ABB Reaktor received an order contract from the Japanese nuclear industry for development and full qualification of a complete NDT system for inspection of core vessel welds and core vessel bolts, including qualification of the testing probes. The core vessel is examined in a water environment for reasons of radiological protection. The core vessel (about 7 m high, 3m across) is a structure made of welded cylindrical segments (austenite). On the inner side of the vessel, core shroud bolts (austenite) fasten the reinforcement plates (ribs) of the core shroud plates building the containment of the fuel elements. Older Japanese PWRs have a cylindrical, thermal shield surrounding the core vessel. The major technical challenge in development of a suitable testing system is the fact that the system has to guarantee safe and reliable performance of complex inspection tasks in the area behind the thermal shield, which involves particularly difficult conditions. Depending on the design age of reactors, the annular gap between core vessel and thermal shield is only between 20 and 50 mm wide. (orig./CB)