CRACKING IN BWR INTERNALS - THE SWEDISH PERSPECTIVE ON PREVENTION, MITIGATION AND REPAIR

The Boiling Water Reactor represents a mature technology and, with the exception of some cases of cracking of various forms, it has been relatively free from serious problems. The most common causes of cracking have been intergranular stress corrosion cracking (IGSCC) and fatigue. A limited number of incidents have occurred in BWRs of ABB Atom's design. In BWRs of different designs the cracking has so far mainly concerned safe ends, nozzles and internal components. A few of these are discussed and commented in the paper, and procedures and methods applied for repair are presented. The examples concern top guides, core spray, thermocouple pockets and a few other internal components. Based on this experience and on information from various other sources, some general conclusions can be drawn. It is discussed how the materials selection, manufacture, plant water chemistry and other factors might influence the probability of future failures. In this context the effect of different preventive and mitigative measures, like hydrogen water chemistry, mechanical and other means to reduce tensile stresses, are critically evaluated. The application of NDE methods is crucial for the early detection of cracks and for the verification of repair and other measures. Some recent development is presented. As an example of available repair methods, the Reactor Weld Repair System (RWRS) is described, which has been developed and demonstrated by Westinghouse based on ROSA or a similar delivery system and a family of end effectors, that perform the repair process. The end effectors are designed for vessel wall and nozzle inlet operation. The repair method and equipment used for repair of a feedwater nozzle in the 940 MWe Forsmark 1 BWR in July 1989 is also reported.