Preliminary Investigation of Allowable Weld Defect Size and Orientation for DEMO Cooling Pipes

Maintenance of the DEMO breeding blanket includes the removal and replacement of plasma-facing components. Due to the components having an active cooling loop, multiple coolant pipes need to be removed to allow access to the tokamak and must be reconnected after the replacement components have been put in place. Due to safety requirements, these pipe connections need to be inspected and pass a safety requirement threshold to be safely used. In a cut-and-weld concept, the resulting weld bead needs to pass a standard in which the allowable defects of certain types need to fall under a certain size. These allowable defect sizes are regulated by industrial regulatory bodies. However, the standards do not cover fusion power plants, which would mean that the values of maximal defect sizes might not be applicable for DEMO. This study analyses the effects of standardized defect forms with variable defect sizes using finite element analysis to determine the maximal allowable defect size for DEMO pipes, using upper port cooling pipes as a baseline case study. This includes the effects such as hydrostatic pressure and external forces. The initial study shows that the current rule of thumb in volumetric testing of detecting defects resolution of 0.5 mm would be sufficient for detecting pipe defects. However, a further study on more specific defect types is required to form a more definitive conclusion.