ROLE OF MICROSEGREGATION IN FRACTURE OF COLD-WORKED ZR-2.5NB PRESSURE TUBES

Results from Zr-2.5Nb pressure tubes removed periodically from CANDU reactors for surveillance tests have revealed significant tube-to-tube variability in fracture toughness. If the microstructural factors responsible for such variations were known, the toughness of future pressure tubes could be improved and their service life extended. Evidence is presented which demonstrates that the variation in toughness for specimens fractured on the radial-axial plane is related to decohesion or fissuring on the transverse-axial plane as a result of microsegregation of Cl and C (as carbide). In the absence of such species and decohesion it is shown that very high toughness can be achieved.