Fracture Toughness Evaluation of Zr-2.5Nb Pressure Tubes Manufactured Employing Double Forging for PHWR700

The need for improvement of the in-reactor performance of Zr-2.5 % Nb pressure tubes arose because of the higher than expected diametral creep rates and variability in axial elongation observed in some of the pressure tubes of 220 MWe Indian pressurized heavy water reactors (IPHWR220). A systematic study revealed that the pressure tubes used in various IPHWR220 had variations in microstructure, mechanical properties, texture, and chemical composition. The forthcoming PHWR700 is expected to operate at a higher temperature than that of IPHWR220 pressure tubes coupled with ∼3 % partial boiling toward the outlet end. The IPHWR220 pressure tubes were manufactured by NFC (Hyderabad, India) using the combination of extrusion-double pilgering with intermediate annealing and with 20 % final cold work. PHWR700 pressure tubes have been manufactured using a combination of forging-extrusion-single pilgering routes. The fracture behavior of cold-worked and stress-relieved Zr-2.5Nb pressure tube material manufactured employing a forging route for PHWR700 is described in this work as a function of hydrogen content (4 and 60 wppm), temperature (25°C–300°C), and sample location. Fracture toughness tests were carried out as per ASTM E1820-06 [ASTM E1820-06: Standard Test Method for Measurement of Fracture Toughness—Designation, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA] using curved compact tension specimens of 17 mm width. The crack growth was along the axial direction of the tube and was measured using the direct current potential drop (DCPD) technique. The fracture toughness of as-received samples show weak dependency on test temperature, whereas samples having 50 ppm hydrogen show typical S curve behavior and regain toughness above 150°C. The variation in fracture toughness across tube length was within scatter band.