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Propagation of an axial crack has been characterized by means of the Pin-Loading Tension (PLT) test. The same experimental arrangement is used for the Delayed Hydrogen Cracking (DHC) and for the J-integral measurements. The DHC tests are performed under variable temperature and/or load conditions simulating a variable PCMI. Tests have been performed in air on unirradiated SRA Zircaloy-4 tubing and irradiated BWR Zircaloy-2 cladding containing from 30–350 ppm of hydrogen. Variable loading facilitates DHC: in the experiment at 300°C a crack velocity as high as 9.3·10−7 m/s (80 mm/day) has been observed for irradiated cladding. V-K curves or diagrams of DHC crack velocity, V, as a function of stress-intensity factor, K, are suggested for different BWR claddings. Despite a limited amount of tests, the V-K curves appear to depend on the type of cladding. DHC does occur at much lower stress intensities compared to the Kc converted from the J0.2 values, confirming DHC to be the dominating secondary failure crack propagation mechanism in BWR cladding.

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Pressure tubes in CANDU and RBMK reactors are fabricated from Zr-2.5Nb alloy. This paper describes the mechanical properties of tubes used in power reactors made by four routes using electrolytic powder as the base material. The microstructures developed by each route are distinct: CW: cold-worked material consists of flattened α-Zr grains surrounded by a skin of β-Zr phase; used in CANDU 6 reactors. CW-A: material that was cold-worked and annealed at 540°C contains elongated α-Zr grains mixed with equiaxed α-Zr grains and particles of β-Nb phase; used in all RBMK 1000 reactors. TMT-1: material quenched from the (α+β)-Zr phase into water follow by cold-working consists of α'-phase and between 10 and 20 % of untransformed α-phase; used in RBMK 1500, Ignalina 1. TMT-2: material quenched from the (α + β)-phase into argon-helium gas mixture followed by cold-working consists of Widmanstätten α-phase and untransformed α-phase. This material is used in RBMK 1500, Ignalina 2. The CW and TMT-2 tubes have a higher proportion of grains with basal plane normals in the transverse direction, FT of 0.52 to 0.57, than in the radial direction, FR of 0.38, while quenched and annealed materials (TMT-1 and CW-A) have similar values of FT and FR, about 0.38 in quenched materials and 0.41 in annealed materials. Transverse tensile strength, crack growth resistance, dJ/da, and axial crack velocity, VH, of delayed hydride cracking (DHC) were evaluated, using standard techniques, between 250 and 300°C on as-fabricated materials. In-reactor creep deformation was evaluated from measurements of tube diameter in RBMK 1000s, RBMK 1500s and two CANDU 6 power reactors. Strength and crack growth resistance were measured on TMT-1 and TMT-2 tubes removed from Ignalina NPP Units 1 and 2 after 12–17 years of in-reactor service. As-received cold-worked material had the highest strength; the annealed material had the lowest strength while the quenched materials had intermediate strength. Irradiation increased the strength by about 200 MPa in all four materials. Although DHC is sensitive to texture and the distribution of the β-Zr phase, the dominating factor controlling crack velocity appears to be material strength: with an increase of strength by a factor of about two, VH increased by a factor of 30. Since harmful trace elements were well controlled during manufacturing, other factors affecting crack growth resistance could be assessed. Again, strength appeared important; dJ/da declined approximately linearly with increase in strength induced by irradiation, decreasing from about 350 to 100 MPa as the strength increased from about 250 to 850 MPa. The exception was TMT-2 material where crack growth resistance was maintained after irradiation. TMT-2 material also had good diametral creep resistance in-reactor, attributed to both its texture and grain structure. The other three materials had similar creep resistance controlled mostly by their texture.