PSI - Issue 14

Priti Kotak Shah et al. / Procedia Structural Integrity 14 (2019) 242–250 Priti et. al. / Structural Integrity Procedia 00 (2018) 000–000

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Table 1. Major specified chemical composition of Zr-2.5Nb pressure tube used for the study

Elements alloyed to Zirconium Specified values (by Weight) Niobium, % 2.4-2.8 Oxygen, ppm 900-1300 Iron, ppm 1500 max Hydrogen, ppm 25 max

Table 2. Specified longitudinal tensile properties for the Indian pressure tubes Properties

Zr-2.5Nb 25C 300 o C

Ultimate tensile strength (MPa) 0.2% Yield strength ( MPa)

-

≥ 462 ≥ 14.0

≤ 586 ≥ 324

% Elongation

-

tension (DCT) test specimens, having curvature same as that of the pressure tube component, have been used for the evaluation of fracture toughness to avoid any flattening or cold working on the material. As pressure tubes see higher hoop stress, most of the fracture studies have been carried out on specimens having axial notch or crack. However, in practice, flaws may be present in material in any orientation. Hence, fracture studies have been carried out on textured Zr-2.5Nb pressure tube to evaluate its fracture properties along axial and transverse directions.

1.1. Material and specimen

The Indian PHWR pressure tubes are made of Zr-2.5Nb alloy and are of ˜82.5 mm average internal diameter, ∼ 3.5 mm thickness. These pressure tubes are fabricated at Nuclear Fuels Complex (NFC), Hyderabad using hot extrusion, cold pilgering and stress relieving treatment. The specified chemical composition and longitudinal tensile properties of the material used for the study is given in Table 1 and Table 2 respectively. Microstructural observation of unirradiated Indian pressure tube showed the lamellar morphology of the α -Zr along with the β -phase present as stingers between two α -laths as well as fine and coarse β globules. The size of α -Zr lamellae was found to be in the range from 0.17 to 0.2 micron, 1.8 to 2.4 micron and 1.7 to 2.8 micron in the radial, circumferential and axial direction respectively (aspect ratio of 1:7:8). Typical TEM microstructure of the Indian pressure tube material having lamellar α -grains separated by thin beta layer is shown in Figure 1 (Srivastava et al., 2012). The pressure tubes have dominant transverse basal pole texture, i.e. the α -grains are oriented such that the large fraction (0.55-0.6) of basal poles of HCP α -phase are oriented close to transverse direction. The basal pole fractions in radial and axial directions are around 0.35 and 0.05 respectively. Idealized orientation of basal poles of α -Zr grains along the three principal directions of the pressure tubes is shown in Figure 2. Tests were carried out on specimens fabricated from a double melted Zr-2.5Nb pressure tube material. Disc compact tension (DCT) specimens (with width W = 22.22 mm) were used for the test. Specimens were fabricated in such a way that some specimens had notches in axial-radial plane and some specimens had notches in the transverse-radial plane. All the dimensions except thickness and curvature of the specimens were as per the ASTM E1820. Typical as-fabricated specimens having transverse notch and axial notches are shown in Figure 3 Four Zircaloy wires were welded to the specimen for use in Direct Current Potential Drop (DCPD) system.

1.2. Fracture toughness test

Tests were carried out using a servo-hydraulic universal testing machine with an attached furnace for high tempera ture test. Specimens were precracked using a load-decreasing method. The precracked specimens were then statically loaded at a cross-head speed of 0.008 mm / sec to required extent. Typical load-displacement-DCPD data plots obtained during the room temperature and 300 0 C fracture toughness tests are shown respectively in Figure 4 and Figure 5. Typ ical J-tested specimens are shown in Figure 6. The J-tested specimens were then heat tinted at 300 o C for an hour