PSI - Issue 60

Rishi K Sharma et al. / Procedia Structural Integrity 60 (2024) 264–276 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Rodgers, & Sagat, 1990). Nevertheless, understanding the fracture behavior of these alloys is of paramount importance to ensure the safety and reliability of nuclear reactors. 2. In-service change in properties of pressure tubes The mechanical and fracture properties of pressure tube materials in the reactor core undergo changes during service in the reactor due to hydrogen absorption and irradiation damage (Rodgers, et al., 2008). As brought out earlier, saturation due to neutron embrittlement occurs after ~1-5 effective full-power years of reactor operation, depending on the position of pressure tube in the reactor. Once it reaches the saturation fluence, properties of pressure tubes are primarily influenced by the hydride embrittlement. Currently, Indian PHWRs use Zr-2.5%Nb alloy pressure tubes in the Cold-Worked and Stress-Relieved (CWSR) state. These tubes were initially made from double-melted ingots, but in newer plants, they are sourced from quadruple melted ingots, reducing trace impurities and in-turn enhancing fracture properties (Theaker, et al., 1994). The structural integrity and Leak-Before-Break (LBB) of these tubes, which are critical for reactor safety, depend on their fracture toughness properties (Moan, Coleman, Price, Rodgers, & Sagat, 1990). Hydrogen embrittlement and Delayed Hydride Cracking (DHC) can compromise their LBB behavior and integrity. To minimize initial hydrogen content, the manufacturing process parameters are closely controlled such that it is within the limit of 25 wppm and 5 wppm for double-melted and quadruple-melted ingot pressure tubes, respectively (Theaker, et al., 1994). A metal water reaction between the Zr metal (fuel clad at higher temperature), and the reactor coolant (heavy water) results in free nascent hydrogen/deuterium in the coolant: + → + + → + ...( 1 ) Zr-2.5%Nb pressure tubes have a surface oxide film which acts as a barrier against the hydrogen absorption. Hence, only ~10% of the hydrogen produced by the metal water reaction is absorbed into the pressure tube material (Motta, et al., 2019). Initially, this Hydrogen, dissolves in the alpha-phase as interstitial atoms, but any of its excess precipitates as hydride, e mbrittling the host α -Zr matrix (Northwood & Kosasih, 1983). The terminal solid solubility of hydrogen in the α -Zr phase decreases with decrease in temperature, following an Arrhenius-type relationship. The degree of embrittlement due to hydride precipitation depends on the shape, size, and orientation of hydride precipitates. Pressure tubes are textured to optimize diametric creep properties, with most α -Zr grains oriented predominantly along the circumferential (~54%) and radial (~43%) directions for CWSR route of 220 MWe pressure tubes (Singh, Mukherjee, Kishore, & Kashyap, 2005). In textured pressure tubes, hydrides can be oriented along the circumferential-axial plane (circumferential hydrides) or the radial-axial plane (radial hydrides) (Singh, Kishore, Singh, Sinha, & Kashyap, 2004). Under unstressed conditions, only circumferential hydrides are formed, but when the tubes are cooled from solution annealing temperature under tensile stress exceeding a threshold value, radial hydrides are formed, enhancing hydrogen embrittlement (Singh, et al., 2006).

3. Experimental Framework 3.1. Gaseous Hydrogen charging

To study the effect of hydrogen on pressure tube material, hydrogen was charged deliberately into the Zr-2.5%Nb pressure tube spool piece in a controlled way such that it did not affect its microstructure and texture. This was achieved using Sievert’s apparatus as outline d in Figure-1, by exposing the polished metal surface to high-temperature hydrogen gas. To ensure the hydrogen penetration into the spool piece without any interference, oxide layers on both the sides were meticulously removed through grinding and polishing successively with final polishing using 1200 grit emery paper. The setup was maintained in a high-purity hydrogen atmosphere under vacuum of ~10 -5 Torr using an