Assessment of Structural Integrity of Pressure Tubes during Cold Pressurization

AbstractPresently, double melted, cold worked and stress relieved Zr-2.5%Nb alloy pressure tubes are employed in some of Indian Pressurized Heavy Water Reactors (IPHWRs) where initial hydrogen content is limited to 25 wppm. Over a period of time, part of hydrogen/deuterium released by the corrosion reaction (metal-coolant reaction) between the Zr-2.5%Nb alloy and D2O may be picked up by the pressure tubes which further embrittles the pressure tube material at lower temperatures. Structural integrity of the irradiated pressure tubes of these reactors during cold pressurization is studied. Studies have been performed to ensure that there is no unstable crack growth and Delayed Hydride Cracking (DHC) during cold pressurization of pressure tubes. For these calculations, reported lower bound fracture toughness properties for the double melted Zr-2.5%Nb irradiated pressure tube material have been used. The calculations indicate that the applied stress intensity factor for the postulated part through wall flaw in the pressure tube is less than the threshold stress intensity factor for DHC initiation. Pressure tubes also meet the required factor of safety for the stability of a postulated leakage size crack in case of cold pressurization and the operator has sufficient time to take the necessary action before the crack reaches the critical size. The paper brings out the details of the safety assessment carried out for the pressure tube during cold pressurization