PSI - Issue 60

P.A. Jadhav et al. / Procedia Structural Integrity 60 (2024) 631–654 P. A. Jadhav et.al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction PHWR type nuclear reactor consists of pressure tubes (PT) which contains the nuclear fuel bundles, details of these are discussed by Bajaj and Gore (2006). The PT serve as the pressure vessel in this type of reactor and hence its structural integrity is of paramount importance. The leakage of coolant from these PT or the break of the PT is an undesirable event. Saibaba et. al. (2013) have given details of the quality control associated with the design, fabrication and operation of these tubes. The fuel bundles in the PT are supported through bearing pads. These bearing pads are welded to the fuel bundles and are designed to take the operational loads. The movement of the fuel bundle during fuel loading and unloading operation and due to flow induced vibrations present in the primary heat transport circuit can cause rubbing of these bearing pads against the PT. This rubbing can generate marks on the PT and in certain cases, can generate shallow flaws as observed by Trelinski (2007). Celovsky and Slade (2002) have observed that such flaws can also get generated when debris present in the coolant circuit finds its way in between the bearing pad and the PT. These flaws are typically volumetric in nature and are aligned along the longitudinal axis of the PT. These flaws are small in size and do not threaten the structural integrity of the PT. According to Langille et. al. (2021), the PT material has some amount of hydrogen present at the beginning of operation and also has a tendency to pick it from the coolant and the end fitting. This hydrogen migrates to regions of high stress concentration like the flaw tip and may cause initiation of a crack leading to Delayed Hydride Cracking (DHC). Cheadle et al. (1987) suggest that, this crack can grow over time and has the potential to cause leakage from the PT. The presence of a flaw does not always result in the formation of crack. The crack formation occurs only when a set of conditions are met. The model for the crack initiation ahead of the tip of a volumetric flaws is prescribed by the CSA Standard (2016). This methodology is developed using the work of Scarth (2002), Scarth and Smith (2001) (2002). The formation of the crack is dependent of the geometry of PT, geometry of the flaw (length, depth and root radius), applied loading, hydrogen concentration in the material and the material tensile and fracture properties. The flaw of interest is the flaw whose length is aligned with the PT longitudinal axis and the loading of interest for this flaw is internal pressure. The PT is joined to the end fitting by a rolled joint. If the flaw exists adjacent to the rolled joint area, the residual stress which gets accumulated during the rolling process needs to be accounted for. The geometry of the PT of interest is the diameter of the tube and its thickness. The PT undergoes dimensional changes in the reactor environment which affects these properties. Thus, these properties are dependent on the fluence, stress and temperature of the PT and vary across location. The geometry of the flaw that gets created is irregular and to measure it with confidence during in-service inspection is extremely difficult. Hydrogen concentration at a location is dependent on initial hydrogen present in the material and the hydrogen which diffuses inside the material during the reactor operation. This parameter again cannot be estimated with certainty. A typical fitness for safety analysis employs the conservative values of the uncertain parameters. Such analysis severely penalizes the system and results in significant economic losses. One way to address this problem and quantify the uncertainty is by using the structural reliability based methods as given by Haldar and Mahadevan (2000) and Nowak and Collins (2013). These methods quantify the uncertainty in the decision parameters using suitable probability density functions. The concept of leak-before-break (LBB) is additionally used do demonstrate the safety of PT for different postulated operating scenarios. The objective is to show that a postulated through-wall crack in a PT will be safely detected by leak detection systems and the plant would be shut-down before the leaking crack grows to a critical length due to DHC.