PSI - Issue 14

Altaf Ali et al. / Procedia Structural Integrity 14 (2019) 273–281 Author name / Structural Integrity Procedia 00 (2018) 000–000

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commission report also, UT has been considered as a potential method for RT replacement, Moran et al. (2010). The nuclear power industry has also indicated that there are many benefits to allowing this substitution including less occupational exposure to radiation and significant inspection cost reductions, Spanner (2005). Radiography is more effective when the flaws are not planar, while ultrasonic is generally more effective when flaws are planar and success of RT depends on parameters such as gap, misorientation to the radiographic beam (tilt) and through wall extent (TWE). Detection tends to be less successful as the gap of a crack becomes tighter, the orientation moves away from being parallel to the radiation source that must be applied normal to the object surface, or the through wall extent is reduced. The planar flaws are the more detrimental type flaws as they are more likely to grow throughout the service lifetime of the reactor and could eventually lead to leaks, Moran et al. (2012). However, Significant technical gaps in knowledge have been identified such as the interchangeability of UT and RT is highly dependent on the acceptance criteria used, and that acceptance criteria for UT with respect to fabrication flaws is essentially non- existent, Forli (1980, 1990, 1998).

Fig. 1. (a) PFBR radial blanket pin schematic; (b-d) Photomicrographs of various end plug welds taken at 50x (b) good weld without LOP; (c) Weld having root pocket defect; (d) LOP due to weld shift

Ultrasonic testing of end cap weld of Pressurized Heavy Water Reactor (PHWR) fuel elements is carried out and is a well-established process but the nature of defect is totally different in comparison to PFBR blanket pin end plug weld. There, a non-fusion line, mostly oriented normal to 450 shear wave, is the most frequent defect, Narang (2014). A study has also been carried out by Mukherjee et al. (2006), to replace RT by UT for PFBR fuel pin end plug welds. But it does not provide a detailed description of defect detectability in RT and various geometrical features of defect that can affect UT response. The UT technique used there is based on A-scan results only. 2. RT experimental For experimentation of end plug weld with RT and UT, some most frequently occurring natural defect samples were prepared by varying different welding parameters especially current for LOP samples. Samples preparation details and expected defects are given in Table 1 below. One Inclusion sample was prepared by applying few powder particles at tube and end plug junction before welding. To have the effect of current variation on the defect characteristic and to prepare LOP samples having different unpenetrated height (or LOP%), four dedicated samples (FWC, TFWC, HWC, and QWC) were prepared as per the current details given in Table 1. The same samples have been radiographed and UT has also been carried out. Presently digital tangential Radiography technique with full compensating block is being used to check integrity of radial blanket pin end plug welds (Fig. 2 (a) and (b)). In a shot, 4 pins are radiographed by putting in a correction block. Compensating block or correction block is used to reduce subject contrast at the edges of blanket tube so that a better interpretation of the indications is carried out in that region and edge burning is prevented, Das et al. (2008). Experimental radiography was done with mini focus of 0.4 mm source focal spot size. Source to Detector Distance (SDD) was fixed to 711 mm and Source to Object Distance (SOD) to 406 mm so that a magnification of about 1.75 was achieved. First, angular position marking was done on blanket tube in anticlockwise manner at 120 0 apart