PSI - Issue 60

D. Sen et al. / Procedia Structural Integrity 60 (2024) 44–59 Deeprodyuti Sen/ Structural Integrity Procedia 00 (2024) 000 – 000

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and is shown in Appendix A.1.All the results discussed above were for 220MWe IPHWR. Results of 540MWe showed similar trends and were evaluated for the corresponding design data and input parameters shown in Table 2 and 3.

a

b

Figure 12: Graphical plot of allowable nominal stress versus flaw tip root radius showing the safe and unsafe regions. The flaw is assumed to be located in a region near the rolled joint in a pressure tube used in 540MWe Indian PHWR. The residual stress is assumed to be 100 MPa for (a) flat bottom BPFF of radial fl aw depth “ a ” is kept as 0.425 mm and (b) for a Debris Fretting Flaw (DFF) of radial flaw depth “ a ” is kept as 0.3 mm. Comparing Fig. 12 (a) and (b) we observe that even for a smaller radial flaw depth ‘ a ’ in case of DFF (Fig. 12 b) and smaller range of Axial flaw length, the ‘Unsafe Zone’ is more pronounced. This is because the stress concentration factor, ‘ k t ’, for DFF is more than BPFF (discus sed in detail in Appendix A.1). Conclusions The process zone model for initiation of delayed hydride cracking (DHC) from a volumetric flaw, developed by Scarth and Smith (2001 & 2002) is implemented in an in-house computational code (ZIPTAS). Earlier CSA (2016) provides only tabular data of critical peak stress for a flaw geometry and size (‘ a ’ and ‘ρ’) whereas the ZIPTAS code provides the allowable nominal stress for a given flaw geometry and size (‘a’, ‘ρ’ and ‘c’). Here, the Allowable nominal stress is directly compared with the design stresses and a flaw can be categorized as safe or unsafe. Using ZIPTAS code, precise calculation of critical flaw dimensions pertinent to Indian PHWRs which are not present in any earlier standards. Parametric studies are performed to assess the influence of flaw geometry, flaw size and service loads on the maximum nominal stress that will not lead to initiation of DHC from a volumetric flaw in a pressure tube. The salient conclusions from this study are as follows, 1. The flaws obtained from the in-service inspection need to be identified as bearing pad fretting flaw (BPFF) or debris fretting flaw (DFF). For the same flaw depth a , the DFF leads to lower values for the permissible nominal stress compared to BPFF. This is essentially due to higher values of stress concentration factor for the former case. 2. The BPFF can be modelled either as a flat bottom or round bottom flaw. The permissible flaw sizes in these two cases are not much different.