PSI - Issue 22

Maksym Zarazovskii et al. / Procedia Structural Integrity 22 (2019) 305–312 Maksym Zarazovskii and Yaroslav Dubyk / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction As a rule, assessment of integrity of RPV is based on calculations of their resistance against fast (brittle fracture). Ukraine is the only WWER country, which has practice of RPV brittle fracture assessment under PTS with using three different modern approaches (normatives, methodologies, codes): national Ukrainian (MT-D.0.03.391-09, 2009), Czech (VERLIFE, 2008) and Russian (MRKR-SKhR-2004, 2004). Besides at WWER design stage the USSR’s rules PNAE-G-7-002-86 was used for RPV lifetime justification in terms of resistance against brittle fracture. The crack initiation criterion, all along the crack front of the ferritic material is based on the following expression: K I ≤ K IC , (1) Here K I is the stress intensity factor (SIF) of postulated or existing defect in RPV wall; K IC is the Fracture Toughness (FT) of RPV metal. As a result of analyzing modern codes, based on our practical experience of RPV lifetime extension, the several issues were observed. Thus, current paper devoted to description of them and possible ways of their solutions. According to criterion (1), the problems are divided onto two groups related to:  Computational fracture mechanics;  Determination of fracture characteristics of metal of RPV beltline zone. In integrity assessment of RPV WWER Units both linear elastic fracture mechanics (LEFM) and elastic-plastic fracture mechanics (EPFM) are used. EPFM analysis is considered to be a refined analysis, but its mindless use can significantly reduce the RPV resource. The problem is in J -integral calculations, generally a modify version should be used, which accounts for residual stresses, non-monotonic loading and incremental theory of plasticity. Compared to other designs, for example French PWR, plasticity correction factor for LEFM is not allowed, but this approach can reduce uncertainties inherent EPFM. Another issue is crack insertion in the residual stress field, instantaneous and progressive nodes release technics can be used, the first one is more conservative, and plastic wake is avoided. Residual crack stress field distribution is also a problem, a conservative way to include weld residual stresses is to apply an additional pressure corresponding to the residual stress amplitude. Two types of cracks are assessed surface crack (more dangerous for LEFM) and sub-cladding crack (more dangerous for EPFM), in second case often a cladding integrity assessment is required. Thus, we need to calculate fracture mechanics parameters on the bi-metal interface, but conv ergence can’t be achieved there, so we must extend our crack inside the cladding. Characterization of RPV beltline zone metal related to the following:  Fracture Toughness curve;  Parameter of cladding integrity.  Critical temperature of brittleness (CTB – analogue of PWRs DTBTT); Fracture toughness curves of the WWER-1000 RPV steel and its welds are specified by the four different documents, that used in Ukraine to RPV brittle strength assessment: PNAE-G-7-002-86 (1989), MT-D.0.03.391-09 (2009), VERLIFE (2008) and MRKR-SKhR-2004 (2004). So, the strange situation we have – for the one and the same material we have for different curves, that ware used for RPV brittle fracture assessment. Nevertheless, that MT D.0.03.391-09 (2009) and MRKR-SKhR-2004 (2004) FT curves are probabilistic ones – they are used for deterministic brittle fracture assessment. In addition, it is important to mention about so called upper shelf of FT curves specified by MT-D.0.03.391-09 (2009), VERLIFE (2008) and MRKR-SKhR-2004 (2004) at 200 MPa √ m level, but, at the same time, it is well known, that within LEFM there is no such limitations, moreover, there is a lot data in the literature for RPV metal FT values exceed this shelf. Due to this issue, in some case the unfulfillment of brittle fracture criteria was received and Utility was forced to implement the mitigation actions for the corresponding PTS scenario, unnecessary actions. In Ukraine cladding integrity assessment is performed according to the VERLIFE’s (2008) criterion only, with 150 kJ/m 2 cladding toughness. It should be noted, that austenitic cladding: is much more plastic in comparison with ferritic RPV steel; characterized ductile type of failure, especially at elevated temperatures; has significantly bigger FT than ferritic steels. As a result, cladding integrity criterion not met for nozzle region of all prolongated Ukrainian WWER 1000 RPVs and, as a corrective actions NPP’s , were oblique to implement the ultrasonic inspection of cladding – again, unnecessary action.