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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In addition, it is important to note, that FT curves experimentally determined within the LEFM. If it would have been determined within plastic J -integral, fracture toughness curve would be much higher. So, criteria (1) with LEFM right part and EPFM left part is more conservative vs LEFM and LEFM correspondingly.

Fig. 3, Classic FT curves indexed CTBs and MC indexed T 0 for BM and weld of RPV of Unit #3 of Zaporizhzhya RPV

3.2. Critical temperature of brittleness As in western countries, in WWER operators a program of surveillance specimens (SS) is implemented. Monitoring of the WWER-1000 RPV metal embrittlement is performed with the usage of the radiation SS sets. For the FT curve indexing the CTB is used, which is obtained by the processing of Charpy impact energy data. The essence of the SS program is to place SS, made of RPV metal, into the reactor zones where the radiation degradation factor affects; periodically extract the SS sets from reactor and determine experimentally the CTB. So, the CTB of RPV metal is a function which depends on time. And, that is why the brittle fracture criterion in nuclear industry (1) is generally considered as temperature margin criterion. Modern codes, that regulate WWER RPVs CTB prediction, are based on the CTB shift ideology. According to this ideology, the CTB is defined as the initial CTB value plus CTB shift due to the radiation, plus CTB shift due to the thermal ageing, plus certain margin. This margin includes: the scatter of CTB in the initial state, scatter of CTB shift due to the radiation and scatter of CTB shift due to the thermal ageing. A tendency of modern WWER RPV integrity assessment codes to provide conservatism with: choosing the CTB in the initial state for the CTB shifts determination; choosing the CTB in the initial state for the CTB determination; taking into account the scatter of CTB in the initial state, scatter of CTB shift due to the radiation and scatter of CTB shift due to the thermal ageing, leads to huge predictive values of CTB and to formal unfulfillment of brittle fracture criterion for RPVs. As an example, Fig. 4 depicts the CTB curves of weld #3 of RPV of Unit 1 of South-Ukrainian NPP built according to VERLIFE (2008), PM- Т.0.03.120 -08 (2009) and PNAE G-7-002-86 (1989). The figure also shows the maximum allowable CTB T ka =67.6º С (parameter determined by computational methods of fracture mechanics, which depends on the size, shape and position of the crack, the FT curve, as well as on thermal-hydraulic conditions of the PTS). Fig. 4 shows that: the VERLIFE (2008) CTB trend reaches to T ka value after about 26.5 year of operation; the PM- Т.0.03.120 -08 (2009) CTB trend reaches to T ka in 35.5 years. Note, that in accordance with conservative design calculations, performed by PNAE G-7-002-86 (1989), resource of WWER-1000 RPV is 40 years. It is seen from the picture, that the use of PNAE G-7-002-86 (1989) approach the CTB prediction provides a margin in relation to experimentally obtained CTB. It is clear from the above examples, that the modern approaches for the CTB determination of WWER RPV, which is based on CTB shift ideology, is not only wrong, but also dangerous for the energetic industry of Ukraine. Thus, we can conclude, that the ideologically correct СTB prediction approaches should be based on the direct usage of SS data.