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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2.2. Cladding integrity assessment Due to the VERLIFE (2008) code, the Ukrainian nuclear regulatory body demands to perform cladding integrity assessment which is based on the use of its J-R curve. The criterion is as follows: J 1mm ≤ [J 1mm ], (2) Here: J 1mm – value of J -integral corresponding 1 mm ductile tearing, [J 1mm ] – maximum allowable value of J integral, which is specified in VERLIFE (2008) and for irradiated WWER-1000 cladding is equal to 150 kJ/m 2 . Our own experience showed us, that due to the proximity of the Bi-metal interface there is no convergence of J 1mm and J -integral in cladding at all, because the integration contours should pass the boundary between two materials with different mechanical and physical properties. Thus, we must extend our crack in cladding, and to build integration contours there, so we artificially increase the size of the crack. It is important to note, that the main function of RPV cladding is the BM protection (which carries all types of loads) from the possible corrosive effects of the reactor coolant. It is well known, that the austenitic steels are characterized by much higher FT in comparison to the ferritic steels and, also well known, that at room, elevated and high temperatures a ductile failure is inherent to the stainless steels and remains so even after irradiation (unlike BM and welds). Experimental observations from the literature data shows, that there is no crack penetration from BM to austenitic cladding. So, there is no possibility of cladding failure as a result of the loading imposed by PTS. Namely because of above reasons, cladding integrity is not considered in the PWR countries (for sake of consistency we have to mention that it was earlier in Japan, but after realizing the mistake they refused from this misconception, see Onizawa et al. (2010)). Moreover, as in case of computation part, there is no experimental validity demonstration of the [ J 1mm ] in the literature. So, RPV cladding integrity assessment is not related to the good practice and must be excluded from consideration. 2.3. Residual stresses The standard Rice developed J -integral with correction for the thermal loading is path-dependent for general residual stress problems (Lei, 2016). Moreover, even modified version may be dependent on the type of residual stress. A special guidance may be applied for J and C(t) calculations (Lei, 2016). Another issue is related to the way of crack inserting in the residual stresses field. If the crack is introduced progressively in the residual stress field causes the formation of a plastic wake along the crack boundaries. Instead, instantaneously introduced crack can be used to minimize a plastic wake (see Probert et al. (2018)). 3. Fracture toughness characteristics 3.1. Fracture Toughness curve The main characteristic of RPV for its resistance against fast fracture assessment is the fracture toughness of RPV metal. In appropriate normative documents of different countries, fracture toughness function is specified for RPV metal in the initial (unirradiated) state. The approaches used for the defining of the fracture toughness curves are very similar. The differences between the curves are not large, but the indexing approaches for using the curves can differ substantially. The general shape of the fracture toughness curves was assumed to express as exponential function with four parameters:

  



K T T

(3)

1 C K A Be  

Here A is the lower shelf asymptote; B and α are parameters defining the shape of the exponential curve; TK is the CTB used to index the fixed curve. It should be noted, that the lower envelope can be described in many other ways. The choice of the exponential function (3) does not mean that in such way the experimental data are described the best. The shape of fracture toughness function (3) – it is more common convention.