PSI - Issue 13

Sergiy Kotrechko / Procedia Structural Integrity 13 (2018) 11–21 Sergiy Kotrechko/ Structural Integrity Procedia 00 (2018) 000–000

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high fluences in low Cu steels, which contain a significant amount of Ni and Mn. Specific feature of formation of these phases consists in the long incubation period and rapid grow thereafter. Therefore, formation of LBPs should result in sudden severe embrittlement, so, it can be dangerous for RPV under long time operation condition. Bergner et al. (2009) found a steep increase in the density of nanoclusters enriched in nickel and manganese on low Cu steel A533B at fluences exceeding 22 2 50 10 m − × . This was accompanied by an almost twofold increase in the intensity of radiation hardening. At the same time, according to the authors of this work, above results can’t be directly transferred to the real operating conditions of RPVs, since this effect was observed at a relatively low temperature (≈ 255° С ). Investigations performed by Miller et al. (2009) for typical RPV operation conditions on high-Ni base metal of WWER-1000 showed accelerated increase in number density of the Ni-Si-Mn-enriched nanoclusters with fluence. However, neither "late" nor "blooming" effects were observed. The use of molecular dynamic simulation to analyse the kinetics of formation of MNPs indicated that the kinetics of formation of MNPs should follow the kinetics of formation of dislocation loops, т . е . no sudden blooming is expected, but gradual accumulation of decorated loops with increasing dose [Terentyev and Malerba (2012), Ngayam-Happy et al. (2012)]. In general, an analysis of these results shows that tradition distinction between matrix damage and precipitates becomes blurred. This should be especially pronounced in low-copper steels, because due to low concentrations of copper, dislocation loops become the main place for nucleation of MNPs. In our opinion, the feature of micromechanism of radiation hardening in these steels under long-term irradiation is that the radiation-induced increase in strength of these steels is due not only to an increase in the bulk density N and the size d of dislocation loops (see (1)), but also to an increase in the concentration of Ni, Mn, and Si atoms segregated on these loops. This results in fixing of the loops and increasing their barrier effect. Significant influence of the impurity concentration on the barrier effect of dislocation loops is confirmed by the results of MD simulation performed by Terentyev et al. (2015). As shown in this study, precipitation of the above atoms on the dislocation loop gives rise to a significant increase in the barrier effect of such a loop for a moving dislocation. Thus, the combination of an increase in the density of dislocation loops and an increase in their barrier effect may be the reason for accelerated hardening of low-copper steels in the later stages of radiation hardening. 2.2. Brittle strength of irradiated RPV metal Fall of the brittle strength of metal after irradiation is the second cause of radiation embrittlement. At present, this constituent of radiation embrittlement is poorly studied. One of the reasons for this is complexity of experimental determination of the value of brittle strength. As is known, the magnitude of the local stress of initiation of brittle fracture in the vicinity of the notch or crack, f σ , is a measure of brittle strength. This stress can’t be directly determined from the experiment. It is necessary to carry out additional labor- and time-consuming calculations using the finite element method, as well as fractographic investigations. The effect of reducing brittle strength after irradiation can be determined from the shift of the critical embrittlement temperature f T ∆ only in the special case when there is no radiation hardening. In particular, such result was obtained by Kuleshova et al. (2017). In this case, radiation-induced segregation of phosphorus at the grain boundaries caused embrittlement. At the same time, the reduction in brittle strength was accompanied by a transition from trans- to intercrystalline fracture. This fact restricts the possibility of using such a technique in the case when the reduction in brittle strength is not due to a change in the fracture micromechanism. This is one of the reasons why the number of works on this issue is negligible. Tanguy et al. (2006) investigated this effect in the framework of the Local approach (LA) to fracture. The effect of fluence on the magnitude of the scale parameter u σ in the Weibull distribution was established. In terms of change in the parameter d σ , characterizing the strength of "carbide-ferrite" interface, this effect is taken into account by Margolin et al. (1999). Physical nature of the effect of neutron irradiation on the magnitude of the local cleavage stress f σ is considered by Kotrechko and Mamedov (2016). Within the framework of the multi-scale Local approach it was shown that the decrease in the local cleavage stress of irradiated metal is due to an increase in the number of crack nuclei formed in the "process zone" (PZ) within the vicinity of a macrocrack / notch tip. In turn, the effect of crack nuclei number on the magnitude of f σ is a consequence of stochastic nature of the cleavage initiation inside the small volume of PZ