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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In essence, this means finding the equivalent of local cleavage stress

f σ for the conditions of uniaxial tension. In

c Φ , are defined as the values of the dose of neutron irradiation, at which the

this case, the critical values of fluence,

( ) σ = Φ f Y

yield strength of the irradiated metal reaches its ultimate value c R , i.e. the radiation hardening curve

( ) = Φ f R c (Fig. 5). At the same time, it is necessary to

intersects with the dependence of brittle strength on fluence

emphasize that the value of c R is determined with a given tolerance for the probability and depends on the conditions of metal loading in the reactor pressure vessel under thermal-shock, i.e., on the crack front length, B , temperature, T , and load level, I K . This is achieved by performing the calibration procedure described above (Fig. 4). Figure 5 shows the dependence of yield strength Y σ and brittle strength c R on the value of fluence for the weld metal of WWER-1000 reactors. To obtain the temperature dependences of fracture toughness, the results of tests of pre-cracked surveillance specimens were used. Values of fracture toughness were recalculated for the crack front length B = 150 mm in the reactor pressure vessel, and were determined for the probability 0 05 . = P . For this we used a standard procedure based on the Master-curve method [VERLIFE (2008)]. Figure 5 clearly demonstrates that along with radiation hardening, the radiation-induced reduction in the level of brittle strength c R has a significant effect on the magnitude of the limiting fluence c Φ .

According to the data given, neglect of this effect, as it is done in many works, leads to incorrect values of the maximum permissible fluence ( 22 100 10 Φ >> × c м -2 ). It should be emphasized that the ascertained decrease in brittle strength c R is not accompanied by a change in the micromechanism of fracture from trans- to intercrystalline. In conclusion, it is necessary to highlight the fundamental difference in micromechanisms governing radiation hardening and reducing in brittle strength. In the general case, radiation hardening is caused by radiation-stimulated changes in microstructure, resulting in increased resistance to movement of dislocations. In contrast, the reduction in brittle strength is associated with an increase in the number of crack nuclei generated in irradiated metal during the local yielding ahead of a macrocrack / notch. At the same time, mechanism of the effect of radiation-induced changes in

Fig. 5. Dependences of the yield stress, brittle strength of WWER-1000 weld metal, R, L, S, P are the power unit designations): c R correspond to fluences, for which the calibration procedure to determine the parameter λ was performed). Y σ , and c R (T, с Φ is critical value of fluences; (Symbols on the curves for

microstructure on the intensity of crack nuclei generation remains unclear to date. This is one of the challenges for physics and mechanics of fracture of irradiated materials. Solution of this problem is necessary both to reliably predict the conditions for safe long-term operation of the RPVs, and to create new RPV steels that provide a significant increase in the life time of nuclear reactors.

4. Conclusions

1. The level of copper content in RPV steels pre-determines both the features of their radiation hardening in the early stages of irradiation and the ultimate levels of hardening in long-term operation. A drastic change in the regularities of radiation hardening is observed at the transition from steels with high ( . . % C 0 4 0 5 Cu ≈ − ) to steels with low ( . . % C 0 05 0 07 Cu ≈ − ) copper content. 2. For real RPV operating conditions, the appearance of a "late blooming" effect is unlikely, but the effect of fixing of dislocation loops by segregations of Ni, Mn, Si, followed by the formation of precipitates enriched with these elements, should determine the regularities of radiation hardening during long-term operation of a new generation of