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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1. Introduction According to statistics, more than 60% of nuclear reactors in the world are operated for 30 years or more. In this situation, estimation of the residual lifetime and the rationale for the possibility of their safe operation are challenges for the nuclear industry. This requires solving two key tasks, namely: ( і ) finding of micromechanisms of radiation embrittlement for RPV metal under conditions of continuous long-term neutron irradiation and ( ii ) development of engineering methods for predicting the radiation lifetime of RPV metal, which allow to take into account the features of radiation embrittlement under these conditions. Radiation embrittlement of metal is the result of two processes: radiation hardening and radiation-induced reduction in brittle strength. The effect of radiation hardening is the most studied issue to date. Currently, the front line of research on this problem lies in ascertainment of differences in the regularities of radiation hardening in RPV metals with a high ( 0.4 0.5% C Cu ≈ ÷ ) and low ( 0.05 0.07% C Cu < ÷ ) content of copper. This is due to two main reasons: ( і ) copper is the nucleation centre for the formation of precipitates enriched with nickel, manganese, silicon and phosphorus, therefore, copper has a decisive influence on the regularities of radiation hardening of RPV steels, with its content 0.40 ÷ 0.50% or more; ( іі ) new generation of RPVs are characterized by the low content (0.05 ÷ 0.07%) of copper, so, the question arises on the micromechanisms of radiation hardening of such RPV steels during long-term irradiation. As compared with radiation hardening, the radiation-induced decrease in the brittle strength of RPV steels is a less studied phenomenon. Usually, a special case of this phenomenon is considered, when radiation embrittlement is not accompanied by radiation hardening. As a rule, this is observed in intercrystalline fracture, caused by phosphorus precipitates at the grain boundaries. The severe drawback of existing engineering approaches to lifetime prediction for RPVs is that they don’t t enable to take into account in an explicit form the simultaneous influence of these two effects on the RPV lifetime. In this paper, a generalization of the physical concepts of two constituents of radiation embrittlement (radiation hardening and radiation-induced reduction of brittle strength) is given. New ideas about micromechanisms of radiation-stimulated decrease in brittle strength are presented. The possibility to overcome the existing gap between physical models of radiation embrittlement and engineering approaches to prediction of RPV integrity is shown. This can be realises within the framework of the proposed engineering version of Local approach to cleavage fracture, which makes it possible to predict the effect of these two constituents of radiation embrittlement on the magnitude of the radiation lifetime of RPV steels.

2. Micromechanisms of irradiation embrittlement of RPV metal

Figure 1 schematically shows the effect of two constituents of the radiation embrittlement of metal on the shift of critical temperature of a ductile-brittle transition. As noted above, usually the radiation-induced decrease in the local cleavage stress ahead of a crack/notch tip is neglected. However, the neglect of this phenomenon can result in a significant underestimation of the critical temperature shift, especially at high fluences.

2.1. Radiation hardening

In general, the radiation hardening of RPV steels is due to two types of radiation-induced defects, namely, ( і ) stable matrix features, and ( іі ) precipitates. Stable matrix defects usually include vacancies (CVCs) and interstitial clusters (CICs), as well as interstitial dislocation loops. Depending on the chemical composition of the RPV steel, the irradiation dose (fluence) and the irradiation conditions (temperature and flux), various kinds of precipitates may form in RPV steel, namely: Cu-rich precipitates (CRPs), manganese-nickel rich precipitates (MNPs), alloy carbo nitride precipitates (CNPs) and alloy phosphide precipitates (APPs). In the last two decades, the issue of the existence of the so-called "late blooming phases" (LBPs) is being actively discussed (for instance, in the research of Odette and Lucas (1998)). These radiation-induced defects are obstacles to the moving dislocations, which gives rise to an increase in the yield strength. In its simplest form, the value of this hardening can be described as [Thomson (1996)]: