SSM perspective
Background
In Swedish Light Water Reactors (LWR), stress corrosion cracking of reactor components and welds occurs from time to time. As the nuclear power plants are ageing, it is essential to study and further understand the mechanism for environmentally induced sensitization. Natural cracking is a phenomenon that is difficult to predict and very hard to study since it occurs suddenly and often unexpectedly. In order to study the crack initiation and growth, the crack is traditionally experimentally provoked and it is not known to what degree these experimental cracks correspond to those that occur naturally. The environment in an LWR contributes to material ageing through chemical reactions with the environment. An in-depth examination has shown that the microstructures of oxide films changes along the crack path and the oxide film in the crack tip is significantly different from what one detects at the crack opening. In this study, 1st principles modelling is used to articulate an environment induced sensitization mechanism for stress corrosion cracking of Ni(Fe,Cr) alloys in LWR conditions.
Results
Density functional theory (DFT) has been used to theoretically describe the processes that occur along a crack in LWR environment. The working hypothesis of the present study takes environmentally sensitized stress corrosion cracking (ES SCC) to be owing to internal oxidation of chromium along alloy grain boundaries. This scenario implies random cracking to be repeatedly healed by outward diffusion of chromium to the crack tip, where sealing becomes subsequently achieved by chromia formation. The water chemistry including the impact of dissolved hydrogen in the reactor coolant, becomes decisive for oxide scale composition, its coherence as well as its adherence to the supporting alloy. The resulting passive layer is understood to control the effective permeability of the water equivalents, i.e., the oxidising agents, along oxide grain boundary interfaces from the water/oxide interface to the crack tip. In cases of limiting chromium mobility in the alloy grain boundaries, the sluggish outward diffusion of Cr risks being overtaken by the inward diffusion of oxygen. Impact of carbide, nitride or hydride precipitates along alloy grain boundaries was taken to have dual detrimental effect. Thus, the precipitates would render Cr locally enriched while also mitigating the Cr mobility along the oxygen activity gradient. Internal oxidation of stationary Cr rich precipitates would render the precipitating additives dissolved. The increased activity of the additives would in turn stabilize corresponding precipitates further away from the sensitization front possibly also intercepting any outward diffusing chromium. Thus, the internal oxidation driven dissolution-reprecipitation process has chromium carbides, nitrides or hydrides acting guides for the sensitization. Were this understanding to be valid then the ES SCC would fundamentally be owing to oxygen competing with carbon, nitrogen, or hydrogen for the chromium in the alloy. It would apparently be resolved by increasing the relative Cr content in the Ni(Fe,Cr) alloys.
However, employing water as oxygen carrier adds the possibility of chromium activity loss beyond the crack tip owing to formation of transient hydride precipitates. Indeed, the DFT study showed how water may be conveyed along chromia and nickel-decorated chromia grain boundaries, to support the oxidation mediated hydrogen pick-up beyond the crack tip. Thus, while any carbon in the alloy would originate from the manufacturing process, the hydrogen uptake would originate from the oxidation process. And while the carbon content would stay constant, the increasing hydrogen content in the alloy with time, in spite of a small pick-up fraction, would cause increased environment sensitization by supporting inward oxidation along the alloy grain boundaries.
Relevance
The method of using 1st principles electronic structure calculations by means of density functional theory (DFT) is a new approach towards understanding possible mechanisms for environmentally sensitized stress corrosion cracking in load bearing structures in LWR environment. The results obtained in this study show that DFT calculations combined with experimental research can offer important information that provides better understanding of ageing mechanisms in LWR environments.
Need for further research
This study was a first step towards understanding if 1st principles electronic structure calculations by means of density functional theory could be employed to formulate and test possible mechanisms for environmentally sensitized stress corrosion cracking (ES SCC) in load bearing structures. The method has been proved to be useful. It can be further developed and used to explain, for example, the effect of hydrogen pick-up on the enrichment of chromium at the alloy grain boundaries in LWR environments, and how Li+ from the coolant becomes enriched at the crack tip by acting H+ equivalent at the oxide grain boundaries. The purpose of the 1st principles electrochemical approach to ES SCC is to offer chemical rates for the various sensitization processes, these time scales in turn serving phenomenological structural mechanics’ based predictive modelling tool.