Fatigue crack growth of austenitic stainless steels can be enhanced significantly in high temperature light water reactor coolant environments and an ASME Code Case, N-809, has recently been developed to provide fatigue crack growth rate curves for these alloys in pressurized water reactor environments. However, under some conditions, the enhanced rates can decrease to rates close to those in air at long rise times, a process referred to as retardation which is not taken account of in the Code Case. An improved understanding of the mechanisms of both enhancement and retardation would be beneficial to determining whether advantage could be taken of these retarded rates in plant assessment. A number of studies have been undertaken to evaluate fatigue crack growth behavior in both air and water environments in order to provide mechanistic insight. Progress on this work will be described. The data from air and inert environments support the proposed mechanism of environmentally enhanced fatigue by environmental enhancement of planar slip, although it is not yet possible to differentiate between the impact of oxidation and corrosion hydrogen on the level of enhancement in aqueous environments. Testing in high temperature water environments suggests that both corrosive blunting and/or oxide-induced closure mechanisms may contribute to crack growth rate retardation under specific circumstances.
There are potentially significant cost benefits through plant simplification if a soluble boron-free lithiated primary water chemistry can be demonstrated to be a viable route for small modular reactor operation. However, the corrosion behavior of the clad material under lithiated conditions remains a concern. High levels of lithium (Li) have been demonstrated to be detrimental to the corrosion behavior of zirconium alloys. Under a thermal gradient, as experienced by the clad in pressurized water reactor operation, this becomes more complex; with increasing oxide thickness, the potential for operation under two-phase (nucleate boiling) conditions increases. The significance of this is twofold: first, the concentration of lithium local to the oxide has been shown to increase as boiling occurs within the thick porous oxide, potentially increasing the Li content beyond the threshold for accelerated corrosion; second, the nucleate boiling can result locally in more aggressive (high lithium) chemistry conditions. This paper presents the results from a test program that has investigated the effect of lithium, temperature, thermal gradient, two-phase flow. and stress on the corrosion behavior of Zircaloy-4 and discusses the results in the context of previous work carried out in this area. The data from this work indicate that in nominal lithium concentrations, under certain combinations of a thermal gradient, two-phase flow, and stress, a significant acceleration in corrosion can be observed. Characterization and modeling of the specimens exposed to a thermal gradient, in comparison to isothermally corroded Zircaloy-4 coupons in a range of Li concentrations, has developed the understanding of the processes occurring during this acceleration.
Laboratory tests on austenitic stainless steel in simulated light water reactor (LWR) coolant environments have been shown to give rise to significant environmental enhancements of fatigue crack growth, especially at low cycling frequencies. The impact of LWR environments on fatigue crack growth has recently been codified in ASME code Case N-809 in terms of parameters such as rise time, stress intensity factor and load ratio. However, plant performance suggests that the application of these predicted environmental effects using current assessment procedures may be unduly pessimistic. This has led to significant number of studies of waveform shape (specifically hold periods) on the corrosion fatigue crack propagation in austenitic stainless steels in LWR environments. The main emphasis of this work addresses the ability of hold periods to cause retardation of environmental crack growth rates. There has been substantial variability in results of these studies with some authors reporting significant retardation whilst others have failed to observe retardation, or even reported additional environmental enhancement of crack growth rates for nominally similar loading waveforms. Although some of the variability may be accounted for in terms of material composition, there remains a considerable uncertainty both on the impact of holds, especially at different positions in the waveform, and the manner in which hold periods should be taken into account in plant assessments (e.g. in assessment procedures such as N-809). The current paper provides a critical review of published data on the effect of hold periods on corrosion fatigue in LWR environments as well as presenting new targeted data generation and analysis in order to rationalise the reported observations.
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