Symposium 18: Mechanical Behavior in Nuclear Materials

Pipe systems in nuclear power plants are subjected to cyclic thermal and mechanical loads from mixing points between hot and cool water and mechanically induced vibrations. The light water reactor (LWR) water environment flowing through the pipes at 300 °C with an internal pressure of 120 bars has a detrimental effect on the fatigue lives of the pipe systems. In this study, an experimental setup has been developed and designed to assess the corrosion fatigue lives of hollow specimens subjected to an alternating uniaxial cyclic load and simulated LWR water environment simultaneously. The corrosion fatigue tests have been conducted for both boiling water reactor (BWR) and pressurized water reactor (PWR) water environments.
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Effect of tension hold on crack propagation under subsequent fatigue loading during creep-fatigue crack propagation (CFCP) in single crystal (SC) and wrought Ni-base superalloys was investigated by conducting crack propagation tests with single tension hold applied during pure fatigue loading. Fatigue crack retardation occurred after the tension hold in the SC superalloy, whereas both retardation and acceleration occurred in the wrought superalloy depending on stress intensity factor, K. The retardation and acceleration were attributed to enhanced crack closure due to creep deformation and grain boundary (GB) embrittlement due to oxygen diffusion, respectively. Transition from the retardation to the acceleration was rationalized based on a comparison between sizes of residual compressive stress field and GB embrittlement area.
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Spent nuclear fuel (SNF) is currently stored across the US in passively cooled stainless steel dry storage canisters (DSC). Due to the design of the DSC, aerosols from the outside environment are able to deposit on the stainless-steel canisters. Over time the deposited aerosols will deliquesce on canisters to form concentrated salt brines resulting in localized corrosion, which when coupled with the high residual stress around welds can lead to stress corrosion cracking (SCC). The scope of the work presented is to investigate the boundaries of SCC to varying sensitivities such as environmental factors, microstructure variability, and material composition. These sensitivities will allow for recommendations to be made for canister monitoring and which variables are of the greatest concern for SCC of the DSCs. The data generated will be used in probabilistic FM predictions of AISCC growth for lifetime management of DSC. These predictions will inform a framework to quantify and manage a risk-based ranking of storage sites.
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The creep lives of enhanced high-temperature strength and creep resistance of 9%Cr 1%Mo P91 steels in boiler and piping systems of high-temperature plants are limited by the formation of cavitation. P91 steels are characterised by various secondary phases and a complex grain boundary microstructure which leads to regions of increased stress accumulation resulting in the initiation of cavities. In order to predict and possibly extend the creep lives of P91 structure components in energy applications, it is important that the processes promoting the initiation and early growth of cavities are understood. This paper employs microscopy techniques as well as image segmentation tools in order to quantify and characterize the cavitation and the secondary phases present in ex-service and creep tested P91 samples.
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Crystal plasticity finite element models can simulate the effect of microstructure on the cyclic behaviour of polycrystalline steels and can simulate the resulting local plastic strain. However, such models are computationally expensive and are therefore limited to simulation on small volume elements of material. In this work, a Gaussian process regression model is proposed as a surrogate model to predict macroscopic quantities of interest based on input parameters relating to the cyclic loading and material microstructure. The advantage with relation to computational expense of the surrogate can be leveraged for the purposes of undertaking uncertainty quantification and sensitivity analysis regarding the effect of the model inputs on the output prediction.
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The influence of the chemical composition on the brittle fracture behaviour of pressure vessel steels was investigated. Two model materials with chemical compositions simulating zones of moderate and severe positive macrosegregation were characterized and compared with the non-segregated material. Fractographic analysis suggests that brittle fracture mechanisms depend on the chemical composition through the induced microstructure.
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Creep-fatigue crack growth is an important failure mechanism for materials operating at high temperatures. While crack growth laws have been developed for hold-time loads and constant-amplitude cyclic loads, load transients must also be considered for determining component lifetimes. In this study, computational fracture mechanics simulations are used to study crack growth in two nickel-base superalloys at elevated temperatures following overloads. The computations demonstrate that post-overload crack growth depends strongly on the magnitude of crack-tip viscoplastic deformation in the bulk material. In some cases, a classical retardation effect is absent. Dynamic recovery and hardening due to viscoplastic strain gradients are also shown to influence post-overload crack-tip fields and crack growth.
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