Session Th1: Thursday, June 15, 10:30-12:30
Thursday Jun 15 2023
10:30 - 10:50
CORROSION FATIGUE OF HOLLOW SPECIMENS IN SIMULATED LWR WATER ENVIRONMENT
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.
10:50 - 11:10
EFFECT OF TENSION HOLD IN CREEP-FATIGUE CRACK PROPAGATION IN NI-BASE SUPERALLOYS: TRANSITION FROM CRACK RETARDATION TO ACCELERATION
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.
11:10 - 11:30
EVALUATING THE SENSITIVITIES OF AISCC SUSCEPTIBILITY IN STAINLESS-STEEL NUCLEAR WASTE STORAGE CANISTERS FOR DEVELOPMENT OF A LIFETIME PREDICTION MODEL
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.
Session Th2: Thursday, June 15, 14:00-16:00
Thursday Jun 15 2023
14:00 - 14:40
MATERIALS PROPERTY CHANGES AFTER IRRADIATION EVLAUATED USING SMALL SCALE MECHANICAL TESTING. [Keynote]
Radiation damage can lead to significant property changes in structural materials. Radiation induced swelling, embrittlement or increase in yield strength are just a few. The dose, dose rate and temperature together determine the effect on the material which can have significant engineering impact. Therefore, it is key to understand how a material changes under radiation and being able to predict the property changes. Small scale mechanical testing offers a wide range of benefits especially when working with materials in nuclear application. The reduced size allows to handle highly radioactive materials while also enabling ion beam irradiations as a surrogate to quantify radiation damage. In this work we will provide examples on how small-scale mechanical testing provided deep insight into the mechanical deformation of materials after irradiation. We investigate how the properties change due the radiation induced dissolution of precipitates or due to the formation of new features such a cavities, dislocation loops or precipitates. We will highlight how the plasticity and associated mechanical property values change. Last but not least we will introduce scaling studies performed in order to extract bulk properties from small scale tests. Mesoscale mechanical tests enabled using laser fabrications are shown.
14:40 - 15:00
HIGH TEMPERATURE CREEP CAVITATION IMAGING AND ANALYSIS IN 9%CR 1%MO P91 STEELS
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.
15:00 - 15:20
PREDICTING THE MACROSCOPIC CYCLIC BEHAVIOUR OF POLYCRYSTALLINE STEELS BASED ON MATERIAL MICROSTRUCTURE VIA SURROGATE MODELLING
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.
Session Th3: Thursday, June 15, 16:30-18:00
Thursday Jun 15 2023
16:30 - 17:00
IN SITU X-RAY TOMOGRAPHY IMAGING OF CRACK INITATION AND PROPAGATION IN NUCLEAR GRAPHITE AT 1000°C [Keynote]
Nuclear-grade graphite is a critically important high-temperature structural material for current and potentially next generation of fission reactors worldwide. It is imperative to understand its damage-tolerant behaviour and to discern the mechanisms of damage evolution under in-service conditions. Here we perform in situ mechanical testing with synchrotron X-ray computed micro-tomography at temperatures between ambient and 1,000 °C on a nuclear-grade Gilsocarbon graphite. We find that both the strength and fracture toughness of this graphite are improved at elevated temperature. Whereas this behaviour is consistent with observations of the closure of microcracks formed parallel to the covalent-sp2-bonded graphene layers at higher temperatures, which accommodate the more than tenfold larger thermal expansion perpendicular to these layers, we attribute the elevation in strength and toughness primarily to changes in the residual stress state at 800–1,000 °C, specifically to the reduction in significant levels of residual tensile stresses in the graphite that are ‘frozen-in’ following processing. A range of other nuclear grade graphite materials were tested and compared with Gilsocarbon graphite.
17:00 - 17:20
BRITTLE FRACTURE MECHANISMS OF THREE MODEL LOW ALLOY STEELS CHEMICALLY REPRESENTATIVE OF A MACROSEGREGATED FORGING
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.
17:20 - 17:40
TRANSIENT CREEP-FATIGUE CRACK GROWTH IN CREEP-DUCTILE AND CREEP-BRITTLE MATERIALS: APPLICATION TO ALLOY 617 AND ALLOY 718
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.
Session F1: Friday, June 16, 10:30-12:30
Friday Jun 16 2023
10:30 - 11:10
IN-SILICO QUALIFICATION OF MATERIALS [Keynote]
Material qualification is an important pre-requisite for design substantiation of any power plant. Historically, this is achieved through large experimental programmes that are eventually collated to support design standards (e.g. ASME) or later in assessment codes (e.g. UK’s R5 and R6). This process is slow and expensive but low risk. In parallel, computer simulations have expanded their roles in the design and assessment process. Advanced physics-based simulations techniques such as crystal plasticity frameworks are increasingly being used to inform the engineering practices. However, they require extensive research to validate and substantial training for the practitioner to ensure the validity of their results. They are therefore considered to be expensive techniques that are deployed at exceptional circumstances. In this paper, a road map to use recent advances in machine learning is proposed that can simplify the complex physics-based simulations and produce high fidelity surrogate models that can be used cheaper, faster, with less stringent training. The surrogate models, because are based on rigorous physics-based simulations, can form part of the material qualification thus accelerating the process and making it more efficient.
11:10 - 11:30
INTEGRITY EVALUATION OF SPENT NUCLEAR FUEL CLADDING IN USE OF MACHINE LEARNED EMBRITTLED PROPERTIES
Yong Gyun ShinWalnut
Integrity of spent nuclear fuel (SNF) cladding should be remained during transportation as well as long-term storage and disposal. At first, this paper addresses machine learning to predict degraded mechanical properties of an advanced zirconium alloy. Subsequently, taking into account the estimated data, finite element analyses of a typical fuel rod were carried out under hypothetical drop accident conditions and resulting integrity was discussed.
11:30 - 11:50
CRITICAL CRACK SIZE OF A PROTOTYPE PIPE BEND UNDER CYCLIC LOADING
The accurate assessment of critical crack size plays a vital role in demonstrating the Leak-Before-Break (LBB) criterion for the safety demonstration of a sodium-cooled Fast Breeder Reactor (FBR) piping system. The advancement of the crack size will increase the stress intensity factor and reduce the load-carrying capacity of the piping system. The prototype-sized pipe bend test revealed that even under a large-size crack growth situation, the ductile pipe bend fails by collapse rather than tearing instability. The critical crack size was realistically estimated based on a prototype-sized pipe bend cyclic test and compared with elastoplastic numerical analysis.