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Local approach methods are becoming increasingly popular as practical tools for cleavage fracture toughness prediction. Their application involves two distinct elements: calculation of "individual" probabilities of failure, dictated by the local mechanical fields; and summation of these failure probabilities to predict the probability of component failure. In this work, we demonstrate that development of the local approach methods to date has been essentially focused on improving the criterion for predicting local failure as a function of the local mechanical fields. Yet, the existing methods fail to predict with sufficient accuracy the effects of irradiation and defect geometry on fracture toughness when the calculations are based on a common set of model parameters. A possible reason for this, common to all methods, is found in the calculation of the cumulative failure probability, which is based on the weakestlink argument. We discuss the implications of the weakest-link assumption, identify those situations where it needs to be reconsidered, and propose future work that will increase our understanding for improving the calculation of global failure probability. INTRODUCTIONThe ability to predict changes in cleavage fracture toughness behaviour of ferritic RPV steels accounting for the effects of irradiation and defect geometry is important to safety assessment and life extension decisions in nuclear plant. Local approaches to cleavage fracture offer a promising methodology to help achieve this. In principle, these are based on current understanding of the micro-mechanisms involved in the cleavage failure phenomenon, such as the nucleation of microcracks at second-phase brittle particles, the propagation of such micro-cracks within grains and the propagation of a critical micro-crack leading to component failure. The scope of this work does not include local approaches that involve changes in

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It is important for safety case justification of the continued use of nuclear power plant that any changes in the ferritic RPV steels' fracture toughness with temperature, irradiation and geometry can be accounted for, particularly with regards to plant life extension. It has been demonstrated that local approach methods have the potential to provide such estimates by assessing the likelihood of cleavage rupture.Here a micro-structurally informed, post-processing, local approach methodology with a new rupture criterion is presented. This has been applied to a base and weld material pair, made available under the EU FP7 PERFORM 60 Programme. This material has been selected as tensile and fracture data are available along with results of recently performed analyses to characterise and quantify the size distribution of fracture initiators for the weld material (predominantly found to be alumino-silicates) which is used as an input to the model.A series of finite element analyses have been performed for two-dimensional three-point bend specimens over a range of temperatures, constraint and irradiation states. Application of the local approach model to these results has then been favourably compared to the experimentally determined toughness. This has been achieved for a range of conditions when using the experimentally determined initiator particle distribution and maintaining the same calibration terms throughout for each material.

This paper considers finite element analyses that have been performed to support a fatigue endurance testing programme. This programme is aimed at understanding the influence of Light Water Reactor (LWR) environment on the fatigue life of austenitic steels under thermo-mechanical loading. Testing has typically been performed on membrane loaded fatigue specimens under isothermal conditions. However, a new test facility at Amec Foster Wheeler has been developed to enable hollow specimens to be subjected to thermal and mechanical loading for a range of thermal cycles. This work has provided a theoretical underpinning of the observed difference in lifetimes between pressurised and unpressurised specimens, and therefore provides a means of mapping data from hollow specimen testing back to solid specimen data. The parameter used to quantify these differences was the Von Mises equivalent strain. This accounts for the additional contributions from radial and hoop strains (which come about due to the internal pressure) and therefore gives enhanced strain amplitude which is fed into the S–N curve. By comparing unpressurised and pressurised lifetimes this way, a direct comparison could be made with test results. This serves two purposes; one is to provide mechanistic understanding of the difference in lifetimes. Secondly the approach will develop an assessment methodology to treat hollow specimens so a direct comparison can be made to bar specimen S–N curves. To provide further confidence in this mechanism being the dominant factor behind this difference, an independent calculation was carried out using multi-axial fatigue life models, namely Brown-Miller and Fatemi-Soci. Good agreement was observed with these models indicating that the Von Mises strain parameter was a valid parameter to characterise the multi-axial strain behaviour at the initiation site. The increase in Von Mises strain between pressurised and unpressurised specimens was found to be a factor of 1.20 on average. This was slightly less than the experimentally derived strain differences of about 1.25. However, there was another aspect of the test results that required investigation. It has been observed from tests that the hollow specimens had a tendency to fail at a region just after the shoulder, which is where the specimen increases in thickness. Upon inspection of the plastic strains from FEA, it became clear that there was a small increase in strain on the inside surface due to this geometric feature of the specimen. The strain at the high strain region was a factor of 1.025 higher than the centre of the specimen after shakedown. Therefore, when the two effects of geometry and incremental plasticity are combined, the agreement between experiment and FEA is better. There is still a slight difference between the observed factor and the predicted factor, and reasons for this discrepancy are discussed in the paper.

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SOTERIA is focused on the ‘safe long term operation of light water reactors’. This will be achieved through an improved understanding of radiation effects in nuclear structural materials. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under agreement No 661913. The overall aim of the SOTERIA project is to improve the understanding of the ageing phenomena occurring in ferritic reactor pressure vessel steels and in the austenitic internals in order to provide crucial information to regulators and operators to ensure safe long-term operation (LTO) of existing European nuclear power plants (NPPs). SOTERIA has set up a collaborative research consortium which gathers the main European research centers and industrial partners who will combine advanced modelling tools with the exploitation of experimental data to focus on two major objectives: i) to identify ageing mechanisms when materials face environmental degradation (such as e.g. irradiation and corrosion) and ii) to provide a single platform containing data and tools for reassessment of structural components during NPPs lifetime. This paper provides an overview of the ongoing activities within the SOTERIA Project that are contained within the analytical work-package (WP5.3). These fracture aspects are focused on the estimates of fracture in both ferritic steels and irradiation assisted stress corrosion cracking (IASCC) in austenitic stainless steels, under irradiated conditions. This analytical development is supported by analytical estimates of irradiation damage and the resulting changes in tensile behaviour of the steels elsewhere in SOTERIA, as well as a wider number of experimental programmes. Cleavage fracture estimates are being considered by a range of modelling estimates including the Beremin, Microstructurally Informed Brittle Fracture Model (MIBF), JFJ and Bordet Models with efforts being made to understand the influence of heterogeneity on the predicted toughness’s. Efforts are also being considered to better understand ductile void evolution and the effect of plasticity on the cleavage fracture predictions. IASCC is being modelled through the INITEAC code previously developed within the predecessor project Perform 60 which is being updated to incorporate recent developments from within SOTERIA and elsewhere.