Modeling and Validation of Irradiation Damage in Ni-based Alloys for Long-Term LWR Applications (US/UK)

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As pressurised water reactor (PWR) plant operating lives are extended, there is an increased need for predictive modeling tools to describe materials degradation in order to ensure safe, reliable operation as well as plan for component replacements. Some models presently exist, but are limited in their applicability, and are not able to predict degradation of all alloys used in PWR systems. Specifically, it is important to be able to predict any degradation in Ni-based alloys used in nuclear power plants, thus, this program represents an integrated approach to address thermal and irradiation-induced transformations mechanisms of to important Ni-base materials used in PWRs, Alloys 690 and 625. Alloy 690 is widely used in existing PWR plants due to its superior SCC resistance compared to Alloy 600. Alloy 625 is currently used in more limited applications but offers the benefits of both high strength (in the aged condition) and corrosion/SCC resistance, and is being considered for use in future reactor systems. Research has shown that both alloys can undergo phase changes due to thermal or irradiation exposure. In the precipitation-hardened condition, Alloy 625 "softens" during neutron irradiation as the strengthening precipitates decompose and metastable precipitates form. However, the nature and rates of these transformations as a function of exposure conditions are not well understood. Similarly, the effects of these thermal and irradiation-induced microstructural changes on mechanical properties require evaluation. The proposed program combines thermal and irradiation experiments, mechanical testing, and advanced microstructural characterization using state-of-the art analytical techniques, the results of which will be combined with atomistic, micro-and macro-scale modelling that can be used to predict materials performance. Such a capability will also greatly aid in research to develop optimised existing alloys or new alloys for future nuclear power plants.