Ten years after the Fukushima Daiichi Nuclear Power Plant accident, the nuclear industry is facing a pivotal moment. As we face growing public concerns in Japan and a highly competitive environment driven by the low cost of fossil energies in the United States, a gradual reduction of the nuclear power fleetʼs capacity may seem inevitable. Overcoming the challenges posed by nuclear plant aging management during decommissioning could add several decades to the service life of the concrete infrastructure. Nuclear energy provides sustainable, carbon-free electricity and the necessary base-load power generation that is indispensable to a safe, dependable, economically viable generation mix of energy sources, including renewable energies. Because replacement of existing concrete structures in reactor buildings is economically unrealistic, the sustained long-term operation of nuclear power plants requires that the structural performance of the concrete structures, systems and components (SSCs) is maintained during an extended service period, possibly for another 80+ years. Extending nuclear power plant operations requires sound, comprehensive, and reliable justifications of plant safety. Therefore, it is imperative that research be conducted to assess the condition, manage aging, and evaluate the performance of the materials, members, and structures currently in service. Furthermore, predictive approaches must be developed to assess future performance of materials, members, and structures. Based on these factors, a second special issue on the aging management of concrete structures in nuclear power plants was created. This issue provides a state-of-the-art review highlighting the current technical challenges for aging management of these structures, and it presents options to address these challenges. This special issue includes 12 pertinent manuscripts addressing the varied technical issues associated with the long-term operation of nuclear power plants. Five of these manuscripts (18-648, 18-618, 18-558, 19-555, 19-668) focus on the effects of gamma and neutron irradiation on concrete and its constituents, bridging the gap between the fundamental understanding of the effect of gamma irradiation (19-555, 18-558) and the advanced modeling techniques using lattice-based models that are applied at varied scales to provide predictive simulation of the physical and engineering properties of concrete-forming aggregates (19-668), concrete (18-648), and the concrete biological shield (18-618). Two papers complement this research by presenting (1) a novel automated petrographic x-ray‒based characterization method (19-395), and (2) a method to implement advanced characterization of the aggregate forming mineral phases into a fast Fourier transform (FFT-based simulation framework (19-149). These techniques may also be used to address other degradation modes, such as the alkali-silica reaction (ASR). Notably, this pathology is addressed in this issue in two original papers focused on the modeling and monitoring of the structural performance of ASR-affected shear walls (19-280, 19-477). The effects of chemical attacks caused either by sulfate ingress (19-796) or carbonation (19-382) are also presented in this special issue, with a focus on the effects of the actual operating conditions in nuclear power plants. A discussion of the structural performance of concrete structures that support vibrating equipment such as that found in a turbine building of a nuclear power plant is also included (19-414). These research works provide invaluable state-of-the-art overview of the knowledge, methodology, and simulation techniques necessary to reinforce public trust and to provide industry stakeholders and regulatory bodies with the guidance and approaches needed to address future nuclear power generation technologies.

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This special issue is planned based on the 6th International Conference on "Construction Materials, Performance, Innovations, and Structural Implications" . ConMat'20. The authors who were awarded "ConMat'20 Best Paper Award" were invited for this Special Issue. Construction industries are large contributors to resource consumption and CO2 emissions. As resource scarcity and global warming are becoming more and more pressing issues for humanity, new demands have arisen. Construction materials now require new, innovative performance features in addition to further improvement of conventional performance characteristics. In our world, sustainability is one of the primary factors for the 21st century. It is well recognized that it is difficult to maintain the current global environment in the future. Therefore, all industries are taking the necessary action to ensure sustainability. For the purpose that the concrete industry itself could be sustainable and for establishing a sustainable society in the 21st century, the concrete and construction sector is making all efforts to make innovations in concrete engineering and construction works. In the ConMat'20, many attempts on this objective were introduced. This special issue includes the selected excellent researches and projects from the ConMat'20, refined and refereed for this issue. We expect this special issue could provide strong and attractive inspirations toward future research and development in the 21st century to concrete researchers and concrete engineers all over the world.

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