Research on Variations of floor response spectra under long-term operation of nuclear power plants

In recent decades, it has been a common international practice to extend the service life of operating nuclear power plants. The aging management of nuclear power plants is one of the key arguments for the life extension of nuclear power plants, and the evaluation of structures and equipment that meet the requirements of “no change in structure and characteristics when performing predetermined functions” and “no replacement based on appraisal life and specified time” in the screening results of aging management objects is the key technology for the life extension operation of nuclear power plants. However, the existing research on the aging of nuclear island plants and equipment mostly focuses on the prediction model of the performance changes of the materials themselves, and there are few studies on whether the nuclear island plants and pressure vessels can meet the original seismic performance evaluation after the aging effect. Based on the typical nuclear island plant structure, we analyzed the response characteristics of the nuclear island plant structure under different material properties in this study. The analyzed results show that the aging of reinforced concrete materials change and broaden the response of the nuclear island structure floor, and exceedances over the original design’s ± 15 % floor response spectra (FRS) uncertainty are significant in X (up to 51 %), Y (up to 22 %), and Z (up to 29 %). The nuclear island structure floor is the seismic input for the seismic performance evaluation of the pressure vessel, but existing studies overly focus on material degradation itself while neglecting seismic safety reassessment of critical structures (nuclear island, RPV). The research is therefore helpful to establish an FRS-envelope methodology for global LTO programs, which provides a technical basis for the subsequent life extension evaluations of nuclear power plants.