Following the accident at the Fukushima Daiichi Nuclear Power Plant resulting from the March 11, 2011 Great Tohoku Earthquake and subsequent tsunami, there was a general concern regarding the beyond design basis capability of existing nuclear power plants. The Condensate Storage Tanks (CST) in the nuclear power plant were originally designed to withstand an earthquake with a peak ground acceleration (PGA) of 0.3g.
The government regulatory commission increased the required PGA to 0.4g, therefore, an upgrade of the design basis for the CSTs was required. Due to the vintage of the existing nuclear power plant, the United States Nuclear Regulatory Commission (USNRC) Unresolved Safety Issue (USI) A-46 methodology may be used for seismic upgrade work of the mechanical and electrical equipment. The Condensate Storage Tanks (CST) belong to the mechanical and electric equipment and therefore were required to have seismic upgrade work because of significant deficiencies that were found in the anchorage of the tanks to the concrete foundation.
A seismic analysis and design upgrade of the Condensate Storage Tanks (CST) was performed to resolve exceedances for base shear, overturning moment and sloshing due to recently updated seismic loads. A detailed analysis of the CST showed that the as-built anchor chair detail did not provide sufficient margin for the Beyond Design Basis Earthquake Event (BDBEE). The as-built anchor chair detail did not provide the required strength to transfer the shear and pull-out loads from the tank shell to the concrete foundation, i.e. no clear load path was provided for the updated seismic loads. Therefore, as a result of the inadequate anchorage, the main issues to resolve in the CST were tank sliding, shell buckling and sloshing due to earthquake loading. The principal challenges encountered during the analysis, design and construction stages were (1) not allowing to loosen the double nut configuration attaching the anchor bolt to the existing anchor chair, and therefore allowing to (2) remove or replace only a few components of the as-built anchor chair, and (3) the retrofit design had to be implemented while tanks were operable, i.e. filled with fluid. An additional challenge faced during the design of the new anchor chair components was the limit in anchor chair height imposed by the numerous interferences in the CST such as nozzles, reinforcing plates and existing welds. A mitigation strategy is analyzed, designed and successfully implemented for retrofitting the ninety-six anchor chairs and allowing for full development of the anchor bolt shear and pull-out strength.