Seismic response of the EBR-II to the Mt. Borah earthquake

On October 28, 1983, an earthquake of magnitude 7.3 occurred in the mountains of central Idaho at a distance of 114-km from the ANL-West site. The earthquake tripped the seismic sensors in the EBR-II reactor shutdown system causing a reactor scram. Visual and operability checks of structures, components, and systems showed no indication of damage or system abnormalities and reactor restart was initiated. As a result of the earthquake, questions arose as to the magnitude of the actual stress levels in critical components and what value of ground acceleration could be experienced without damage to reactor structures. EBR-II was designed prior to implementation of present day requirements for seismic qualification and appropriate analyses had not been conducted. A lumped-mass, finite element model of the primary tank, support structure, and the reactor was generated and analyzed using the response spectrum technique. The analysis showed that the stress levels in the primary tank system were very low during the Mount Borah earthquake and that the system could experience seismic loadings three to four times those of the Mount Borah earthquake without exceeding yield stresses in any of the components