ALBERT

Description: The M w  5.8 Mineral, Virginia, earthquake was recorded at an epicentral distance of about 18 km at the North Anna Nuclear Power Plant (NPP) by the magnetic tape digital accelerographs installed inside the plant’s containment at the foundation and deck levels. The North Anna NPP is operated by the Virginia Electric and Power Company (VEPCO) commonly called Dominion Power Company and has two pressurized water reactor (PWR) units that began operation in 1978 and 1980. Following the earthquake, both units were safely shut down. The strong-motion records were processed to get velocity, displacement, Fourier amplitude, and 5%-damped response spectra. The basemat record demonstrated relatively high amplitudes of acceleration of and velocity of 13.8 cm/s with a relatively short duration of strong motion of 2–3 s. The higher elevation containment-deck record had peak acceleration of and peak velocity of 26.1 cm/s. These accelerograms are the strongest ground motions ever recorded at a U.S. nuclear power plant. Basemat response spectra exceed the Design Basis Earthquake for the existing Units 1 and 2, whereas comprehensive plant inspections performed by Dominion and the U.S. Nuclear Regulatory Commission have concluded that the damage to the plant was minimal and not affecting any structures and equipment significant to plant operation. This can be explained in part by the short duration of the earthquake ground motion at the plant. The North Anna NPP did not have free-field strong-motion instrumentation at the time of the earthquake. Because the containment is founded on rock, there is a tendency to consider the basemat record as an approximation of the free-field recording. However, comparisons of deck and basemat records demonstrate that the basemat recording is also affected by structural resonances in the frequency range of 3–4 Hz and higher. Therefore, future seismological interpretation of these recordings should take into account the effect of structure.