Recent Experiences Related to Natural Phenomena Hazards
Since 2007, three earthquake events have occurred resulting in ground motions larger than the design basis ground motions at the affected plants. These three events are:
- (1) The Niigataken Chuets-oki (NCO) earthquake of July 16, 2007, with epicenter 16 km from Kashiwazaki- Kariwa (KK) NPP site, Magnitude 6.8 (JMA Magnitude)
- (2) Tohoku Pacific earthquake of March 11, 2011 affecting various sites including Fukushima-Daiichi, Magnitude 9.0 Mw
- (3) August 23, 2011, Mineral, Virginia earthquake, epicenter about 11 miles from North Anna Power Station, Magnitude 5.8
Figure 8.5 shows observed maximum accelerations at the base mat of Reactor Buildings of all seven KK units with the corresponding design basis values. As seen in the figure, all seven units recorded significant but different motions and the observed motions significantly exceeded the design basis in some cases.
OBSERVED AND DESIGN BASIS MOTIONS AT KK UNITS
Figure 8.6 shows similar comparison of observed motion and design motion for Fukushima Daiichi units during the Tohoku Pacific earthquake. The highest peak acceleration measured at Fukushima Daiichi was 0.561 g in the horizontal direction and 0.308 g in the vertical direction at Unit 2. In three instances, at Units 2, 3, and 5, the observed motion exceeded the design basis. These exceedances were much smaller compared to the exceedances at the KK units.
North Anna earthquake:
At 1:51 p.m. EDT on August 23, a magnitude 5.8 earthquake occurred near Mineral, Virginia, close to the North Anna Power Station. The earthquake caused the reactor plants to automatically shut down and resulted in a loss of off-site power. The plant declared an Alert, the second lowest of the four emergency classification levels used by U.S. nuclear plants. No damage was reported to systems required to maintain the station in a safe condition. Several aftershocks felt in the region later that day did not affect the station.
FIG. 8.6 OBSERVED AND DESIGN BASIS MOTIONS (IN GAL) AT FUKUSHIMA DAIICHI UNITS (NUMBER SHOWN IN PARENTHESES ARE DESIGN VALUES)
NORTH ANNA NUCLEAR POWER PLANT
As designed, the plant’s four emergency diesel generators started and supplied power to important electrical equipment while off-site power was unavailable. One of the four emergency diesel generators was taken off-line to repair a coolant leak, and another generator available at the station was started to replace it until the off-site power was restored. Later that evening, off-site power to the plant was restored, eliminating the need to rely on the back-up generators. The North Anna Power Station has two Safe Shutdown Earthquake ground motions, one for structures, systems, and components located on top of rock, which is anchored at 0.12 g, and the other for structures, systems, and components located on top of soil, which is anchored at 0.18 g. The plant has two corresponding Operating Basis Earthquake ground motion spectra, anchored at 0.06 g for rock and 0.09 g for soil. At several frequencies, the spectral and peak ground accelerations as a result of the August 23, 2011 earthquake were greater than those used for the Operating Basis and Design Basis Earthquakes. Figures 8.8 and 8.9 below show recorded motions at the basemat of Unit 1 containment building. The observed horizontal spectrum (North- South component) and vertical spectrum show exceedances when compared to the corresponding SSE spectra.
UNIT 1 BASEMAT SPECTRUM COMPARISON (HORIZONTAL) BETWEEN THE DESIGNED AND THE OBSERVED
UNIT 1 BASEMAT SPECTRUM COMPARISON (VERTICAL) BETWEEN THE DESIGNED AND THE OBSERVED
Several salient observations can be made from these three events:
- (1) Common-cause effects of an earthquake, in-terms of affecting all of the units at a site, are clearly evident. Also, the observed damage of some of the non-safety related components at KK units highlighted a need for consideration of potential for significant correlation of earthquake responses and capacities of SSCs so that seismic risk is more realistically assessed. This is particularly an important issue in considering site risk for a site with multiple units.
- (2) The recorded motions at units from the same event are significantly different, thus highlighting spatial variations and important effects of site topography, layering, and subsurface conditions under each unit that affect local site response.
- (3) Although, the observed motions were higher, both at KK and North Anna Nuclear Power Plant, there was no observable damage to safety-related SSCs and these SSCs were able to perform their safety functions. The information available to date also indicates that there is no evidence of significant seismic induced damage to the safety related SSCs at the Fukushima units. This observation confirms the existence of margins and conservatisms associated with the seismic design processes.
- (4) The occurrence of events that challenge or exceed a design basis could lead to extended shutdowns. Existence of procedures to deal with the post-earthquake inspections and conducting necessary plant evaluations can reduce the down time.
- (5) Existence of functioning seismic instrumentation is also very important to fully understand the ground motions and other earthquake parameters, and to evaluate the plant responses.