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| Fig. 1: The Kashiwazaki-Kariwa Nuclear Power Station prior to the earthquake event. (Courtesy of Tokyo Electric Power Co. Source: Wikimedia Commons.) |
Nuclear power plants (NPP) use certain intensity measurements of seismic events to determine the need for shutdown. [1] Although they mainly use peak ground acceleration or spectral acceleration as the deciding measurement, numerical simulation results that use previous earthquake events and the corresponding seismic response on NPP's show that other measurements are more apt for determining shutdown. Specifically in higher frequency events (larger than 10 Hz), measurements of specific energy density and characteristic intensity (which is a function of the root-mean-square of acceleration and the total time) have a better correlation with the potential damage to a NPP than the more commonly used intensity measurements. [1]
Intensity measures have been used to shutdown reactors in several events around the world. It happened for the first time in the United States of America at the North Anna power plant during the 2011 Mineral Virginia US earthquake. [1] Although there have been other examples in Taiwan and South Korea, probably thet best-known earthquake event relevant to NPP is the Great East Japan Earthquake that resulted in the Fukushima nuclear disaster in 2011. [2] However, it is important to note that the earthquake itself did not directly cause the eruptions and radiation release at the plant. The earthquake, with a 9.0 magnitude on the Richter scale, initiated this situation when the reactors first shut down in response to measurements of ground acceleration signaling the seismic event. [2] With the earthquake incapacitating grid electricity to the plant, the pumps for cooling the reactor were powered with backup diesel generators. However, the earthquake also created an immense tsunami that impacted the NPP with a wave estimated to be 46 ft in height. The tsunami disabled the backup generators, resulting in overheating of the reactors. This led to a series of events that caused several explosions and major leakage of radiation to the air and sea. [2]
Another significant event was the 2007 earthquake the directly caused radiation leakage at the Kashiwazaki-Kariwa NPP in Japan. Shown in Fig. 1, this facility is the world largest nuclear power plant in electricity production with a direct supply of cooling water from the Sea of Japan. [3] Although the plant was rated to withstand a large amount of seismic acceleration, a 6.6 magnitude earthquake occurring around 17 km from the plant generated ground motion approximately twice as intense as the upper design limit for the reactor. [3] The plant's location on top of a folding geological structure lead to much larger pulse effects. [4] Nevertheless, after the shutdown of the reactor only minor radiation leakage occurred. [3]
Seismic protection systems (SPS) are a well-studied and successful method of protecting civil structures from damage due to earthquakes. However, SPS for NPP have been implemented in very few cases, most notably in the Koeberg and Cruas NPP in South Africa and France, respectively. [5] For the special case of nuclear reactors, SPS have been designed to include both vertical and horizontal isolation - in contrast to SPS for civil structures, which focuses on horizontal isolation. The reactor SPS also show the need for multiple forms of isolation devices to improve the safety factor. [5] Unlike civil structures, which are meant to sustain an acceptable amount of damage, SPS for reactors must be designed to withstand no damage, given the higher risks involved in a damaged reactor. The most common isolation methods proposed and used for NPP are passive devices, such as lead-rubber bearings and viscous dampers, which provide damping levels up to 30% each. [5] These methods would be used to provide an isolated foundation that covers all of the components of the NPP to create what is referred to as a "nuclear island", which aims to insulate the reactor from seismic activity regardless of the local geological factors. [5]
The low number of SPS currently used in NPP can be attributed to the low number of actual applications that deems the protection necessary. As previously discussed, there are few cases in which earthquakes damaged NPP - where the most noteworthy case led to a disaster due to a chain of events that included a 46 ft high tsunami. Yet, the increased incorporation of SPS to NPP could provide many benefits and opportunities to the nuclear energy industry. Firstly, the newer generation of nuclear reactors show signs of increased susceptibility to seismic events. For example, the high mass density coolant used in lead-cooled fast reactors makes potential sloshing motions from earthquakes an important factor. [5] Also, estimates indicate that the SPS would have a small increased cost to the construction of NPP. Finally, the standardization of NPP design and construction with SPS has the potential to increase the approval of reactors in regions that were previously deemed unsafe due to seismic activity. [5]
© Andy Castillo. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] D. D. Nguyen et al., "Identifying Significant Earthquake Intensity Measures for Evaluating Seismic Damage and Fragility of Nuclear Power Plant Structures," Nucl. Eng. Technol. 52, 192 (2020).
[2] A. Labib and M. J. Harris, "Learning How to Learn from Failures: The Fukushima Nuclear Disaster," Eng. Fail. Anal. 47, 117 (2015).
[3] O. V. Pavlenko and K. Irikura, "Nonlinear Soil Behavior at the Kashiwazaki-Kariwa Nuclear Power Plant During the Niigata Chuetsu-Oki Earthquake (July, 16, 2007)," Pure Appl. Geophys. 169, 1777 (2012).
[4] F. Gatti et al., "On the Effect of the 3-D Regional Geology on the Seismic Design of Critical Structures: the Case of the Kashiwazaki-Kariwa Nuclear Power Plant," Geophys. J. Int. 213, 1073 (2018).
[5] C. Medel-Vera and T. Ji, "Seismic Protection Technology for Nuclear Power Plants: a Systematic Review," J. Nucl. Sci. Technol. 52, 607 (2015).
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