Fukushima Daiichi Nuclear Power Plant: A Retrospective Evaluation P. Carydis Professor Emeritus, National Technical University of Athens, Athens, Greece A. Pomonis Cambridge Architectural Research Ltd, United Kingdom K. Goda University of Bristol, Department of Civil Engineering, United Kingdom SUMMARY: Based on available data and personal experience in Japan at the time of the design and construction of the Fukushima Daiichi nuclear power plant (NPP), the basis of its earthquake resistant design is revisited. The relevant knowledge and technology of the time (late 1960’s to early 1970’s) are reviewed to illustrate the technical environment in which this and other plants around the world were designed. The initial structural design of the plant was based on static analysis and for structures and components needing high seismic protection, a seismic coefficient of 0.54 was used. A comparison is carried out between the design seismic loads and the inferred seismic excitations that were likely to be experienced at the Fukushima Daiichi NPP during the 11 th March 2011 earthquake. It is indicated that the allowable checking stress reached the yielding material stress. The piping system and the containment structure were dynamically designed based on a pre-described response spectrum shape with peak ground acceleration of0.27g. It is concluded that structural components’ upgrading should be as much as around three times the initial seismic resistance in order for the plant to safely withstand the 2011 ground motion. Keywords: March 11 th 2011 Tōhoku earthquake, Fukushima Daiichi nuclear power plant, seismic design 1. INTRODUCTION On Friday 11 th March, 2011, at 14:46 Japan Standard Time, a M w 9.0 earthquake occurred at the Japan Trench off the coast of Tōhoku in north-east Japan. Ground shaking was felt as far as western Japan and lasted for almost four minutes (220 sec), generating large, unprecedented tsunami waves that caused the loss of around 19,200 lives (incl. circa 3,000 people missing). This M w 9.0 earthquake is the largest event that has been recorded in Japan since the beginning of instrumental seismology circa 1900. The Fukushima Daiichi nuclear power plant (NPP) is situated on the Pacific coast of northern Honshu Island near Okuma Town and Futaba Town, Futaba County, Fukushima Prefecture and consists of six reactors that commenced operation between 1971 and 1979 (IAEA, 2011a). There were three reactors, one, two and three, operating at the time when the 2011 earthquake occurred, while reactors four, five and six had been shut down as part of routine maintenance work. The strong ground shaking triggered the immediate shut down of the three operating reactors. The control rods were automatically inserted into the reactors, halting the fission reaction that generates electricity. The same shutdown process happened at other eight reactors in Japan (EEFIT, 2011), causing a sudden loss of power across Japan's national power grid, widespread power failures and cutting vital electricity supplies to Fukushima Daiichi. In addition there was seismic damage to the external grid infrastructure that resulted in the loss of external power supplies (EEFIT, 2011). This was critical because when it is shut down, it is reliant on the external grid, on-site emergency diesel generators, or other reactors at site for AC power (ONR, 2011). The design tsunami height in Fukushima Daiichi was 5.4 to 5.7 m (JSCE, 2002) and to include a margin of safety the elevation of the plant was set higher (IAEA, 2011b) – the site elevation of Fukushima Daiichi NPP is 10 m at Units 1 to 4, and 13 m at Units 5 and 6 (EEFIT, 2011). About 45 minutes after the earthquake, 14 to 15 m high tsunami waves overpowered the plant’s sea protection wall and flooded much of the facility (TEPCO, 2011), disabling the critical back-up power generators that were placed at elevation lower than the tsunami flood zone.