Safety goals for seismic and tsunami risks: Lessons learned from the Fukushima Daiichi disaster

• Published in: Nuclear engineering and design Vol. 280; pp. 449 - 463
• Main Author: Saji, Genn
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.12.2014
• Subjects: Design engineering; Disasters; Earthquake design; Earthquake engineering; Nuclear engineering; Nuclear reactor components; Seismic phenomena; Tsunamis; INE, Pacific, San Andreas Fault
• ISSN: 0029-5493; 1872-759X
• DOI: 10.1016/j.nucengdes.2014.09.013

•Reviewed why the Fukushima disaster was not anticipated among seismologists.•Reviewed Fukushima Daiichi's preparedness against the earthquake and tsunami.•There was a large “cliff edge” in radiological consequences from the design basis tsunami.•By including earthquakes as an “external event” resulted in insufficient “defense in depths”.•Proposes a new probabilistic seismic and tsunami safety goal be developed. This paper first reviews why the potential occurrence of the Tohoku-oki earthquake with momentum magnitude Mw of 9.0 earthquake was not anticipated by Japanese seismologists, and to clarify our limitations in predicting rare but severe earthquakes at our current knowledge in the field of geosciences. Although there was a large volume of historical records related to earthquakes and tsunamis, generally this data infer high plate coupling in regions where earthquakes were known to have already occurred, with only partial or even no coupling from the Japan Trench to a point approximately midway between the trench and the coastline—precisely the region where the 2011 Tohoku-Oki earthquake occurred. This phenomenon has been explained as a “silent earthquake” or a fault creep as observed at the San Andreas Faults in the US. Considering the large uncertainties in seismic events, nuclear power plants should be conservatively designed with adequate safety margins. TEPCO's preparedness against seismic and tsunami hazards were reviewed in order to clarify why the established safety margin was not sufficient during the Fukushima Daiichi. It was found that the plant incorporated the necessary safety margins against seismic oscillation however, there was a large “cliff edge11A cliff edge effect in a nuclear power plant is an instance of severely abnormal plant behavior caused by an abrupt transition from one plant status to another following a small deviation in a plant parameter, and thus a sudden large variation in plant conditions in response to a small variation in an input (IAEA Safety Standards Series No. SSG-2, 2009).” in which the radiological consequences surged by several orders of magnitude from the design basis tsunami. Since the tsunami's height was greater than the ground level of the turbine hall, a large amount of the tsunami water leaked through the truck entrance shutters and louver windows for the Diesel Generators’ air intakes. In view of the difficulties in predicting natural events when establishing the design basis for nuclear facilities, a drastic reappraisal of the safety design approach is essential when considering risks and uncertainties. The author proposes a new probabilistic seismic and tsunami safety goals be developed on the basis of lessons learned from the Fukushima disaster which would fortify the vulnerable systems thereby reducing seismic and tsunami risks as low as practical. The safety goal should also be used to enable stakeholders to find an answer to the question of ‘how safe is safe enough’. Through the development of the safety goals it is demonstrated that the risks of tsunami hazards are by far the largest risk to nuclear facilities in Japan due to its high recurrence period in certain regions of the country. It is essential to guard against tsunami-induced flooding and the need for more robust emergency power supply systems as well as special provisions for the disposal of hydrogen gas in the event of severe accidents.