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Assurance of Seismic Safety by Defense-in-Depth and Safety Goals: Lessons Learned From the Chûetsu-Oki Earthquake (2007)

[+] Author Affiliations
G. Saji

Nuclear Safety Commission of Japan, Yokohama, Japan

Paper No. ICONE16-48309, pp. 519-532; 14 pages
  • 16th International Conference on Nuclear Engineering
  • Volume 1: Plant Operations, Maintenance, Installations and Life Cycle; Component Reliability and Materials Issues; Advanced Applications of Nuclear Technology; Codes, Standards, Licensing and Regulatory Issues
  • Orlando, Florida, USA, May 11–15, 2008
  • Conference Sponsors: Nuclear Engineering Division
  • ISBN: 0-7918-4814-0 | eISBN: 0-7918-3820-X
  • Copyright © 2008 by ASME


Although the basic safety concerning control of reactivity, residual heat removal and confinement, was assured in the Kashiwazaki–Kariwa Nuclear Power Plants at the time of the Chûetsu-Oki Earthquake (2007), the accident caused great public concern as to the seismic safety of NPPs. The earthquake resulted in severe economic impacts, far exceeding the actually negligible environmental effect. The public is calling for a reassessment of the seismic safety of NPPs, as they are unable to understand the basic safety approaches of the Japanese seismic design practice. The earthquake significantly exceeded the design basis ground motion in all units. Due to this the seismic consequences, especially those malfunctions and damages in the lower seismic class items, are not surprising. The following three topics are highlighted from the lessons learned and the author’s reappraisal of the current seismic safety approach, namely: (1) Prevention of seismic consequences of the lower seismic class items, (2) Measures to ensure the seismic safety by including defense-in-depth, and (3) Reduction of seismic safety risks as low as reasonably achievable referring to ‘safety goals for seismic events.’ The author believes that ‘reduction of seismic risks as low as practical (S-ALAP)’ should be a new guiding principle for lower safety class items, in line with the new concept (gensai) developed in light of the Great Hanshin-Awaji Earthquake (1995), acknowledging difficulties of coping with earthquakes just by conservative design. For a reasonable reduction of seismic risks, it is necessary to answer to question of ‘how safe is safe enough.’ The author developed a safety goal for seismic consequences by integrating the International Nuclear Event Scale (INES) and Farmer’s probabilistic siting criteria. It is an extension of the author’s quantitative safety goals for non-seismic events already published in a series of previous papers (including RESS Vol. 80-2, pp. 143–161, 163–172; PSA’05-139985, 139989, 139990: ICONE14-89351).

Copyright © 2008 by ASME



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