The safety of NPPs has to be maintained in any condition which has to be considered. Rational criteria are especially needed for adequate classification of external hazards. Characteristics of external hazards, e.g. Tsunami, Earthquake, Land slide etc, are changed by features of the site. It is necessary to make a rational and clear process of classification to appropriate method of risk assessment. This study treated of various external hazards included natural phenomena and man-made events. Much disaster information was gathered from historical documents these one thousand and five hundred years (1). Published standards of US and IAEA were referred. This study provided some viewpoints for classification, e.g. hazard frequency, range of impact, basic design, and difficulty of accident management etc. The classification included several paths; a full-scope PRA, a hazard analysis, or a comparison between an impact of the hazard and the design condition against it.

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