External Flooding

This chapter overviews the techniques for incorporating external events into a PSA. The discussion was primarily aimed at nuclear power plants but is equally applicable to chemical process plants. The types of external events discussed were earthquakes, fires and floods. Notably absent were severe winds and tornados. Tornados are analyzed as missiles impacting the structures and causing common-cause failures of systems (EPRINP-768). Missile propagation and the resulting damage is a specialized subject usually solved with computer codes. 

The analysis methods are similar for all external events probability of the external event, probability of failures, effects of failures on safety systems, and estimating the effects of failures for the workers, public and environment. 

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Forest fire Fire cannot propagate to the site because the site is cleared plant design and fire-protection provisions a adequate to mitigate the effects Seiche Included under external flooding... 

Hail Subset of missile analysis Snow Plant designed for high loading, snow melt causing river flooding is induui-i under external flooding... 

High water Included under external flooding rstinami Included under external floo-tin. ind -c-ismic events... 

External Flooding Extreme Winds Extreme Suimner Temperature Fog... 

Whereas Appendix 1 of DS 394 hsts many hazards related to harsh weather conditions, only the hazards high winds and external flooding are addressed explicitly in the text. The discussion of these hazards covers the aspects... 

This Safety Report addresses only the internal events that originate in the reactor or in its associated process systems. Some initiating events that affect a broad spectrum of activities at a nuclear power plant (often referred to as internal or external risks), such as fires (internal and external), flooding (internal and external), earthquakes and local external impacts, such as aircraft crashes, are not discussed in detail. Nevertheless, the guidance provided may be used for analysing the consequences of such events from the viewpoint of neutronics and thermohydraulics. 

Protection should be provided to prevent an external flood leading to the failure of safety system equipment. ... 

For further information on external flooding refer to Safety Series Nos 50-SG-S10A, Design Basis Flood for Nuclear Power Plants on River Sites (1983) 50-SG-S10B, Design Basis Flood for Nuclear Power Plants on Coastal Sites (1983). 

Systematic external assessment of external flooding has been carried out in 1995. Systematic assessment for establishing seismic parameters for review has been initiated. 

External Flooding Aircraft Impact External Explosion Extreme Temperature Drought Extreme Wind External Fire External Missiles Biological Fouling EMI and Lightning Malicious Activities Surrounding Influences... 

The withstand required by plant systems, structures and components against external hazards, such that appropriately safe operation can be maintained, has been addressed comprehensively in the DCD, primarily in Chapter 3 and will be discussed in more detail in the sub-section following. On the basis of the requirements for withstand being directly incorporated into the Design Basis for the plant systems, structures and components, only those postulated hazards with significant requirements on management are taken forward for further assessment in the PRA, to make sure that the requirements to address any potential vulnerabilities are captured. The external hazards taken forward to the PRA for this purpose are external flooding, extreme winds, seismic and transportation accidents (external explosion). 

The basic steps involved in an external flooding analysis are similar to those followed for internal flooding in the individual plant examination. However, the focus of attention is on areas, which due to their location and grading may be susceptible to external flood damage. This requires information on such items as dykes, surface grading, locations of structures and locations of equipment within the structures. It is expected that the generic envelope will bound site hazard parameters. 

APIOOO assessment parameters for external flooding exceed those associated with the UK Design Basis events, even when account is taken for climate change. 

No other SSC could fail following an external flooding event such that a safety-significant SSC is consequently adversely affected. 

External hazards - for example seismic, extreme weather, and external flooding. 

The external hazards considered are seismic events, external floods, extreme weather, lightning, external electromagnetic interference, and aircraft crash analysis demonstrates that the design features of the API000 provide appropriate protection against these hazards (External Hazard Report, Reference 14.30). Therefore, it is demonstrated that external hazards do not present a significant probability of a major accident to the environment. Hence, the risk of acute environmental pollution as a result of external hazards is not considered further in this section. Plant faults and internal hazards are discussed further below. 

There are many operational records of experience of external flood induced accidents in which the functionality of safety related equipment has been impaired. Most of these are related to insufficient measures for site protection, to poor maintenance of the drainage systems and to effects of ice on river sites. 

Originally, a more comprehensive update of the section on external hazards was planned. To date, the supplementary document, however, will focus on seismic events and external flooding. 

Needs for room and floor drainage should be considered and provisions for these should be commensurate with the maximum level of external flooding for the site. 

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