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Proceedings of the Eighth International Conference on Probabilistic Safety Assessment & Management (PSAM)

Michael G. Stamatelatos
Michael G. Stamatelatos
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Harold S. Blackman
Harold S. Blackman
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ASME Press
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A comprehensive PSA Level 1 and Level 2 has been performed for the NPP Ringhals 2 from 2001 to 2004. After this there have been some minor updates by RAB. In this paper the analysis of external events will be presented.

The work started with identifying a large number of potentially relevant external events, both natural and man-made. In total more than 50 different single events were identified.

The potential plant impact from external events was also systematically assessed by grouping events according to the type of impact from each event on the basic interfaces between the plant and its surroundings, i.e., structure, air supply (ventilation, combustion, etc.), ultimate heat sink, external power, integrity to external flooding or fire, and integrity to electric impact.

Based on this systematic mapping of potential threats, a four-stage screening analysis was performed aiming at identifying events that would need to be analyzed in detail in the plant PSA. Each stage involved collection of additional information concerning external events (strength and frequency) and plant structures (capacity and vulnerabilities). The aim was to exclude as many events as possible as early as possible, thus making optimal use of the resources available for the analysis.

As a result of the screening analysis the following single external events were selected for detailed analysis and (possibly) inclusion and quantification in the PSA:

• Strong winds

• Extreme snow

• Explosion in the on-site hydrogen production plant

• Airplane crash

• Extreme sea water levels (low or high)

• Organic material in intake water

In addition two multiple external events were identified:

• Strong winds (causing loss of external grid) and organic material in water (causing loss of ultimate heat sink)

• Strong winds (causing loss of external grid) and extreme snow (causing loss of ventilation)

A number of recommendations were also made in order to verify the status of the plant protection against certain external events.

A detailed analysis was then performed which aimed at collecting sufficiently detailed information on the events to make possible a final analysis of the strength and frequency of the events, and to judge the plant impact. After the detailed analysis, three further events could be excluded from analysis (airplane crash and high/low sea water level). For the remaining events, the scenarios to be modeled were defined, modeled in the plant PSA, and quantified.

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