External events, hazards earthquakes

Recommendations on design features relating to all external events, excluding earthquakes, have been incorporated into this publication, which makes reference to the Safety Guides on external human induced events, flood hazards and extreme meteorological events [2-4],... 

It is preferable to evaluate the external event hazard on a probabilistic basis. The frequency of occurrence of the parameters describing the severity of the external hazard (such as earthquake ground acceleration, wind speed, water elevation) is estimated by probabilistic methods. Statistical parameters used for extreme events include return period and annual probability of exceedance. The hazard from other rare external events such as accidental aircraft crashes or explosions reflects the frequency of occurrence of an event with postulated characteristics (quantity of explosive material, weight and velocity of the missile, etc.), as proper statistics may not be available for the area of interest. Performance goals depend on the external event categorization as defined in Section 2. For practical use they can be approximated by deriving the product (for continuous hazard levels it can be the convolution) between the annual probability of exceedance of an external event and the Pp induced by that specific external event. Probability values for performance... 

It is clear that the precautions taken by the designer against theso extern random hazards depend not only upon the likelihood of their occurrence, bu also upon the consequences of the structure being overcome by them. Fo example, as stated previously, fire is a major hazard to all buildings an 1 must b considered, but in the U.K. the likelihood of an earthquake is so remote tha such an event can be discounted. However, if a nuclear reactor is to bt- built, th consequences of failure are so enormous that even such a remote po.ssibilit must be considered. 

External event Event caused by (I) a namral hazard—earthquake, flood, tornado, extreme temperature, lightning, etc or (2) man-induced events— aircraft crash, missile, nearby industrial activity, sabotage, etc. 

The external events considered in this report include both natural hazards and human induced hazards from sources external to the site or external to the safety related buildings. Explicit reference is made to the most common external event scenarios considered in the design of research reactors (earthquake, wind, precipitation (snow, rain, hail), flood, explosions and aircraft crash, external fire), for which special recommendations are provided. However, the approach to the safety evaluation discussed in the present publication can be applied to any scenario included in the facility s safety analysis report. 

The probability of failure of structures, systems or components as the result of external events is computed as the product of the full range hazard curve of the external events convoluted with the derivative of the fragility of the structure, system or component under consideration, as shown in Section 4. The fragility of structures, systems and components is defined as the cumulative conditional Pp (unacceptable performance) versus the selected hazard parameter. The hazard parameter is typically represented by factors such as the peak ground acceleration (PGA) for earthquakes, the water depth for floods and the maximum wind speed for winds. 

The selection of the P(EE) should follow the considerations of Section 2.4 that some events show higher destructive potential, or the potential for common cause failures, and therefore their return period may be longer. However, physical considerations may also affect the choice of P(EE). For some events, evaluation of a very low probability hazard is feasible because physical evidence is available (typically earthquakes), but for some scenarios this may not be the case (e.g. precipitation). A feasible choice for P(EE) is therefore suggested in the following the values have to be interpreted as minima in order to rehably estimate the associated physical description of the external event scenario. 

In addition to the traditional in-plant (internal) initiators discussed above, there are external initiators that can occur with variable magnitudes. Hazard analyses are performed to assess the likelihood of such events as functions of their magnitudes. Such analyses may indicate that the risk contribution of some initiators is clearly negligible. For example, the frequency of aircraft-impact damage to any one of the vulnerable structures whose failure could lead to core melt is often found to be much lower (e.g., by a factor of 100) than the frequency of other large external events, such as earthquakes. (If the consequences of severe accidents induced by aircraft impact are comparable to those for severe accidents induced by more likely external events, then detailed assessments of aircraft-impact accidents may be unnecessary.) Some unique characteristics of particular initiators are discussed in more detail below. 

At least one bounding accident from each of the major types has been selected unless the bounding consequences are low. Accident categories are internally initiated operational accidents (fires, explosions, spills, and criticality) natural phenomena events for the site (e.g. earthquakes, tornadoes) that could affect the facilify and externally initiated, man-made events (e.g. airplane crashes, transportation accidents, and adjacent facility events). Criticality assessments have indicated that criticality is an incredible event for isotope processing operations. Based on these evaluations, criticality has not been included in the hazard analysis and will not be included in the accident analysis. 

The failure of non-structural elements such as block walls, stairs and scaffolding could have consequences for SSCs. External hazards (such as earthquakes, high winds, explosions or impacts of aircraft) could be the cause of such a failure and they are usually evaluated on the basis of Ref [5]. However, there may be situations in which the failure of non-structural elements may be caused by internal initiating events such as operator error or accidents during maintenance. The consequences for SSCs should be evaluated in these cases. Care should be taken either to avoid such failures or to minimize the potential damage to SSCs by means of proper location and adequate barrier design. 

Currently, the assessment of seismic capacity is being carried out to comply with the relevant IAEA recommendations. In addition, a probabilistic seismic hazard analysis is included in the Temelin PSA Project scope in order to address the contribution from earthquake induced accident sequences to the overall CDF of the plant. The seismic hazard curves for the Temelin site has been developed and seismic fragility analysis has been performed for the structures and components. Based on the preliminary results (annual frequency of O.lg PGA (SSE) earthquake is lE-6/year), it is expected that the contribution of seismic events and the consequential accident sequences to the overall CDF will be negligible (i.e. less then 1% of overall CDF). The independent review of this PSA task is to be carried out in the framework of the 2nd IAEA IPERS (Level 1 - external initiating events) in August/September 1995. 

In a similar way to the first report in this series. Ref. [3], this PHWR specific volume also deals only with internal events originating in the reactor or in its associated process systems. It does not cover originating events affecting broad areas of the plant (often called internal and external hazards), such as fires, floods (internal and external), earthquakes and aircraft crashes. However, analysis of the consequences of these events from a thermohydraulic point of view is partially covered by the present guidance. The emphasis in this guidance is on the transient behaviour of the reactor and its systems, including the containment and/or confinement. 

The layout of safety systems should be such that the minimum required capability is maintained in the event of a failure in one train of protection or in the event of needing to survive any internal and/or external hazards (e.g. earthquake, fire and flooding). 

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