External Events

In this section the design criteria for the resistance of structures, systems and components to external events should be presented. These may include  

Additional information on siting requirements is presented in Section 4 of Safety Series No. 35-Sl. 

In this section, all codes and standards to be employed in the design of 

If different codes and standards are used for different aspects of the same item or system, the consistency between them shall be demonstrated. Typical areas covered by codes and standards are  

For items important to safety for which no appropriate established codes or standards exist, an approach derived from existing codes or standards for similar equipment should be applied. In the absence of such codes and standards, the results of experience, tests, analysis, or a combination thereof, may be applied, and an explanation of these results should be given. 

Instant in time of cooling failure Final quantity of TCDD in kg Maximum temperature in K Final quantity of TCDD in kg Maximum temperature in K 

According to [8] the operator of a plant does not only have to make provisions against plant internal hazard sources but also against external ones (literally in Germany environmentally caused hazard sources). This concerns impacts from outside the establishment (and hence on the plant) which may jeopardize the function of safety-relevant parts of the plant [27]. Such impacts may originate from  

Impacts from neighbouring establishments may result from fires and explosions (vid. Sect. 10.6), missile generation (vid. Sect. 10.9), ground motion or releases of toxic materials (vid. Sect. 10.5). 

Neighbouring transport installations (roads, rails, waterways) are to be regarded as external hazards, if the increased risk is caused by the trafiic conditions in the vicinity of the establishment (e.g. traffic density, traffic routing, type of transports, weather conditions). Under certain circumstances civil and military air traffic is included. 

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Two types of initiators are internal and external. Internal initiators result from failures within a plant or the plant s support utilities. Thus, vessel rupture, human error, cooling failure, and loss of offsite power are internal events. All others are external events earthquakes, tornados, fires (external or internal), and floods (external or internal). Event trees can be used to analyze either type of initiator. 

RISKMAN is an integrated Microsoft Windows , personal computer software system for [H. i forming quantitative risk analysis. Used for PSAs for aerospace, nuclear power, and chemical [iroccsses, it has five main modules Data Analysis, Systems Analysis, External Events Analysis, Event Tree Analysis, and Important Sequences. There are also modules for software system maintenance, backup, restoration, software updates, printer font, and page control. PEG has also integrated the fault tree programs CAFTA, SETS, NRCCUT, and IRRAS into RISKMAN. 

Analyzes shutdown and full-power operations for internal and external events ... 

External events are accident initiators that do not fit well into the central PSA structure used for "internal events." Some "external events" such as fire due to ignition of electrical wires, or flood from a ruptured service water pipe occur inside the plant. Others, such as earthquakes and tornados, occur outside of the plant. Either may cause failures in a plant like internal events. External initiators may cause multiple failures of independent equipment thereby preventing action of presumably redundant protection systems. For example, severe offsite flooding may fli 1 the pump room and disable cooling systems. An earthquake may impede evacuation of the nearby populace. These multiple effects must be considered in the analysis of the effects of external events. 

A characteristic of "external" events is their propensity for causing multiple failures. This is simplified somewhat for internal flooding by assuming equipment fails when it is submerged, hence, failure is determined by the water level and the location of equipment. 

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. 

CR-2300 (PRA Procedures Guide) with actual expenditures (Joksim-ovich, 1983). In this comparison of analyses. Grand Gulf is a level 1 PSA Arkansas Nuclear 1 (ANO-1) is an IREP level 2 PSA Big Rock Point (BRP) is a level 3 PSA with limited treatment of external events but thorough in consideration of environmental effects on equipment. The Zion PSA thoroughly treats both... 

It is more common for a complete PSA to take 16 to 24 months with several rnuiilhs tor preparation, review, and revision of the final report. The final report for a level 3 nuclear plant PSA, includes an analysis of external events, in several large volumes. Completeness and consistency in such a large document requires several months of team leadership and selected analysts. Given these resources, it may be possible to complete the technical analyses for a Level 1 PSA in a year or less, but the final report will take several more months to prepare. 

A Level 1 PSA is a core-melt analysis that may or may not include external events a Level 2 PSA encompasses Level 1 and extends to the physical processes of the accident and their effect on containment a Level 3 PSA encompasses Levels 1 and 2 and extends to public risk. 

Emergency Flan Responses Sirens/warning, Emergency prtKedures, Personnel safety equipment. Sheltering, Escape and evacuation External Events Early detection. Early warning, Specially designed sfn- ctures Training... 

Analysis of External Events uses the models developed in the plant system analysis with considerations for seismic, fire, flood, high winds and missiles on the plant. Additional c cm trees or their equivalent may be needed for the external events. 

This section reflects on the limitations of the PSA process and draws extensively from NUREG-1050. These subjects are discussed as plant modeling and evaluation, data, human errors, accident processes, containment, fission product transport, consequence analysis, external events, and a perspective on the meaning of risk. 

The methodology for assessment of external events is qualitatively satisfactory but not quantitatively. Little confidence should be placed in any estimates of the risk from external initiators compared to those from internal initiators. This is exacerbated by the fact that the external risk is the larger of the two in many cases. 

Risk-based information provides a foundation for regulation of severe accidents. Early PRAs, with large uncertainties, indicated risk that was above or below the Safety Goals depending on containment performance. Consequently the NRC developed an Integration Plan for Closure of Severe Accident Issues (SECY-88-47) with six main elements to this plan 1) individual plant examinations (IPE), 2) containment performance improvements, 3) improved plant oper itions, 4) severe accident research, 5) external event considerations, and 6) accident management. 

In 1988, an internal events PRA was published (Johnson, 1988) this was followed by an external events analysis. The results were reported by Johnson (1991), and Flanagan (1990). The basic approach to risk is that of Kaplan (1981) that asks the questions "What can happen, and How likciy is it and what are the consequences " These are organized as triplets to characterize the risk... 

The external events PSA was based on standard methods used for commercial reactor PSAs, Fire risk was estimated from commercial nuclear power plant data combined with industrial fire information. The seismic hazard was evaluated using a combination of the EPRI and LLNL ( UREG/CR-.3250) databases. Wind hazards were analyzed by EQE, Inc., using NRC-based nicihodulogy. 

The results are summarized in Table 11.3-3 for both internal and external results, the CDF from all causes is 7.4E-4/y of which internal events contribute 3.1E-4 and external events 2.9E-4. This number has been reduced by design, administrative, and operational procedure changes. 

Brandyberry, M. D. and H. E. Wingo, 1990, External Events Analysis for the Savannah River Site K Reactor, ANS Topical Meeting, The Safety, Status and Future of Non Commercial Reactors and Irradiation Facilities, Boise ID, Sept. 31 - October 4, 1990... 

The German Gesellschaft fur Reaktorsicherheit (GRS) has a private arrangement with Rheinische Westalisches Elekrizitatswerke (RWE) to compile reliability data from an operating power plant, Biblis B. The data base contains failure rate, maintenance, and operational event data. External event data (floods, earthquake, fire, etc.) are compiled through a separate utility-sponsored data base. The data base provides information on repair and maintenance, and equipment performance. 

Other natural disasters and external events are listed below in Table 6.7.1., along with an abbreviated comment. 

Failure sequence modeling techniques such as fault tree analysis or event tree analysis are used to estimate tlie likelihood of incidents in facilities where historical data is unai ailable, or is inadequate to accurately estimate tlie likelihood of the liazardous incidents of concern. Otlier modeling tecluiiques may be required to consider tlie impact of external events (eartliquakes, floods, etc.), common cause failures, and human factors and hmnan reliability. 

Although natural selection is the only evolutionary agent that adapts organisms to their environments, the course of evolution has been profoundly influenced by major environmental changes, some of which had catastrophic effects. Some of these events resulted from Earth s internal processes, such as the activity of volcanoes and the shifting and colliding of continents. Others were the result of external events, such as collision of meteorites with Earth. 

External events have also triggered important changes. At least 30 meteorites hit Earth each year, but collisions with very large meteorites are very rare. One, about 10 km in diameter that collided with Earth 65 million years ago, caused massive firestorms and tidal waves and triggered the extinction of many species of marine organisms and all terrestrial animals larger than about 25 kg in body weight. 

The electrochemical detector must be zeroed after each analysis just before the next sample injection. This procedure is necessary owing to the drifting baseline associated with the electrochemical detector. The detector is equipped with this baseline zero capability, and the adjustment can be activated through an external event output signal sent from an autosampler. 

The number of alcohol PCP - and heroin PCP-related deaths was higher than expected. Users may prefer taking these drugs in combination, or there may be an interaction in the effect of the combinations. Many of the PCP-related deaths are the consequence of some external event rather than a direct consequence of the drug itself. 

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