External events, hazards

According to Ref [1], a periodic safety review should be conducted to assess whether or not the safety performance of the research reactor meets the applicable safety requirements, to account for external event hazard changes and research reactor configuration changes. The method discussed in Section 4 for the overall safety review may be used to this end. 

The external event hazard is evaluated on a probabilistic basis ... 

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... 

Development of a site dependent median (if mean values are chosen, a target probability one order of magnitude lower can be chosen) external event hazard definition in terms of the probabihty of exceedance for the annual frequency ... 

Evaluation of (structural or functional) demand associated with the external event hazard ... 

Equation (1) could first be used to check if the external event hazard and the component fragility are consistent with the performance goal. This means that the calculated total Pp corresponding to an accident sequence (damage state) as result of the loads induced by the external event has to be less than the performance goal ... 

Step 1 Define an approximate external event hazard curve applicable for the site. This step requires a site-specific investigation as described in Sections 5 and 6. 

After completion of the design process, the failure probabihty both at the structure, system and component level and at the facility level is evaluated and an overall performance evaluation carried out. At the end of the design process, the external event hazard is available in detail. The performance evaluation in relation to external events includes the following steps ... 

For hazard category 1 and 2 research reactors, the design basis wind can be evaluated on the basis of the selected probability of exceedance for the external event hazard according to the performance goal assigned to their safety class 1 and 2 structures, systems and components. For more sophisticated investigations and analysis, further guidance is provided in Ref. [22],... 

The decision to install monitoring instrumentation and to safety classify it is usually taken on the basis of the relevance of the external event hazard for system design and, in general, on the basis of the instrumentation s significance for the plant s emergency procedures. Seismic monitoring and automatic scram systems, when installed, need to be properly classified for safety and adequate redundancy according to their objectives. 

Process hazards Hazards of utilities Hazards of water Hazards of steam Hazards of ice Hazards of compressed gas Hazards of air Hazards of external events Hazards of equipment and instruments Hazards of piping, valves, and hoses... 

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. 

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. 

The Level 1 LPS PSA of Mochovce Unit 1 includes as internal events LOCAs, transients and internal hazards, i.e. fires and floods. External events included seismic events, air craft crashes, influences of external industrial facilities, and extreme meteorological conditions. Operational experience and results obtained from PSA for Bohunice V-2 NPP and Dukovany NPP as Mochovce NPP are used as much as possible. 

In addition, DOE 5480.28 addresses natural phenomena hazards protection for DOE-owned facilities it defines the design-basis external events for DOE-owned reactors. 

A formal hazard analysis of the anticipated operations was conducted using Preliminary Hazard Assessment (PHA) and Failure Modes and Effects Analysis (FMEA) techniques to evaluate potential hazards associated with processing operations, waste handling and storage, quality control activities, and maintenance. This process included the identification of various features to control or mitigate the identified hazards. Based on the hazard analysis, a more limited set of accident scenarios was selected for quantitative evaiuation, which bound the risks to the public. These scenarios included radioactive material spills and fires and considered the effects of equipment failure, human error, and the potential effects of natural phenomena and other external events. The hazard analysis process led to the selection of eight design basis accidents (DBA s), which are summarized in Table E.4-1. 

DOE Order 5480.23, Chg. I, Nuclear Safety Analysis Reports, paragraphs 8.b.(3)(e) and 8.b.(3)(k), as amplified in Attachment 1 to the Order, paragraphs 4.f.(3)(d)5 (Topics 5 and 11), require the analysis of both hazards and the facility classification, along with the assessment of operational, natural phenomena, and external events postulated to occur at the facility (DOE 1994a). 

This section summarizes the methods used to evaluate and quantify the consequences of operational accidents, natural phenomena events, and external events selected in Section 3.3.2.3.5, Accident Selection as DBAs. Consequences to the public and the environment stem from airborne releases of radioactive material since no liquid or solid radioactive material would be released in the selected accidents. The airborne pathway is of primary interest for releases from nonreactor nuclear facilities according to DOE-STD-1027-92 (DOE 1997). Exposure to direct or scattered radiation is a hazard only for workers due to the distance to the public. 

Spill (liquid radioactive materials-toxic constituents) Various hazardous radioactive liquids -Forklift puncture -Toppling of container (mishandling, energetic external event) -Fatigue ftiilure of container -Liquid radioactive material would be contained in spQl control pallet. -Spill would be limited to area immediately surrounding container. -Procedures and training direct woikers to leave area immediately. 

This appendix presents the potential external events that are considered in the hazard analysis of the Hot Cell Facility (HCF) and its associated radioactive material storage facilities. From this list of events, a screening assessment was performed to eliminate from further consideration any of e events that posed little or no hazard to the HCF and associated radioactive material storage facilities or their contents. Events that were not eliminated in this screening process were to be analyzed more closely as part of the qualitative analyses contained in Appendix 3C or 3D (and are summarized in Section 3.3.2, Hazard Analysis Results ). 

Potential external events were identified by reviewing previous Safety Analysis Reports of similar DOE facilities (Restrepo 1995) and the recommended list of external events used to evaluate commercial nuclear power plant risks (NRC 1983). In addition, an attempt was made to identify any other potential external-initiating event unique to the site that had not been considered in previous studies. It is important to note that operational accidents (e.g., criticality, internal fires) occurring inside the HCF and assodated radioactive material storage facilities are not considered in this screening process. These types of "internal initiating events are identified separately using preliminary hazard checklists (see Appendix 3A). 

All of the events listed in Table 3B-1 were eliminated from further consideration by using this screening process. (Some events were eliminated based on multiple criteria.) This does not mean they are all eliminated from all further consideration in the HCF SAR. Some external events are considered in Appendix 3C, Preliminary Hazard Analysis," or Appendix 3D, Failure Modes and Effects Analysis," as potential initiators to internal hazard events. 

In nuclear safety there is a clear distinction between internal events (loss-of-coolant accidents and transients) and external events. External events is an umbrella term for internal hazards (e. g. fires) and external hazards. Up to now, most PSAs have focnsed on internal events and internal hazards. With the exception of seismic PSAs, PSAs for external hazards are not yet very common. 

The answers to the questionnaire indicate that in regulatory requirements there is a general trend towards full-scope PSA, including external events. But even though some standards and guides on external events PSA are available, the analysis of harsh weather conditions carmot be considered a well established practice internationally. In a few countries external events PSAs with a fairly wide spectrum of initiating events are underway. But mostly the analysis is limited to specific weather conditions which are well known to be hazardous due to the regional conditions. 

Xe, Kr and tritium (T) that are released from molten salt fuel could be effectively trapped in an activated charcoal bed and/or other trapping materials. The container vessels in which the trapping equipment is installed have thick and heat-resistant steel walls and they can be isolated from the off-gas lines by passively operating valves. The production rate of T is estimated at about 6.2><10 Bq/(100 MW day) in normal operation. More than 90% of the T is transferred into the secondary coolant salt and finally, about 98% of the T is transferred to the trapping equipment through an off-gas line [XXX-22]. In this way, the hazard of radioactive gas release from the core under internal and external events and combinations thereof can be decreased ... 

The Rovno NPP site has been assessed with respect to man-induced external events. There are no gas lines, pipe lines or chemical or industrial plants near the plant site which could pose a potential hazard. However, there is a railway line about 500 m from the site. A detailed analysis of this will be conducted at a later date. For the present, a pressure wave with a value of 30 kPa will be considered in the design of the reactor building. If the analysis to be conducted at a later date shows that there are hazards resulting in loads higher than those resulting from this value of 30 kPa, then appropriate administrative measures will have to be enforced. 

The most extensive method for analysing external hazard risk contribution to core damage is PSA. The methods have been used since the very beginning as in NUREG-1150. These methods are presented in NUREG-4840, which provides a relatively straightforward and, in some cases, simplified set of techniques for the analysis of the full range of external events. However, the methods are continuously... 

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