Selection of Initiating Events

This section shall list the postulated initiating events that are treated in the safety analysis. The list shall be comprehensive, and justification for the rejection of particular initiating events shall be provided. Annex I provides some information on methodologies. The selection should consider the points mentioned in paras A. 1611 and A. 1612. 

The initiating events in each group shall be evaluated to identify the events that would be limiting, and the events selected for further analysis shall be indicated. Such events would include those having potential consequences thdt are limiting for all other initiating events in the group. 

The detailed information listed below should be given for each initiating event selected in para. A. 1612. This information is organized under the following headings  

For each event evaluated, a description of the occurrences that led to the initiating event under consideration should be included. 

The step by step sequence of events, from the event initiation to the final stabilized condition, shall be described. The following should be provided for each event sequence  

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Identification of hazards and selection of initiating events are the first steps in any risk analysis. Hazards are generally related to the physical properties of the material being transported. The properties represent the inherent risk and the potential adverse consequences if the material were released. The hazards to be considered when evaluating different movements may include ... 

Consistent with these publications, the IAEA in 2002 issued a detailed report on Accident Analysis for Nuclear Power Plants (Safety Reports Series No. 23) that provides practical guidance for performing accident analysis. That report covers the steps required for accident analyses, i.e. selection of initiating events and acceptance criteria, selection of computer codes and modelling assumptions, preparation of input data and presentation of the calculation results. It also discusses aspects that need to be considered to ensure that the final accident analysis is of acceptable quality. Separate IAEA Safety Reports deal with specific features of individual reactor types, such as pressurized water reactors, boiling water reactors, pressurized heavy water reactors and RBMKs. 

Selection of initiating events that includes cascading failures— this is important since the specific initiating events of interest here are those where cascading failures played a major role. 

Selection of initiating event types to consider this is important since what types of events to consider will affect the types of cascading failures and contexts that will be covered, which in turn will affect which decision contexts the generated knowledge can be used in. 

In the present framework a number of criteria can be defined that can be used as a point of departure for selection of initial event types to consider. These include the following ... 

Selection of initiating events that include cascading failures... 

Based on the selection of initiating event types outlined in previous section, a selection of specific events should be made. With few exceptions, the existing frameworks do not address this function in great detail. In the present framework the following criteria are suggested for the selection of events ... 

The report deals with the usual approach to accident analysis adopted for present CANDU reactors. This means that the use of best estimate computer codes and tools is encouraged in such a way that sufficient safety margins are ensured by means of a conservative selection of initiating events and then-combinations, and specifying conservative plant input data for analysis. 

Layer of protection analysis (LOPA) is a simplified form of event tree analysis. Instead of analyzing all accident scenarios, LOPA selects a few specific scenarios as representative, or boundary, cases. LOPA uses order-of-magnitLide estimates, rather than specific data, for the frequency of initiating events and for the probability the various layers of protection will fail on demand. In many cases, the simplified results of a LOPA provide sufficient input for deciding whether additional protection is necessary to reduce the likelihood of a given accident type. LOPAs typically require only a small fraction of the effort required for detailed event tree or fault tree analysis. 

Table 1 lists a selection of these events and processes from the time of accretion to c. 2 Ga. This is perhaps a somewhat subjective list of events and only about half relate to the Archaean Eon sensu stricto (2.5-c. 3.9 Ga), whereas others relate to initial accretion, the Hadean, or the transition to the Proterozoic Eon. Yet other events or processes were diachronous or remain poorly dated. 

The appendix provides a list of the selected postulated initiating events to be considered in the safety analysis for a research reactor. 

Clearly, it will be necessary to demonstrate, first, relief of translational competition in an efficient and more native cell-free system, such as the micrococcal nuclease-treated reticulocyte lysate, and under conditions where the total number of initiation events does not change, and second, selective and specific binding of eIF-4A or CBP-II to individual mRNA species, before it can be concluded with certainty that these proteins can act as targets of mRNA competition. 

For each selected postulated initiating event, a list of safety related structures, systems and components needs to be developed and safety requirements for these items established. Spatial and other possibiUties of interaction between items have to be examined since an external event can alter the... 

The text above describes the selection of initial conditions when calculating the rate constant for reaction with an isolated site on the surface. For simulating other gas-surface events, such as energy transfer to the surface " or to the projectile, " initial conditions must be chosen so that the projectile is aimed at a random point on the surface. This is accomplished in the initial conditions, by randomly sampling the points in a unit cell on the surface. 

STATUS reevaluates plant status for a specific plant configuration. The user can update the plant status according to the components that are out-of- service. It can change the data by selecting basic events according to systems, initiating events, event codes, or attributes. 

Event Tree Analysis (ETA) A method for illustrating the intermediate and final outcomes that may arise after the occurrence of a selected initial event. 

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