Initiation events

Lifetime measurements have elements in eommon with both eounting and time-of-flight experiments [4, 5]. In a lifetime experiment there is an initiating event that produees the system tliat subsequently deeays witli the emission of radiation, partieles or both. Deeay is statistieal in eharaeter taking as an example luielear deeay. 

Event Trees. Event trees use an inductive logic approach to consider the effects of safety systems on an initiating event. The initiating event is propagated through the various safety functions. Branching is dependent upon the success or failure of the safety function. 

Consider again, for example, the case of the flat tire on an automobile. The initiating event in this case is the flat tire. There are two safety functions which can be defined a spare tire and an emergency road patrol. Other safety functions might be included depending on the particular situation. 

It is not coincidental that the top event of the fault tree is the initiating event for the event tree. The fault tree shows how an event is decomposed into basic events whereas an event tree demonstrates the effect of the various safety functions. The disadvantage of event trees is that the outcomes are difficult to predict. Thus the outcome of interest might not arise from the 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. 

Initiating Event Feed Shuts Off Reactor Dump Works Accident Sequence Number Frequency (events/yr) Consequence (impacts/event)... 

Frequency Phase 1 Perform Qualitative Study, Typically Using HAZOP, FMEA, or What-if Analysis. To perform a qualitative study you should first (1) define the consequences of interest, (2) identify the initiating events and accident scenarios that could lead to the consequences of interest, and (3) identify the equipment failure modes and human errors that could contribute to the accident... 

Frequency Phase 3 Use Branch Point Estimates to Develop a Ere-quency Estimate for the Accident Scenarios. The analysis team may choose to assign frequency values for initiating events and probability values for the branch points of the event trees without drawing fault tree models. These estimates are based on discussions with operating personnel, review of industrial equipment failure databases, and review of human reliability studies. This allows the team to provide initial estimates of scenario frequency and avoids the effort of the detailed analysis (Frequency Phase 4). In many cases, characterizing a few dominant accident scenarios in a layer of protection analysis will provide adequate frequency information. 

Certain kinetic aspects of free-radical reactions are unique in comparison with the kinetic characteristics of other reaction types that have been considered to this point. The underlying difference is that many free-radical reactions are chain reactions that is, the reaction mechanism consists of a cycle of repetitive steps which form many product molecules for each initiation event. The hypothetical mechanism below illustrates a chain reaction. 

After having studied the peer comments about some important classes of initiating events, we remain unconvinced of the WASH-1400 conclusion that they contribute negligibly to the overall risk. Examples include fires, earthquakes, and human accident initiation. 

Identify initiating events and event sequences that might contribute significantly to risk,... 

One of the products of a nuclear power plant PSA is a list of plant responses to initiating events (accident starters) and the sequences of events that could follow. By evaluating the significance of the identified risk contributors, it is possible to identify the high-risk accident. sequences and take actions to mitigate them. 

Function event trees are concerned with depicting functions that must happen to mitigate an initiating event. The headings of the function event tree are statements of safety functions that are required but that may fail in an accident sequence. 

Function event trees are developed to represent the plant s response to each initiator. The function event tree is not an end product it is an intermediate step that provides a baseline of information and permits a stepwise approach to sorting out the complex relationships between potential initiating events and the response of the mitigating features. They structure plant respoases to accident conditions - possibly as time sequences. The transition labels of function event trees (usually along the top of the event tree) are analyzed to provide the probability of that function occurring or not occurring. 

Some initiating events affect either the function or the availability of mitigating systems. Therefore, the set of systems available for mitigating events will be reduced from the full complement because of interactions. 

RDB, thej eliability database module, creates a user-defined database or retrieves data from the IAEA generic reliability database. The design facilitates RDB development for components, human actions, initiating events and the attributes of components. Component unavailabilities can be calculated from the database of reliability parameters using 10 types of predel mcd leliabiliiv models. 

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. 

Appendix HI, of WASH-1400 presents a database from 52 references that were used in the study. It includes raw data, notes on test and maintenance time and frequency, human-reliability estimates, aircraft-crash probabilities, frequency of initiating events, and information on common-cause failures. Using this information, it assesses the range for each failure rate. 

The frequency of anticipated transients is addressed by EPRI NP2330, 1982 io give information on the type and frequency of initiating events that lead to reactor scram. 

The first step-in plant-system and accident-sequence analysis is the identification of earthquake-induced initiating events. This is done by reviewing the internal analysis initiating events to identify initiating events relevant to seismic risk. For example. Table 5,1 -5 shows the initiating events that were used in the Seismic Safety Margins Research Program for a PWR plant (Smith et al., 1981)... 

I). iOtoO.459). Thejoint frequency distribution of responses at different critical locations in the piping is calculated. The convolution of the frequency distribution with the fragilities yields the conditional frequency of the initiating event. In preparing the list of the initiating events, it is necessary to consider the possibility lif multiple initiating events rather than single,... 

The construction of event trees for eaithquake-induced initiating events follows the metliods described in Section 4.3. From... 

Identify critical locations where fires can cause an initiating event that could fail redundant engineered safety functions, or disable redundant and diverse safety-related equipment. 

Assess the effects of initiating fires and subsequent fire growth on the initiating events, such as LOCAs and transients. 

Assess the effects of these fire scenarios on accident sequences in event trees for fire-induced initiating events. 

The plant internal PSA can be used to identify critical equipment that could be damaged by fire. This form of screening was employed in the fire-risk portions of ZIP. At each location considered, the loss of all the equipment in the zone is postulated regardless of the size or position of the fire in the zone. If this does not show the occurrence of an initiating event (LOCA or transient) or if the safety functions are not damage to required for safe shutdown, the location is eliminated from consideration. If the location is found to be critical, it is considered furilier lot-detailed fire growth and fire suppression analyses. 

TABLE 6.1-1. List ofPWR Transient Initiating Events (from NUREG-2300) ... 

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