Initiating Event Frequencies

Zaoh specific initiating event frequency history. 

The Millstone Unit 1 PRA contains component failure and maintenance unavailability data, and initiating event frequency data, including typical BWR anticipated operational occurrences and LOSP. 

Event trees are used to perform postrelease frequency analysis. Event trees are pictorial representations of logic models or truth tables. Their foundation is based on logic theory. The frequency of n outcomes is defined as the product of the initiating event frequency and all succeeding conditional event probabilities leading to that outcome. The process is similar to fault tree analysis, but in reverse. 

Risk Analysis—The development of a quantitative estimate of risk based on engineering evaluation and mathematical techniques for combining estimates of initiating event frequency and independent protection layers and consequences. 

For the initiating event, frequencies per operation year are determined, as a rule, by evaluation of statistics, or, if no experience is available, by justified assessments. The probability of non-availablities of safety installations or measures is determined by fault trees or immediately by values derived from operational experiences. The total frequencies of the event sequences are then calculated by multiplication of the frequency of the initiating event with those of the non-availabilities. 

Identify the initiating event of the scenario and determine the initiating event frequency... 

When an organization implements LOPA, it is important to establish tools, including aids like look-up tables for consequence severity, initiating event frequency, and PFD for standard IPLs. The calculation tools must be documented, and users trained. All LOPA practitioners in an organization must use the same rules in the same way to ensure consistent results. 

Parameter imcertainty is that which relates to the parameters of the PRA, given a choice of model. Even with a known model, the parameter values may stiU be unknown. Examples of parameter uncertainties include equipment failure rates, initiating-event frequencies, and human error probabilities. 

After selecting a HAZOP scenario and assigning the consequence severity category, the LOPA team had to establish the initiating event frequency. For the out-ofservice coke drum safety rehef valve leaking scenario, a frequency of 10 per year was assigned. All other scenarios were related to operator error. 

For the scenarios that used a safety relief valve as an IPL, the risk reduction required to eliminate the risk gap was achieved without the need of any instrumented function. For these cases, considering an initiating event frequency of 10 per year, a permissive logic IPL (implemented in the BPCS) with a RRF of 10 and a safety relief valve with a RRF of 100, the tolerable frequency of 10 per year was achieved with no need of a SIR... 

Step 1. If an initiating event occurs due to a room fire, then replace the initiating event frequency with the room fire occurrence frequency in the fault tree model or MCSs. 

In spite of the diverse uncertainty sources, the uncertainty analysis is commonly restricted to the consideration of uncertainties on initiating event frequencies and on reliability parameters for technical safety systems and for human actions. In a Level 2 PSA, the uncertainty analysis is additionally focused on uncertainties on physical / chemical phenomena... 

For a more extensive uncertainty analysis within a PSA, a systematic consideration of potentially relevant uncertainty sources is necessary. As a step in this direction, other sources of uncertainties than the conventional ones (like the uncertainties about initiating event frequencies, reliability parameters and about physical/chemical phenomena) are identified and discussed. The paper focuses particularly on model uncertainties. Additional papers on the characterization and quantification of model uncertainties can be found in (Mosleh et al. 1995). 

Initiating event Frequency (1/reactor-year) Characteristic Identification... 

The operational data should also include information on component and system performance, initiating event frequencies, component failure rate data, modes of failure, system unavailability during maintenance or testing, and component and system repair times. 

The initiating event frequencies and equipment failure probabilities used should be appropriate to the design or operation of the plant If possible, plant specific data should be used. When this is not possible, data from the operation of similar plants should be used. Again, when this is not possible, generic data should be used when these can be shown to be relevant. Eor initiating events with a low frequency, a judgement should be made. 

Human errors which can lead to initiating events should be identified and included as part of the initiating event frequency. 

Data should be collected by the plant operators throughout the lifetime of the plant to check or update the analysis. These should include statistical data on initiating event frequencies, component failure rates and plant unavailability during periods of testing, maintenance or repair. The analysis should be assessed in the light of the new data. 

Initiating event frequencies and component failure rates where no operating experience data exist,... 

The question of what can go wrong is answered by identifying the accident initiators (called the initiating events IE). The PRA analyst then needs to answer the questions of "With what likelihoods " and "With what potential consequences " These latter two questions can be answered by quantifying the initiating events frequencies and the risk associated with each accident sequence, respectively. 

Comparison of observed demand rates, from user trip reports and near-miss evaluations, versus assumed initiating event frequencies -5.2.5.3... 

This PFDavg provides the amount of risk reduction that was determined to be required from the site risk criteria. The frequency of the scenario of loss of containment is calculated by multiplying the initiating event frequency by the probability of the enabling event and the PFDavgS of any protection layers ... 

The LOPA team looked at the LOPA rules. The initiating cause, control valve CV-1, is shared by the control loop and by the SIS. The protection layer is not completely independent of the initiating cause. The simple math of multiplying the initiating event frequency by the probability of failure of each protection layer would not yield correct hazard rate. 

It is important to note that only those protection layers that can mitigate the initiating cause are given credit in the analysis. The calculation results in an optimistic result when the calculation is done by simply multiplying the initiating event frequency of the entire BPCS loop by the probability of failure of the SIF without regard for the common mode existing between them. This is shown in Table F.3 as Incorrect... 

---