Failures initiating events, frequency

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. 

Description of the failure (initiating event) Expected frequency of the initiating event hj in aT Unavailability of the system function Expected frequency of the undesired event Hj in a ... 

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. 

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. 

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 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... 

Where the initiating event is caused by the failure of a person fo carry out a task correctly and in a timely manner, the initiating event frequency is calculafed as fhe product of the number of times the task is carried out in a year and the human error probability (HEP) for the task. In this case, the time at risk (see Annex 4) is already included in the number of times the task is carried out in a year and no further factor should be applied. 

The frequency associated with any particular outcome of the event tree is the product of the initiating event frequency and the successive, often dependent success or failure probabilities at each branch. For example, the risk of core damage due to an accident initiated by a small LOCA (S2) and compounded by failure of both High Pressure Injection (fHPI) and Automatic Depressurization (FADS) is... 

The validity of PRA results is determined in part by the quality of the data that is used in the quantification. Collection and analysis of data is therefore an important part of a reactor PRA. Data needed in order to perform a core damage frequency analysis include component failure rates, test and maintenance unavailabilities, initiating event frequencies, and human error rates. When possible, it is generally best to use... 

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. 

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 an initiating event is usually based on industrial experience. If the process is new or rare, it may be estimated by a system model of the process steps (e.g., a fault tree) and using data from similar experience to give the probability of failure of the steps. Either of these estimates should consider the possibility of mitigating actions to prevent the hazard from having detrimental effects. 

The failure rates for the safety functions are written below the column headings. The occurrence frequency for the initiating event is written below the line originating from the initiating event. 

The failure frequencies for the common initiating events of an accident scenario are shown in Table 11-3. 

The initiating event corresponds to a leakage in any mechanical equipment, pipe or flange, located between the vacuum column C-201 and the decomposition reactor E-207, as shown in Figure 6. Since the pipeline will be under severe conditions of acidity, pressure and temperature, an occurrence frequency of 5.23E-5/year is assumed, which is the generic value for catastrophic pipe failure (including straight pipe and connections) (Bari, 1985). 

Step 1 - Complete the LOPA without taking any credit for the SIF. First, determine the initiating events from HAZOP/What-if/EMEA study. Next, evaluate frequencies of all initiating events from company database and industry experience. Then, determine the probability that each IPL will function successfully from an industrial database. PFO yg of some typical protection layers are (CCPS, 2000) BPCS control loop = 0.10 Operator s response to alarm = 0.10 Rehef safety valve = 0.01 to 0.001 and vessel failure probability at maximum design pressure = 10 ". Finally, compare the calculated risk with the tolerable risk target... 

If control system equipment failure can cause an initiating event, then quantitative analysis must be done for all components where failure might initiate a hazard. For those failures with no other protection layer, the frequency of failure will result directly in an incident. The detailed quantitative analysis must show that these failures will not increase risk beyond tolerable levels. 

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